August 29, 2023 • 53 mins
We encounter sticky surfaces and sticky substances everyday, but what exactly IS “stickiness?” In this episode of Stuff to Blow Your Mind, Robert and Joe discuss the physical properties of stickiness and look at some very sticky examples from the natural world.
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Speaker 1 (00:03):
Welcome to Stuff to Blow Your Mind production of iHeartRadio.
Speaker 2 (00:12):
Hey, welcome to Stuff to Blow Your Mind. My name
is Robert Lamb.
Speaker 3 (00:16):
And I'm Joe McCormick, and today we are coming back
with part two in our series on stickiness. This is
a topic I got interested in after feeding my ten
month old various fruits and fruit based smoothies and stuff,
and noticing the way that a kind of stealth stickiness
can easily migrate outside of the direct vicinity of eating
(00:41):
and contaminate surfaces beyond. There's a kind of stickiness fallout
effect after the initial meltdown. Rob I think you said
last time, you've had similar experiences.
Speaker 2 (00:52):
Oh, yes, and I'm still finding sticky spots in the house,
you know. Now here's a question about your daughter. Has
she used the stickiness to scale walls yet?
Speaker 3 (01:02):
No, she has not figured this out yet, but she's
crafty and I think she could be getting there very soon.
Speaker 2 (01:10):
More on wall climbing in a bit.
Speaker 3 (01:12):
Yeah, the creature will not be contained. But so in
the last episode we started off talking about difficulties and
even defining stickiness rigorously. This is one of those subjects
where I expected there to be a pretty simple, straightforward,
well understood physics or chemistry answer to what makes things sticky?
(01:33):
And it turns out no, the answer is super complex
and in some ways not fully understood, and we'll be
talking more about that some at the beginning of today's episode.
But in the last episode we also discussed one of
the most glorious of sticky foods, glutinous rice aka sticky
rice aka sweet rice, the unstoppable amylopectin avalanche.
Speaker 2 (01:56):
That's right, I hope if nothing else, we inspired everyone
out there to seek out some sticky rice or sticky
rice derived food products in wake of listening.
Speaker 3 (02:07):
Did you have some sticky rice over the weekend after
we did the episode?
Speaker 2 (02:11):
I had some mochi, and I really I think it
enhanced my enjoyment of the mochi, and I'm looking forward
to having sticky rice the next opportunity I get nice.
Speaker 3 (02:22):
Well, today, we wanted to further explore the concept of stickiness,
and one place I wanted to start was with a
chapter in a book that I've been reading. It's a
very good book called Sticky The Secret Science of Surfaces
by Laurie Winkless. This is a popular science book from Bloomsbury,
twenty twenty three. So this is just recently published. And
(02:43):
this book is not just about stickiness. It's about all
different kinds of surface interaction, so it covers slipperiness and
friction and all that stuff too. But there is a
great chapter early in the book about stickiness. And this
book is full of interesting stuff about the world that
I never thought of before. For example, in this chapter
(03:04):
on stickiness or adhesion, Winkless is talking about how oil
based paints are made. Typically, so an oil based paint
is usually going to be a solid pigment of some kind,
little colored particles that are suspended in a liquid medium
that might be something like linseed oil. And then it'll
also have additives in it that will be things like
(03:25):
stabilizers to help thicken the mixture and disperse the pigment
particles evenly. But the thing that really caught my attention
is she gets to an interesting fact, which is when
you use an oil based paint. Maybe you're painting a
painting on a canvas with some oil based paint. What
do you do when you're done painting? You let it dry.
We say that it dries, but actually the paint does
(03:48):
not dry drying would mean that it loses moisture through evaporation,
but moisture does not leave the oil based paint. Instead,
what oil based paints do is polymerize. This is sometimes
called curing. So instead of losing water molecules to the
air through evaporation, these paints steal oxygen from the air
(04:10):
and use that oxygen to form bonds between molecules, which
allows the paint to harden into a solid film and
form these layers of hard solid films. And because oil
based paints remove oxygen from the air rather than drying
by losing moisture, they actually get heavier, not lighter, as
they cure. So Winkles says that paints based on linseed
(04:33):
oil can sometimes increase their weight by over fifteen percent
during curing, so a painting would be heavier after it's
dry than when it was wet if you use one
of these paints. Now you can contrast this to water
based paints, which actually do dry by losing moisture, and
these will leave behind only the solid pigments held in
(04:55):
place by binder compounds. So when you buy a can
of water based paint, most of the volume of that
can is not actually going to end up on your wall.
It will instead evaporate. And in illustrating this, she writes,
quote Colin Gooch, technical director of paint manufacturer Racine, told
me that a four liter can of high quality waterborne
(05:17):
paint might contain just over one point five leaders of
volume solids. That's what actually forms the film that stays
on the surface. The job of the other two point
five leaders is to keep those solids dispersed. It allows
us to carry the pigment from the can to the wall.
Speaker 2 (05:33):
This is crazy. I'd never thought about this before either.
It makes me wonder if I were using oil based
paints in painting minis instead of the acrylic paints that
I use. Like what the difference would be like all
the you know, sort of the fine art of the thing.
Do you have to factor that in to how much
(05:53):
paint you're applying. I don't know. I'd love to hear
it doesn't out there with experience painting anything, you know,
minis or canvas or houses.
Speaker 3 (06:01):
Yeah, the book doesn't get into how the material the
differences and the materials affect the craft itself, but I
expect they probably do.
Speaker 2 (06:11):
Yeah.
Speaker 3 (06:12):
But anyway, coming to the more general topic in the
same chapter of What Stickiness Is, I wanted to talk
about some of the things that Winkless explains here. And
there are two very important concepts for stickiness. One is
adhesion and the other is cohesion. Adhesion is the attraction
between two different materials, and it's defined usually by the
(06:37):
work or the effort that it takes to separate the
two materials. And then cohesion is the attraction of a
material to itself. How the molecules of the substance, how
well they stay stuck together to each other. Now, to
discuss the different physical models of adhesion, how two different
substances stick together, Winkless starts by imagining a hypothetical interface
(07:01):
between two different things. So you imagine a drop of
some kind of liquid, some kind of adhesive liquid, sitting
on top of a clean, flat block of solid material.
So you can think of a drop of water sitting
on a piece of wood, or a piece of metal,
or a drop of whatever of glue. You know you're
going to be measuring how well this thing adheres. And
(07:22):
coming back to the level of uncertainty that I've found
surprising in this subject, she writes that the general consensus
is that there are quote three or four ways that
these materials could interact.
Speaker 2 (07:37):
Yeah, this is a familiar thread, it seems that one
finds when you start researching stickiness is that, Yeah, we
don't necessarily know On one l hand, we might not
necessarily know like the one thing that is making something stick,
and it may more honestly be various things, and there's
disagreement on to what extent each thing is contributed to
(08:01):
the stickiness.
Speaker 3 (08:02):
That's right. So the first one of these models of
interaction for adherents is chemical adherents, and this means there's
an actual chemical reaction. There are molecular bonds between the
adhesive and the surface, so some kind of reaction has
taken place, resulting in essentially a new compound where they
(08:23):
meet and winkless. Here uses the example of paint. Quote.
In paint, this sort of adhesion is enabled by the
binder molecules that surround the pigment particles. So the pigment
particles are the little solid particles that give the paint
its color, and then they have these binder molecules that
surround those pigment particles and help stick them to the surface.
(08:46):
She says, quote they react with molecules on the surface
sharing and borrowing electrons, effectively forming a new compound at
the interface. So this is actual chemical interaction here. Second
model of action is mechanical adhesion. Here there's not a
chemical reaction going on between the adhesive and the surface. Instead,
(09:08):
Winkless uses the analogy that one sticks to the other,
like the way a rock climber clings to the surface
of a cliff by like sticking fingers and toes into
cracks and crevices in the rock. So the drop of
the adhesive wants to stick to itself. Of course, it
wants to stay intact, much like the rock climber's body
(09:30):
wants to stay intact and stick to itself. And parts
of this adhesive are jammed into recesses in the surface
of the solid material. Now you might think, well, okay,
but what about smooth surfaces. And the fact is that
basically all surfaces, even if they're pretty smooth as far
as you can tell at the macro scale, they've got
(09:51):
lots of little irregularities when you zoom in. If you
get a high sensitivity microscope, you can zoom in and
see mountains and ravines the microscale, most surfaces you will
find in the real world are like this. Third model
of adhesion is diffusion. This typically happens when the solid
(10:12):
material in the scenario is a polymer, for example, rubber
or cellulose or nylon. There are lots of different kinds
of polymers in the world, and structurally, polymers are long
molecules with a repeating structure, sort of like the links
in a chain. In diffusion, these long chains can sort
(10:33):
of intermingle and tangle with several nanometers of material on
the other side, on the other object or the other substance,
even though they're not reacting chemically. And this might be
a kind of crude analogy, but for a rough picture,
just sort of like imagine surfaces that have a bunch
of chains and ropes and strings at the edge of
(10:54):
them getting tangled up with the edge of the other object. Now,
remember you said there were three or four. This last
one seems more debatable, but Winkless notes that the adhesive
product manufacturer three M also identifies a fourth possible type
of adhesion, and this is electrostatic. Sometimes, if you are
(11:15):
about to attach a strip of adhesive tape to a
piece of paper, you will see the paper actually kind
of move through the air like it's reaching out toward
the tape. You know, they want to you know, it
wants to be found, like the one ring, and it's
kind of like it's being attracted by a magnet because
it's sort of is This is because the tape accumulates
(11:37):
charged particles as you peel it off of the roll,
and there is attraction due to static electricity there same
reason you know, you get static electricity on a balloon
or something and then it lifts your hair off of
your head.
Speaker 2 (11:51):
Yeah, or occasionally in that scenario where there's some little
bit of plastic garbage or packaging that you are trying
to throw away and it just clings to your hand
and you can't fling it away, even though there's obviously
nothing sticky on your hands. You weren't just messing around
with glue, you haven't, you know, you've washed your hands.
But yeah, it's sticking to you because of the static.
Speaker 3 (12:11):
So that is a real force. But Winkles says that
she doubts whether this should really be considered a true
adhesive force that like meaningfully sticks objects together, because while
it might cause an initial attraction, this is not really
what would like hold a you know, an adhesive tape
firmly to the paper, what prevents it from peeling off?
(12:32):
But it's important to note she says that no single
one of these forces fully explains stickiness, and she writes quote,
for any given adhesive product, it's almost impossible to determine
exactly which model or models might be operating.
Speaker 2 (12:50):
That's that's interesting just even to think of in terms
of hobbyists, you know, because there's so many different types
of glues, and you get into any given hobby, you
might think, well, all clues are the same, but of
course they're most certainly not. You know, some expand some don't.
You know, some are more about like essentially melting one
bit of plastic into another. So yeah, it's I guess
(13:14):
the thing we keep running up against is like everything
involving stickiness. There's like this level of language you have
to get past, as well as sort of the level
of human perspective, you know, like we're just we're just
too big. We're not on the same level where all
of this is actually taking place.
Speaker 3 (13:29):
Yes, And I think also when we think about it,
we're usually thinking, we're trying to think too simply, we
think about it in one direction. We think, like why
does X stick to why? Like why does duct tape
stick to the wall, And we think that there would
be one answer for that. There is one cause. But
it might be better to think about stickiness as an
(13:50):
emergent system that relies on many different things interacting, rather
than a property one thing does to another. Now, remember
all of everything I've been saying so far was about adhesion,
(14:11):
how two different materials cling to each other. Another important
factor in stickiness is cohesion, the tendency of a material
to stick to itself. So in order for a sticky
material like paint to work the way that it's supposed to,
it has to be both adhesive meaning it sticks to
the wall, and cohesive, meaning it sticks to itself. If
(14:33):
either of these properties fails, then it's not going to
be good paint. Another interesting factor that Winkless discusses in
this chapter is what's known as surface energy. So when
you come back to that image of you put a
drop of some kind of liquids, some adhesive liquids, sitting
on a flat, solid surface, the nature of the solid
material that the liquid is sitting on also influences how
(14:57):
well the adhesive sticks and one of the carearacteristics that
matters is that solid material's surface energy. So technically, the
surface energy of a material is the extra energy present
at the outside of a solid mass compared to the
energy present inside the mass. And this extra energy is
(15:18):
there because of imbalances of molecular bonds on the surface
of the object. But for a more intuitive understanding of
surface energy, you can think about it basically as equivalent
to wet ability. Usually, you can predict how well and
adhesive will stick to a surface by looking at how
easily the surface gets wet, how well it attracts water.
(15:41):
And a common test used to measure surface energy is
to look at what's known as the contact angle of
a drop of water on the surface of the material.
That might sound kind of abstract, but it's actually when
you see it illustrated, it's pretty easy to understand. So
imagine you put a drop of water flat on a
(16:01):
solid surface, and it can be it could be wood, metal, cardboard, whatever.
Watch what the water does. Does it spread out and
form a wide, flat disk or kind of a lens shape,
or does it stand up a little bit taller and
form kind of a flat bottomed hemisphere or does it
sit way high up in almost kind of a full
(16:23):
sphere or bead shape. The first one I talked about,
the wide flat lens of water is what or has
what's known as a low contact angle. If you measure
the angle between the water droplet and the flat surface,
it's going to be pretty narrow, well below ninety degrees.
A low contact angle means that this surface likes water.
(16:46):
It attracts water, and it has a high surface energy.
And you can kind of see that because the water's
like spreading out. It's the water wants to touch the surface. Meanwhile,
the bead of water that sits almost like a sphere
or a ball up on the surface has a very
high contact angle. It minimizes contact with the solid, and
this indicates that the solid has a low surface energy,
(17:09):
and it's a substance that wants to fight off wetness.
It wicks away the moisture. So you might think of
like a think of like a waxy leaf, you know,
where that you see water hidden and it just rolls
right away. It's like what you know, it almost seems
like it's impossible for it to get wet. In most cases,
this quality the surface energy correlates with stickiness. Surfaces with
(17:34):
a high energy that attract water then and form that
flat disc shape are also usually easier to stick things too,
and Winkless writes, this is kind of interesting. Surface energy
also tends to correlate with the coefficient of friction. It's
not a hard and fast rule by any means, but
if a material has low surface energy, if it's slippery
(17:55):
to liquids, it is often also low friction slippery to solids.
So I thought that's kind of interesting. As she says,
it doesn't hold in every single case, but generally, the
easier it is to kind of like slide smoothly over
something without rubbing or sticking to it, also that surface
(18:15):
that you're sliding over is going to be harder to
like stick sticky things to. But also on the subject
of surface energy, this coming back to what we were
talking about earlier. I was really not expecting a book
chapter about the science of stickiness and adhesion to contain
much controversy. But you know, again, this just feels like
one of those topics that you would find already pretty
(18:38):
much settled and more just kind of standardized textbook explanations,
like we all know how this works, but once again,
this topic still does contain a lot of mystery and
room for disagreement. And one of those areas of disagreement
that Winkless documents is disagreements between materials scientists and chemists
about how important surface energy actually is when you're making
(19:01):
things like glues or adhesives that are supposed to hold
together to different solid materials. So she cites a professor
named Stephen Abbott, who is a fellow with the Royal
Society of Chemistry, who argues that, and these are abbots
words quote, surface energy is undoubtedly useful for paint, but
for practical adhesive systems where we're actually sticking stuff together,
(19:24):
it's basically irrelevant, thousands of times too small to give
us what we need, and yet people are obsessed with it.
So he seems almost mildly annoyed by this. Maybe I'm
reading too much tone into that, but either way, I
thought it was fascinating that this topic is has so
many more questions within it than I realized, Like and
the fact that we have lots of adhesive products that
(19:48):
work pretty well, Like you know, they're companies that make
tapes and glues and sticky notes and all kinds of things.
They stick to one another pretty effectively. And we know
at least some and may all of the underlying mechanisms
by which these by which these products work, and they
do work, and yet you still can't say in every
(20:09):
case which mechanism or mechanisms are making the difference. And
part of this is because stickiness is not merely a
property of one material, but, as we were saying, an
interaction between two or more materials and the conditions under
which they interact. And at the later on in this chapter,
(20:30):
Winkless quotes the same researcher, Stephen Abbott, saying, quote, adhesion
is a property of the system. It's not a property
of like the individual substance. But it's all these things
interacting together. They have adhesive behavior as a whole or not.
Speaker 2 (20:51):
Yeah, I think that's key. That's key to keep in mind,
and it's it also factors into what I'm about to
discuss here in a minute, because originally the plan was
that I thought, well, we'll roll through a few different
animal examples of which ones are sticky and why are
they sticky? Foolishly thinking again that we must have have
this settled. It must be settled how this particular animal
(21:13):
sticks to a wall. And we do know a lot
about this topic, but it's maybe not as perfect of
knowledge as one might expect. And and and I do
have to, you know, to admit that. Of course, this
is the case with plenty of plenty of other things
about animals that we are even very familiar with. I mean,
even a house cat has its secrets. There are things
(21:34):
about house cat behavior that we have varying ideas about
and we haven't completely worked out. But yeah, with stickiness,
it's easy just to again to completely take it for granted,
to approach it from our you know, look at it
through the fog of our scale and our language and think, oh, yeah,
well it just sticks right. It's like, you know, it's
(21:56):
it's like a suction cup.
Speaker 3 (21:57):
Well, speaking of suction cups, that brings up an interesting thing,
which is that on the macroscopic scale, you can get
stickying forces that have nothing to do with any of
the stuff we've been talking about so far, because we
were just talking about like surfaces sticking to one another
due to properties of the surfaces. But in the case
of a suction cup, like if you want to stick
a suction cup to a window and climb a skyscraper
(22:19):
like Tom Cruise in one of those Mission Impossible movies.
That's a totally different mechanism. What's going on there is
the suction cup. By evacuating the air from the space
between the cup and the smooth surface, you are creating
a low pressure area inside, which allows the atmosphere to
press on the outside of the cup and hold it
(22:40):
against the wall. So when Tom Cruise is doing that,
what he's really doing is letting the atmosphere, the weight
of the atmosphere press his handholds against the building.
Speaker 2 (22:50):
Is Tom Cruise still climbing up buildings with the suction cups?
Is that still happening in the current movies.
Speaker 3 (22:55):
I think he was in the most recent one of
those I saw.
Speaker 2 (22:58):
Maybe that's the way. I can't remember did that, And
I think I only saw the first Mission Impossible movie,
And if you told me that he used suction cups,
I would just assume, Well, I guess he probably used them.
It seems like the sort of thing he would do.
Speaker 3 (23:10):
I think in the first one he uses magnets, because
that's when he's on the top of that train, you know, Oh, no,
John Voight, and they're fighting for a helicopter on top
of a train.
Speaker 2 (23:22):
I remember John Voight, And don't they lower him into lasers?
Is that the movie with lowering endo lasers.
Speaker 3 (23:27):
Lower him in the lasers.
Speaker 2 (23:28):
Yeah, like room full of like laser trip wires and
they have to like it comes down from.
Speaker 3 (23:33):
The cel Oh I don't know, Yeah, yeah, yeah. The
first one is where they put Tom Cruise on a
rope and they lower him down through out of an
air vent into a room that the floor is lava.
You can't touch the floor or the alarms will golf.
And that was so he could steal the knock list,
you know, the knocklst I had VHS to the first Yeah.
Speaker 2 (23:56):
I guess I only saw it once. Then. Yeah. I
don't have any strong feelings about it one one or
the other, but I mean I admire, I admire the
commitment to amazing stunts.
Speaker 3 (24:07):
Apart from like the coronal mass ejection of star power
that is Tom Cruise. Most of those movies have a
pretty cool supporting cast. Oh you know the first one
it had Ving Raims and John Renau.
Speaker 2 (24:19):
Yeah, yeah, I believe you're right. Yeah, yeah, strong casts
all right. Well, speaking of star power, and speaking of
daring stunts. The main creature we're going to talk about
here today is going to be the gecko. I'm also
going to get a little bit into Spider Man later,
but Spider Man's not real. The gecko is real, and
the gecko is quite amazing, one of the most famous
(24:42):
sticky animals of all, though to be clear, not all
geckos have this ability. For instance, the oft domesticated leopard gecko.
There's actually one of these living in my house. It's
my son's pet, is a ground dwelling lizard and does
not have this ability. But geckos are found on every
continent except Antarctica various species of the infraorder Gekota. Of these,
(25:08):
some sixty percent have adhesive toepads that enable climbing, although
this ability has apparently been lost and gained many times
over their evolution.
Speaker 3 (25:18):
Now, when you hear that a gecko has adhesive toepads,
first of all, that might make sense because you've probably
seen a gecko climbing up a flat surface like a
you know, a window pane or a sliding door, or
the wall of a building or a tree trunk, or
even climbing on the underside of things. Again, much like
tom cruise kind of doing overhang free climbs, going around
(25:40):
on the ceiling. It is pretty amazing what they're able
to do, and across the range of surfaces across which
they're able to do it. So you might just assume, okay,
well they got you know, toes that feel like glue.
Like if you were to go put your hand on
a gecko's toe, it would be a gross, sticky you know,
almost kind of like a like a sugar seer or
(26:00):
something like that. But no, no, that's not the case.
If you go and look at the toe and feel
the toe of a gecko, the kind that has these
amazing climbing powers and clinging powers, it feels totally smooth.
It is not covered in a sticky adhesive like the
back of duct tape.
Speaker 2 (26:18):
Yeah, it's one of the things that's long fascinated us
about the gecko. And again, geckos are widespread, so they've
been there. They're around for great thinkers of different ages
to potentially ponder them, and in fact, Aristotle mentions them
in the History of Animals, the fourth century BCE text.
If you start looking searching around in that text for
(26:39):
mentions of the gecko, they do come up several times.
But he also writes of the amazing climbing ability of
the quote unquote gecko lizard to quote run up and
down a tree in any way, even with the head downwards.
So even Aristotle thought that this was pretty interesting, though
I don't believe he offers any possible explanation for it,
(27:02):
is just kind of an observation about you know, I
think he's comparing it to another creature. Here.
Speaker 3 (27:06):
The quest to understand how gecko toes cling to all
these different surfaces and allow it to walk straight up
vertical walls of any roughness or smoothness, to cling to
ceilings and overhangs. This goes way way back, and people
have put forward tons of hypotheses over the years. Again,
coming back to our mission, impossible digression was due to
(27:28):
talking about suction cup climbing, and for a long time
a lot of people thought that the geckos did climb
by essentially using suction cup grips. But that's not the case.
Speaker 2 (27:39):
Yeah, At different points they you know, hypotheses included sticky
secretions like we've mentioned, suction cups, also tiny hooks. You know,
these are all decent hypotheses to work with but it
didn't take too long for various naturalists and ultimately scientists
and thinkers in general to realize, well, none of these
are really explaining it. Something else is goinging on, and
(28:01):
it seems like it's something maybe more you know, more
physical certainly than chemical. But once again, like it's our
perspective and our language kind of get in the way
of like simple understanding of what the get go is doing.
And I find my experience with researching this it was
kind of like like the Powers of ten sort of
experiences ZUSA in you know, yes, because you start off
(28:24):
with like a simple question, well, why can gecko's climb walls?
And the answer is, well, because they have special flexible
adhesive ridges on their toes. And if you wanted, you
could stop there and be like, okay, question answered, I
can go home now. But but then you might ask
the follow up question, well, how do their adhesive toes work? Well,
the answer is the toes have special tiny hairs like
(28:46):
even you know, far smaller than what we think of
as hairs called ceta on them. And then you might say, okay,
I got it. I can go home now, that's what's
doing it.
Speaker 3 (28:56):
But wait, yeah, how are the tiny hair What do
the tiny hairs do? Or how are they sticky? How
do they stick well?
Speaker 2 (29:03):
Each of these has even smaller bristles called spatula on them.
Speaker 3 (29:08):
Right, So, one way I've seen this described is all
these tiny little hairs. If you can zoom way way in,
like smaller on a scale smaller than you can see
with visible light. You have to use like a scanning
electron microscope, you can see that each hair like frayse
like a rope that just goes out to like millions
of little fibers that are beyond microscopic. They're down on
(29:30):
the nanoscale. They're super super tiny, frayed out to uncountable ends.
Speaker 2 (29:35):
Yeah, I mean, there's so many that a given gecko
i've read have somewhere in the neighborhood of two billion
of these on the pads of their toes combined.
Speaker 3 (29:45):
Okay, so on their toes they have these ridges. The ridges,
by the way, are called lamelli, and then on the
lamelly they have the setae, and the setae are little hairs,
and those hairs fray out into these little frayed bristly
ends called spatchel. But what does the spacially do. What
are those how do they work?
Speaker 2 (30:04):
Well? The answer here is well, it's complicated, but the
basic idea is that their stickiness seems to depend on
a combination of intermolecular forces.
Speaker 3 (30:16):
Yeah, it does seem like there are maybe multiple forces involved,
though the sources I was looking at zeroed in on
the most agreed upon explanation having to do with what
are known as Van Derval's forces.
Speaker 2 (30:28):
Yes, van der Waals force named for Johannes Diedrich van
der Waals who lived eighteen thirty seven through nineteen twenty three.
There's an excellent two thousand and two article I've been
shouse that I that I One of the sources I
turned to for this section is you can find on
science dot org has the just perfect title how Geckos
(30:50):
stick on de valls at Waals spelling for walls. So yes,
that as of two thousand and two is to gused
in this article like that is the primary hypopsis for
how the gecko's toepads are ultimately sticking to the wall.
But another one that was being looked at at this
(31:12):
time in an article I'm going to side here is
this idea that it could have been water based capillary forces,
so like a thin film of water between pads and
the climbing surface. Quote. Because water molecules are polar, their
electrical charges aren't evenly distributed. They might stick to some
polar molecule in gecko's feet.
Speaker 3 (31:33):
Oh okay. Now on the opposite side of that, what
I've been reading about gecko's feed is that they seem
to be composed largely of water repelling hydrophobic materials. But
that again, I think it's complicated.
Speaker 2 (31:46):
Yes, yeah, Now to come back to vander Walls here,
science writer Eleanor Nelson put together a great explainer on
this whole topic for ted D several years ago. You
can look this up. There's a video on ted ED
about the gecko to search for ted ed gecko and
you can watch it as great visuals to help explain
all this. But she stresses that what we're talking about
(32:10):
here isn't attraction between charged molecules positive or negative, but
rather attractiveness between uncharged molecules that experience patches of negative
and positive charges due to the movement of atoms with
different electro negativities in the same molecule. So we're talking
about flickering patches of weak attraction. It's not strong in
(32:33):
isolation like each of those you know, tiny spatula are
not you know, pulling things across the room or anything
remotely like that. But when you have two billion of
these on a single gecko exploiting the force, it adds up.
In fact, it adds up so much that a gecko
can hang by a single digit, you know, from a
(32:54):
like a ceiling, you know. And it has complete control
over this as well, so you know, because they can
manipulate the angle and surface area of those tow ridges
in just the right way to activate and release that
stickiness on demand.
Speaker 3 (33:08):
Yeah, it is amazing, especially when you notice like the
motor activities that the gecko has to coordinate to orient
these little harris correctly to always cling the right way.
But I wanted to clarify one thing because it took
me a minute to understand what was going on with
the Vandervals forces. The vanderwalls or Vanderval's forces are something
that is different from like the attraction you get in
(33:30):
a polar molecule like water, which is permanently polarized in
terms of a charge, so it's always going to be
able to attract one way or another because it's got
a you know, negative charge on one side, positive on
the other. The Vanderval's forces are, Yeah, they're due to
the fluctuations in the orientation of electrons around an atom
(33:53):
that are just random. So they just randomly change over time,
even when an atom is electrically stable, so this atom
is not polarized, but things just sort of flicker in
and out of existence. A charge attraction going one way
or the other flickers in and out of existence because
of just random fluctuations of where the electrons are lined up.
(34:14):
And that in cases where where atoms are packed close
enough together can actually have an attractive effect, but usually
atoms are not packed close enough together for that to matter.
Speaker 2 (34:28):
Yeah, this is a if you're still foggy on this,
I do recommend that ted Ed video because they have
a nice animation that kind of that illustrates what's going
on here. At least for me, it helped clarify the
whole thing.
Speaker 3 (34:40):
Yeah, and so the reason the spatuely are able to
take advantage of the Vanderwolts force is because they're so
tiny and there's so many of them.
Speaker 2 (34:47):
Forests of them, I've seen it described a right. So
coming back to the two thousand and two time period here,
there was a paper that came out in a two
thousand and two edition of Integrative and Comparative Biology titled
(35:11):
Mechanisms of Adhesion in Geckos. This was by Keller autumn
at All and in this they were exploring the vendor
val and the water based capillary forces explanations to you know, see, okay, well,
let's look at these two possible ideas and see which
one seems to seems more evident when put to the test.
(35:34):
And they used the aid of semiconductors, so they ruled
out water droplet hypothesis as it depends on polar surfaces.
They found in their experiments that geckos could equally climb
a non polar surface. In this experiment they used a
semiconductor called gallium arsenide, which is a compound of gallium
(35:54):
and arsenic. By the way, it is also interesting that
gecko feet can not adhere to teflon. You may have
heard this or seen some headlines about this over the years,
but according to Sarah Chodesh on Popular Science back in
twenty eighteen, teflon disrupts the Vanderval's force effect here that
(36:16):
enables them to climb other or seems to be one of,
if not the primary factors in their ability to climb
other surfaces. Quote. Vanderval's forces depend on being able to
induce a positive and negative end of a molecule or atom.
But the layer of fluorines in Teflon just have this
permanent negative charge to them. The electrons stay in one place.
Speaker 3 (36:37):
Teflon's not having any of it. Yeah.
Speaker 2 (36:40):
So anyway, that twenty and two study backs up the
vandervals explanation. In their own words, their findings at the
time provided quote direct evidence that Vanderval's force is the
mechanism of adhesion in get Go ceta and that water
based capillary forces are not significant. So again not to
say it couldn't be doing something, but it's not the
(37:01):
significant factor.
Speaker 3 (37:02):
Yes, though it's interesting. In uh the chapter in Winkless's
book on gecko toes, she talks about, how you know,
it seems like most researchers in this area kind of agree, Yeah,
it's Vanderval's force. That's that's the explanation or the main explanation.
But she says, they all kind of have this bit
of hesitation when they talk about it, like maybe there's
(37:25):
something else going on too that we haven't fully figured
out yet. Yeah.
Speaker 2 (37:29):
I kept thinking about like the experience of trying like
a fancy cocktail somewhere and you're trying to pick out
the ingredients just on taste. You know, generally you can
you can pick out some of the major flavors. But
are you gonna, you know, absolutely put your money down
that there are only three ingredients in this cocktail and
you're there are not four, they're not five, and so forth.
Speaker 3 (37:52):
That's right. What's that tingling in the back of your mouth?
Could that be an absentthe rense?
Speaker 2 (37:59):
So one sample of this concerning gecko feed This comes
up in a twenty fourteen article that I was looking
at role of contact electrification and electrostatic interactions in gecko
adhesion by A. Zadi at All published in the Journal
of the Royal Society Interface, and this one discusses the
(38:19):
CE driven electrostatic interaction. This refers to the contact electrification phenomenon,
and I understand that this is still controversial. For example,
in a twenty twenty three paper by song at All,
in published in the journal Friction. The author's cite the
controversial aspect of the CE driven explanation, though to be clear,
(38:39):
it's ultimately more about the level of contribution that's controversial,
because again it's like saying, well, yes, I think there's
absinth in this cocktail, but maybe it's just a drop.
It's like three drops of absinthe I'm not saying it's
anything more than that, Whereas there might be someone else arguing, actually,
I think they put an ounce in there, and then
(38:59):
the fighting and controversy. But anyway, in this song at
all paper the way they explain it, it seems like
CE may be part of the situation, but their study
suggests that its impact is low, perhaps contributing only around
three percent of the adhesiveness, and this may also be
(39:21):
why it doesn't help them out with something like teflin,
which again essentially cancels out Vanderval's force, which, as far
as we can tell, seems to be the primary factor
in play here. However, they note that quote long range
electrostatic forces may play other roles in a distance range
where the Vanderwals interaction cannot function, So again there could
(39:43):
be certain situations where it's more important to some degree
than other times.
Speaker 3 (39:47):
But it does seem clear that whatever else is going on,
the main ingredient, the whiskey in the cocktail is the
vandervals interaction. Yes, and another interesting thing here, Rob you
alluded to this earlier, but it's something that you can
notice if you watch a gecko actually climbing, which is
the question of wait, how do they make their toes
(40:10):
stick and then unstick? So they've got this really powerful
sticking force, and in some cases the sticking force it
can hold so much more than the weight of a
gecko's body. There's one estimate in Winkliss's book about a
test showing that a gecko I can't remember, I didn't
write down the number, but it's like a gecko's grip
could hold hundreds of pounds up on the wall, despite
(40:33):
the fact that the gecko itself only weighs like half
a pound. Now, on one hand, you might think, well,
that's kind of overkill, but of course that would be
testing it in sort of ideal conditions in a lab,
whereas in nature a lot of times the surfaces it's
climbing on are going to be dirty, which will reduce
the adhesive potential, might be wet, which would potentially reduce
adhesive potential. Some of their toes might be injured or something.
(40:56):
Some of the lamilly might come off. In nature, you know,
it's better safe than sorry. When you're a gecko, have
a lot of sticking power. But yeah, anyway, the question
is how does it stick and unstick and climb in
different directions? And a lot of this comes down to
how it lifts and attaches its toes to the surface
and how it orients them. So like you, if you
(41:18):
watch a gecko crawling up a wall, it will tend
to sort of like peel its peel its toes up
and then lay them down with all of the toes
facing upward on the wall. But if you see a
gecko climbing down a wall, it will rotate its back
feet so that its toes are facing up. Because the
(41:39):
ceta and the spatually on them, they sort of only
grip effectively when laying in one direction.
Speaker 2 (41:47):
Yeah, one way to think about all this is that again,
our understanding of what's going on when a gecko climbs
has not reached a level of perfection, but their exploitation
of it has reached perfection, like they are the perfect
manipulators of this force. So it's pretty amazing to watch.
Speaker 3 (42:08):
Now, whenever evolution achieves something this ingenious, you will bet
that human engineers come running to see, how are they?
How are they doing that? What's going on there? Could
I make robots that could do something like that?
Speaker 2 (42:21):
Yeah? Yeah, So of course biomimicry has gazed longingly at
the gecko for a while now. There have been various
efforts to create artificial ct with some working prototypes and
robotics and even human climbing suits popping up in headlines.
I think there was a DARPA project that made the
news in twenty fourteen. These have certainly not reached a
(42:44):
level of perfection yet, but you know, there are some
interesting applications out there potentially for this, and they're not
all of them involved like super soldiers climbing the windshield
or robots cleaning you know, high rise buildings, for instance.
It's been put far that they could be used in
low gravity environments. They could even be used, you know,
(43:05):
to collect space debris in orbit, that sort of thing.
Speaker 3 (43:08):
So the amount of weight that can be supported by
a gecko's toes and the range over the range of
different surfaces that they can crawl over is truly astounding
and sort of a superlative in nature. But there are
lots of other animals that can crawl up walls, and
I wonder do many of them make use of similar
mechanisms or forces well.
Speaker 2 (43:29):
Spiders climb in a similar way and also depend on
tiny set that make use of Vanderval's force. Though they're
ultimately like masters of these structures, they also use them
for other purposes as well, like like sensing sense sounds, vibrations,
and air currents. But this does lead us back to
(43:49):
the topic of Spider Man, because, as everyone knows, Spider
Man can do whatever a spider can, and it's this.
Speaker 3 (44:00):
He can liquefy your innards and slurp them out through
his mouth orifice.
Speaker 2 (44:05):
Maybe he can, and he chooses not to, But I
guess that's more of a Isn't there a man There's
a man spid. I know there's a batman and a
man bat there's also like a spider man monster creature,
but I don't know if that's man spider or not
off the top of my head. But it gets it
gets so complicated when you start comparing Spider Man to
spiders because, of course there are all these things that
(44:26):
don't match up. For example, it's easy to think of
Spider Man. What does Spider Man do well? He swings
around on webbing through the through the streets, though he
does so in a way that's not particularly spider esque,
and while in some adaptations of Spider Man he's using
like organic like mutations, like he has like he's you know,
he's been by a radioactive spider and now he can
(44:49):
he can shoot webbing out of his wrists. Most versions
of Spider Man you encounter he has these little gadgets
that he built that shoot some sort of nylon like webbing.
So he's this weird mix of like, Okay, I'm part
Spider but also I have like super crazy sci fi
weapons as well, and all of these combined to make
me able to do whatever a spider kit.
Speaker 3 (45:09):
It's not really satisfying that way, is it. He's one
foot in each world. He's half a Charles B. Griffith
script where yes it was atomic radiation. But he's half
Batman and we already have Batman. He makes gadgets.
Speaker 2 (45:21):
Yeah, I mean, I guess I'm not a Spider Man
purist or anything, but I like the mutant version. I
like the idea that he's shooting it out of his wrist,
even though that doesn't really match up. Like spiders don't
they have spinnerets, they don't have wrist openings or whatever.
So you know, it's still not one to one.
Speaker 3 (45:40):
I guess Spider Man fans, please don't hate me. I'm
just joking around. I don't know much about Spider Man.
Speaker 2 (45:45):
I mean, Spider Man is great. But one of Spider
Man's abilities that is or seems to be universally accepted
as an ability brought on by the radioactive spider bite
is his ability to climb walls and stick to ceilings
with out the aid of his wedding. How does he
do this well? On one hand, we have to admit
(46:06):
like this is a fool's errand to try and make
sense of this. Spider Man climbs walls because he's Spider Man,
and any kind of like scientific explanation to this comes
after the fact they were not thinking long and hard
about the anatomy of a spider when they came up
with this character. As fun as this character is, so
(46:26):
any attempt to scientifically explain everything about him is is.
It can be very fun, but it's not going to
get you to a place of absolute certainty.
Speaker 3 (46:36):
You might as well ask, exactly scientifically, how do the
crabs in attack of the crab monsters eat your brains
and gain your knowledge exactly?
Speaker 2 (46:45):
But like I say, it's a fun exercise, and I've
been enjoying this book marvel Anatomy by Sumeruk and Wallace,
and the authors point out several things about Spider Man's
adhesive abilities and how they match up or don't match
up with the natural world. One of the main ishu
that they keep talking about, though, is Okay, Spider Man
can climb, and perhaps he does this because he has
(47:07):
little CD on his hands at least or I guess,
over his entire body, and it allows him to adhere
to walls. But also he's wearing a full body costume.
Spider Man is almost never naked when he's clinging to
walls and so forth, so it raises questions like would
vander walls work through a pair of spider gloves or
(47:29):
would that weaken it to some degree.
Speaker 3 (47:32):
I don't think it would work, because the whole thing
about Vanderval's force is you've got to be incredibly close,
closer than solids can usually get to each other. That's like,
that's why the gecko's foot works. It's the tiny little
thing that can get in there in the cracks and
crevices in the surface, unlike anything else.
Speaker 2 (47:48):
Well, the author suggests that, well, maybe the ceed poke
through the costume, but that raises all sorts of questions,
like then when he takes the costume off, when he
peels off the sweaty Spider Man costume at the end
of the day, does it just rich them all off
and he has to regrow them or I don't know.
It's a wonder fabric that Iron Man made for him,
so it lets them out.
Speaker 3 (48:08):
I don't know.
Speaker 2 (48:09):
Again, it just raises more questions than anything.
Speaker 3 (48:13):
I think Spider Man should have finger gloves like a bandit.
Speaker 2 (48:17):
The authors here suggest that maybe Spider Man can actually
quote consciously control the intra atomic attraction between molecular boundary layers,
which I suppose sounds reasonable, though I mean this raises
the question about to what extent this still makes sense
as a sixties at comic book. Radiation Advancement of natural
World spider abilities, but I'll take it. I'll take it.
(48:41):
Sounds good to me. Now. They do point out that
another character, Spider Woman. I don't know if you're familiar
with Spider Woman. She was introduced in nineteen seventy seven.
She just kind of like a red outfit. But she
is said to be able to climb via secreted biological
adhesive substances that are on her hands and her feet
(49:04):
that I guess swiftly penetrate tiny pores in the climbing surface. Also,
she wears gloves and booties as well, so this secretion
would have to like rapidly soak through the fabric in
these cases as well or through the booties and then
allow her to stick to the walls. And again this
raises more questions because when you look to the natural world,
(49:25):
you do see adhesive secretions in some organisms, but they
tend to be defensive rather than climbing aids. These are
things that you would extrude when threatened, so that whatever
was trying to eat you might decide, oh, I don't
want any of this.
Speaker 3 (49:39):
Yeah, well, maybe I'm not thinking creatively enough, but it
seems like if you had to secrete enough sticky stuff
to hold you to a wall every time you wanted
to climb. That'd just be a lot of stuff that
sounds like you're secreting, Like do you get dehydrated? Do
you run out of energy doing that? Well?
Speaker 2 (49:57):
I mean, Spider Man is leaving nylon thread all over
the city, all over New York as he's fighting crime,
and I guess somebody has to clean that up. They
probably didn't think about it much in the sixties, and
I guess it was acceptable in the eighties, but nowadays,
like it's what's the what's the environmental impact of this
crime fighting?
Speaker 3 (50:16):
But once again the genius of the gecko. The gecko
climbs without goop.
Speaker 2 (50:20):
Yes, yeah, And the authors here they do include a
bit on one of Spider Man's many enemies. You've perhaps
heard of, the lizard. I think the lizard was in
at least one of the movies. I can't remember which
Spider Man this was, which Spider Man regime, But this
is a man, a scientist who turns into a great,
big green lizard and chases Spider Man around. And in
(50:44):
many of the depictions, the lizard can climb around on
walls and on the ceiling. In this particular book that
I was referring to, they discuss it as more of
a claw thing, which I think makes sense for a
super villain, you know, big nasty claws. Can you know
claw into the concrete? Is the creatures climbing around. But
I've also seen some descriptions that discuss the lizard as
(51:08):
having gecko abilities.
Speaker 3 (51:10):
Oh okay, so either way claw or gecko, that would
be San's goop. That would be a mechanical grip of
some sort or of Vandervohal's force. Yeah.
Speaker 2 (51:20):
But again, I think with a super villain like the Lizard,
it makes more sense that his climbing ability is visibly destructive,
though it raises the question like, where's the love for
the gecko? Then in the creation of Superheroes, I was
looking around looking at the various databases, and it looks
like they're a couple of minor Marvel Gecko characters, or
(51:43):
maybe it's two versions of one character that have existed.
And it looks like DC Comics also has a Gecko
or the Gecko. But I don't get this sense that
these are based on anything, you know, true to the
get go. But comic book fans out there let us
know perhaps you have more detail on these Geckos.
Speaker 3 (52:03):
Who is Gordon Gecko? Is he some kind of gecko supervillain?
Speaker 2 (52:08):
I don't know. Maybe it's part of the shared cinematic universe.
Speaker 3 (52:11):
Okay, I think that's all we got for today.
Speaker 2 (52:13):
All right, We're going to go ahead wrap it up here,
but yeah, write in let us know what you think
about stickiness in general, the examples of stickiness that we've discussed,
and yes, even fictional sticky comic book characters and so forth.
Just a reminder that we're primarily a science podcast here
at Stuff to Blow Your Mind, with core episodes on
Tuesdays and Thursdays, listener mail on Mondays, a short form
(52:35):
monster fact or artifact episode on Wednesdays, and on Fridays.
We set aside most serious concerns. Who just talk about
a weird film on Weird House Cinema.
Speaker 3 (52:42):
Huge thanks to our excellent audio producer JJ Posway. If
you would like to get in touch with us with
feedback on this episode or any other, to suggest a
topic for the future, or just to say hello, you
can email us at contact at Stuff to Blow your
Mind dot com.
Speaker 1 (53:03):
Stuff to Blow Your Mind is production of iHeartRadio for
more podcasts from iHeartRadio, visit the iHeartRadio app, Apple podcasts,
or wherever you listen to your favorite shows.
Welcome to Stuff to Blow Your Mind production of iHeartRadio.
Speaker 2 (00:12):
Hey, welcome to Stuff to Blow Your Mind. My name
is Robert Lamb.
Speaker 3 (00:16):
And I'm Joe McCormick, and today we are coming back
with part two in our series on stickiness. This is
a topic I got interested in after feeding my ten
month old various fruits and fruit based smoothies and stuff,
and noticing the way that a kind of stealth stickiness
can easily migrate outside of the direct vicinity of eating
(00:41):
and contaminate surfaces beyond. There's a kind of stickiness fallout
effect after the initial meltdown. Rob I think you said
last time, you've had similar experiences.
Speaker 2 (00:52):
Oh, yes, and I'm still finding sticky spots in the house,
you know. Now here's a question about your daughter. Has
she used the stickiness to scale walls yet?
Speaker 3 (01:02):
No, she has not figured this out yet, but she's
crafty and I think she could be getting there very soon.
Speaker 2 (01:10):
More on wall climbing in a bit.
Speaker 3 (01:12):
Yeah, the creature will not be contained. But so in
the last episode we started off talking about difficulties and
even defining stickiness rigorously. This is one of those subjects
where I expected there to be a pretty simple, straightforward,
well understood physics or chemistry answer to what makes things sticky?
(01:33):
And it turns out no, the answer is super complex
and in some ways not fully understood, and we'll be
talking more about that some at the beginning of today's episode.
But in the last episode we also discussed one of
the most glorious of sticky foods, glutinous rice aka sticky
rice aka sweet rice, the unstoppable amylopectin avalanche.
Speaker 2 (01:56):
That's right, I hope if nothing else, we inspired everyone
out there to seek out some sticky rice or sticky
rice derived food products in wake of listening.
Speaker 3 (02:07):
Did you have some sticky rice over the weekend after
we did the episode?
Speaker 2 (02:11):
I had some mochi, and I really I think it
enhanced my enjoyment of the mochi, and I'm looking forward
to having sticky rice the next opportunity I get nice.
Speaker 3 (02:22):
Well, today, we wanted to further explore the concept of stickiness,
and one place I wanted to start was with a
chapter in a book that I've been reading. It's a
very good book called Sticky The Secret Science of Surfaces
by Laurie Winkless. This is a popular science book from Bloomsbury,
twenty twenty three. So this is just recently published. And
(02:43):
this book is not just about stickiness. It's about all
different kinds of surface interaction, so it covers slipperiness and
friction and all that stuff too. But there is a
great chapter early in the book about stickiness. And this
book is full of interesting stuff about the world that
I never thought of before. For example, in this chapter
(03:04):
on stickiness or adhesion, Winkless is talking about how oil
based paints are made. Typically, so an oil based paint
is usually going to be a solid pigment of some kind,
little colored particles that are suspended in a liquid medium
that might be something like linseed oil. And then it'll
also have additives in it that will be things like
(03:25):
stabilizers to help thicken the mixture and disperse the pigment
particles evenly. But the thing that really caught my attention
is she gets to an interesting fact, which is when
you use an oil based paint. Maybe you're painting a
painting on a canvas with some oil based paint. What
do you do when you're done painting? You let it dry.
We say that it dries, but actually the paint does
(03:48):
not dry drying would mean that it loses moisture through evaporation,
but moisture does not leave the oil based paint. Instead,
what oil based paints do is polymerize. This is sometimes
called curing. So instead of losing water molecules to the
air through evaporation, these paints steal oxygen from the air
(04:10):
and use that oxygen to form bonds between molecules, which
allows the paint to harden into a solid film and
form these layers of hard solid films. And because oil
based paints remove oxygen from the air rather than drying
by losing moisture, they actually get heavier, not lighter, as
they cure. So Winkles says that paints based on linseed
(04:33):
oil can sometimes increase their weight by over fifteen percent
during curing, so a painting would be heavier after it's
dry than when it was wet if you use one
of these paints. Now you can contrast this to water
based paints, which actually do dry by losing moisture, and
these will leave behind only the solid pigments held in
(04:55):
place by binder compounds. So when you buy a can
of water based paint, most of the volume of that
can is not actually going to end up on your wall.
It will instead evaporate. And in illustrating this, she writes,
quote Colin Gooch, technical director of paint manufacturer Racine, told
me that a four liter can of high quality waterborne
(05:17):
paint might contain just over one point five leaders of
volume solids. That's what actually forms the film that stays
on the surface. The job of the other two point
five leaders is to keep those solids dispersed. It allows
us to carry the pigment from the can to the wall.
Speaker 2 (05:33):
This is crazy. I'd never thought about this before either.
It makes me wonder if I were using oil based
paints in painting minis instead of the acrylic paints that
I use. Like what the difference would be like all
the you know, sort of the fine art of the thing.
Do you have to factor that in to how much
(05:53):
paint you're applying. I don't know. I'd love to hear
it doesn't out there with experience painting anything, you know,
minis or canvas or houses.
Speaker 3 (06:01):
Yeah, the book doesn't get into how the material the
differences and the materials affect the craft itself, but I
expect they probably do.
Speaker 2 (06:11):
Yeah.
Speaker 3 (06:12):
But anyway, coming to the more general topic in the
same chapter of What Stickiness Is, I wanted to talk
about some of the things that Winkless explains here. And
there are two very important concepts for stickiness. One is
adhesion and the other is cohesion. Adhesion is the attraction
between two different materials, and it's defined usually by the
(06:37):
work or the effort that it takes to separate the
two materials. And then cohesion is the attraction of a
material to itself. How the molecules of the substance, how
well they stay stuck together to each other. Now, to
discuss the different physical models of adhesion, how two different
substances stick together, Winkless starts by imagining a hypothetical interface
(07:01):
between two different things. So you imagine a drop of
some kind of liquid, some kind of adhesive liquid, sitting
on top of a clean, flat block of solid material.
So you can think of a drop of water sitting
on a piece of wood, or a piece of metal,
or a drop of whatever of glue. You know you're
going to be measuring how well this thing adheres. And
(07:22):
coming back to the level of uncertainty that I've found
surprising in this subject, she writes that the general consensus
is that there are quote three or four ways that
these materials could interact.
Speaker 2 (07:37):
Yeah, this is a familiar thread, it seems that one
finds when you start researching stickiness is that, Yeah, we
don't necessarily know On one l hand, we might not
necessarily know like the one thing that is making something stick,
and it may more honestly be various things, and there's
disagreement on to what extent each thing is contributed to
(08:01):
the stickiness.
Speaker 3 (08:02):
That's right. So the first one of these models of
interaction for adherents is chemical adherents, and this means there's
an actual chemical reaction. There are molecular bonds between the
adhesive and the surface, so some kind of reaction has
taken place, resulting in essentially a new compound where they
(08:23):
meet and winkless. Here uses the example of paint. Quote.
In paint, this sort of adhesion is enabled by the
binder molecules that surround the pigment particles. So the pigment
particles are the little solid particles that give the paint
its color, and then they have these binder molecules that
surround those pigment particles and help stick them to the surface.
(08:46):
She says, quote they react with molecules on the surface
sharing and borrowing electrons, effectively forming a new compound at
the interface. So this is actual chemical interaction here. Second
model of action is mechanical adhesion. Here there's not a
chemical reaction going on between the adhesive and the surface. Instead,
(09:08):
Winkless uses the analogy that one sticks to the other,
like the way a rock climber clings to the surface
of a cliff by like sticking fingers and toes into
cracks and crevices in the rock. So the drop of
the adhesive wants to stick to itself. Of course, it
wants to stay intact, much like the rock climber's body
(09:30):
wants to stay intact and stick to itself. And parts
of this adhesive are jammed into recesses in the surface
of the solid material. Now you might think, well, okay,
but what about smooth surfaces. And the fact is that
basically all surfaces, even if they're pretty smooth as far
as you can tell at the macro scale, they've got
(09:51):
lots of little irregularities when you zoom in. If you
get a high sensitivity microscope, you can zoom in and
see mountains and ravines the microscale, most surfaces you will
find in the real world are like this. Third model
of adhesion is diffusion. This typically happens when the solid
(10:12):
material in the scenario is a polymer, for example, rubber
or cellulose or nylon. There are lots of different kinds
of polymers in the world, and structurally, polymers are long
molecules with a repeating structure, sort of like the links
in a chain. In diffusion, these long chains can sort
(10:33):
of intermingle and tangle with several nanometers of material on
the other side, on the other object or the other substance,
even though they're not reacting chemically. And this might be
a kind of crude analogy, but for a rough picture,
just sort of like imagine surfaces that have a bunch
of chains and ropes and strings at the edge of
(10:54):
them getting tangled up with the edge of the other object. Now,
remember you said there were three or four. This last
one seems more debatable, but Winkless notes that the adhesive
product manufacturer three M also identifies a fourth possible type
of adhesion, and this is electrostatic. Sometimes, if you are
(11:15):
about to attach a strip of adhesive tape to a
piece of paper, you will see the paper actually kind
of move through the air like it's reaching out toward
the tape. You know, they want to you know, it
wants to be found, like the one ring, and it's
kind of like it's being attracted by a magnet because
it's sort of is This is because the tape accumulates
(11:37):
charged particles as you peel it off of the roll,
and there is attraction due to static electricity there same
reason you know, you get static electricity on a balloon
or something and then it lifts your hair off of
your head.
Speaker 2 (11:51):
Yeah, or occasionally in that scenario where there's some little
bit of plastic garbage or packaging that you are trying
to throw away and it just clings to your hand
and you can't fling it away, even though there's obviously
nothing sticky on your hands. You weren't just messing around
with glue, you haven't, you know, you've washed your hands.
But yeah, it's sticking to you because of the static.
Speaker 3 (12:11):
So that is a real force. But Winkles says that
she doubts whether this should really be considered a true
adhesive force that like meaningfully sticks objects together, because while
it might cause an initial attraction, this is not really
what would like hold a you know, an adhesive tape
firmly to the paper, what prevents it from peeling off?
(12:32):
But it's important to note she says that no single
one of these forces fully explains stickiness, and she writes quote,
for any given adhesive product, it's almost impossible to determine
exactly which model or models might be operating.
Speaker 2 (12:50):
That's that's interesting just even to think of in terms
of hobbyists, you know, because there's so many different types
of glues, and you get into any given hobby, you
might think, well, all clues are the same, but of
course they're most certainly not. You know, some expand some don't.
You know, some are more about like essentially melting one
bit of plastic into another. So yeah, it's I guess
(13:14):
the thing we keep running up against is like everything
involving stickiness. There's like this level of language you have
to get past, as well as sort of the level
of human perspective, you know, like we're just we're just
too big. We're not on the same level where all
of this is actually taking place.
Speaker 3 (13:29):
Yes, And I think also when we think about it,
we're usually thinking, we're trying to think too simply, we
think about it in one direction. We think, like why
does X stick to why? Like why does duct tape
stick to the wall, And we think that there would
be one answer for that. There is one cause. But
it might be better to think about stickiness as an
(13:50):
emergent system that relies on many different things interacting, rather
than a property one thing does to another. Now, remember
all of everything I've been saying so far was about adhesion,
(14:11):
how two different materials cling to each other. Another important
factor in stickiness is cohesion, the tendency of a material
to stick to itself. So in order for a sticky
material like paint to work the way that it's supposed to,
it has to be both adhesive meaning it sticks to
the wall, and cohesive, meaning it sticks to itself. If
(14:33):
either of these properties fails, then it's not going to
be good paint. Another interesting factor that Winkless discusses in
this chapter is what's known as surface energy. So when
you come back to that image of you put a
drop of some kind of liquids, some adhesive liquids, sitting
on a flat, solid surface, the nature of the solid
material that the liquid is sitting on also influences how
(14:57):
well the adhesive sticks and one of the carearacteristics that
matters is that solid material's surface energy. So technically, the
surface energy of a material is the extra energy present
at the outside of a solid mass compared to the
energy present inside the mass. And this extra energy is
(15:18):
there because of imbalances of molecular bonds on the surface
of the object. But for a more intuitive understanding of
surface energy, you can think about it basically as equivalent
to wet ability. Usually, you can predict how well and
adhesive will stick to a surface by looking at how
easily the surface gets wet, how well it attracts water.
(15:41):
And a common test used to measure surface energy is
to look at what's known as the contact angle of
a drop of water on the surface of the material.
That might sound kind of abstract, but it's actually when
you see it illustrated, it's pretty easy to understand. So
imagine you put a drop of water flat on a
(16:01):
solid surface, and it can be it could be wood, metal, cardboard, whatever.
Watch what the water does. Does it spread out and
form a wide, flat disk or kind of a lens shape,
or does it stand up a little bit taller and
form kind of a flat bottomed hemisphere or does it
sit way high up in almost kind of a full
(16:23):
sphere or bead shape. The first one I talked about,
the wide flat lens of water is what or has
what's known as a low contact angle. If you measure
the angle between the water droplet and the flat surface,
it's going to be pretty narrow, well below ninety degrees.
A low contact angle means that this surface likes water.
(16:46):
It attracts water, and it has a high surface energy.
And you can kind of see that because the water's
like spreading out. It's the water wants to touch the surface. Meanwhile,
the bead of water that sits almost like a sphere
or a ball up on the surface has a very
high contact angle. It minimizes contact with the solid, and
this indicates that the solid has a low surface energy,
(17:09):
and it's a substance that wants to fight off wetness.
It wicks away the moisture. So you might think of
like a think of like a waxy leaf, you know,
where that you see water hidden and it just rolls
right away. It's like what you know, it almost seems
like it's impossible for it to get wet. In most cases,
this quality the surface energy correlates with stickiness. Surfaces with
(17:34):
a high energy that attract water then and form that
flat disc shape are also usually easier to stick things too,
and Winkless writes, this is kind of interesting. Surface energy
also tends to correlate with the coefficient of friction. It's
not a hard and fast rule by any means, but
if a material has low surface energy, if it's slippery
(17:55):
to liquids, it is often also low friction slippery to solids.
So I thought that's kind of interesting. As she says,
it doesn't hold in every single case, but generally, the
easier it is to kind of like slide smoothly over
something without rubbing or sticking to it, also that surface
(18:15):
that you're sliding over is going to be harder to
like stick sticky things to. But also on the subject
of surface energy, this coming back to what we were
talking about earlier. I was really not expecting a book
chapter about the science of stickiness and adhesion to contain
much controversy. But you know, again, this just feels like
one of those topics that you would find already pretty
(18:38):
much settled and more just kind of standardized textbook explanations,
like we all know how this works, but once again,
this topic still does contain a lot of mystery and
room for disagreement. And one of those areas of disagreement
that Winkless documents is disagreements between materials scientists and chemists
about how important surface energy actually is when you're making
(19:01):
things like glues or adhesives that are supposed to hold
together to different solid materials. So she cites a professor
named Stephen Abbott, who is a fellow with the Royal
Society of Chemistry, who argues that, and these are abbots
words quote, surface energy is undoubtedly useful for paint, but
for practical adhesive systems where we're actually sticking stuff together,
(19:24):
it's basically irrelevant, thousands of times too small to give
us what we need, and yet people are obsessed with it.
So he seems almost mildly annoyed by this. Maybe I'm
reading too much tone into that, but either way, I
thought it was fascinating that this topic is has so
many more questions within it than I realized, Like and
the fact that we have lots of adhesive products that
(19:48):
work pretty well, Like you know, they're companies that make
tapes and glues and sticky notes and all kinds of things.
They stick to one another pretty effectively. And we know
at least some and may all of the underlying mechanisms
by which these by which these products work, and they
do work, and yet you still can't say in every
(20:09):
case which mechanism or mechanisms are making the difference. And
part of this is because stickiness is not merely a
property of one material, but, as we were saying, an
interaction between two or more materials and the conditions under
which they interact. And at the later on in this chapter,
(20:30):
Winkless quotes the same researcher, Stephen Abbott, saying, quote, adhesion
is a property of the system. It's not a property
of like the individual substance. But it's all these things
interacting together. They have adhesive behavior as a whole or not.
Speaker 2 (20:51):
Yeah, I think that's key. That's key to keep in mind,
and it's it also factors into what I'm about to
discuss here in a minute, because originally the plan was
that I thought, well, we'll roll through a few different
animal examples of which ones are sticky and why are
they sticky? Foolishly thinking again that we must have have
this settled. It must be settled how this particular animal
(21:13):
sticks to a wall. And we do know a lot
about this topic, but it's maybe not as perfect of
knowledge as one might expect. And and and I do
have to, you know, to admit that. Of course, this
is the case with plenty of plenty of other things
about animals that we are even very familiar with. I mean,
even a house cat has its secrets. There are things
(21:34):
about house cat behavior that we have varying ideas about
and we haven't completely worked out. But yeah, with stickiness,
it's easy just to again to completely take it for granted,
to approach it from our you know, look at it
through the fog of our scale and our language and think, oh, yeah,
well it just sticks right. It's like, you know, it's
(21:56):
it's like a suction cup.
Speaker 3 (21:57):
Well, speaking of suction cups, that brings up an interesting thing,
which is that on the macroscopic scale, you can get
stickying forces that have nothing to do with any of
the stuff we've been talking about so far, because we
were just talking about like surfaces sticking to one another
due to properties of the surfaces. But in the case
of a suction cup, like if you want to stick
a suction cup to a window and climb a skyscraper
(22:19):
like Tom Cruise in one of those Mission Impossible movies.
That's a totally different mechanism. What's going on there is
the suction cup. By evacuating the air from the space
between the cup and the smooth surface, you are creating
a low pressure area inside, which allows the atmosphere to
press on the outside of the cup and hold it
(22:40):
against the wall. So when Tom Cruise is doing that,
what he's really doing is letting the atmosphere, the weight
of the atmosphere press his handholds against the building.
Speaker 2 (22:50):
Is Tom Cruise still climbing up buildings with the suction cups?
Is that still happening in the current movies.
Speaker 3 (22:55):
I think he was in the most recent one of
those I saw.
Speaker 2 (22:58):
Maybe that's the way. I can't remember did that, And
I think I only saw the first Mission Impossible movie,
And if you told me that he used suction cups,
I would just assume, Well, I guess he probably used them.
It seems like the sort of thing he would do.
Speaker 3 (23:10):
I think in the first one he uses magnets, because
that's when he's on the top of that train, you know, Oh, no,
John Voight, and they're fighting for a helicopter on top
of a train.
Speaker 2 (23:22):
I remember John Voight, And don't they lower him into lasers?
Is that the movie with lowering endo lasers.
Speaker 3 (23:27):
Lower him in the lasers.
Speaker 2 (23:28):
Yeah, like room full of like laser trip wires and
they have to like it comes down from.
Speaker 3 (23:33):
The cel Oh I don't know, Yeah, yeah, yeah. The
first one is where they put Tom Cruise on a
rope and they lower him down through out of an
air vent into a room that the floor is lava.
You can't touch the floor or the alarms will golf.
And that was so he could steal the knock list,
you know, the knocklst I had VHS to the first Yeah.
Speaker 2 (23:56):
I guess I only saw it once. Then. Yeah. I
don't have any strong feelings about it one one or
the other, but I mean I admire, I admire the
commitment to amazing stunts.
Speaker 3 (24:07):
Apart from like the coronal mass ejection of star power
that is Tom Cruise. Most of those movies have a
pretty cool supporting cast. Oh you know the first one
it had Ving Raims and John Renau.
Speaker 2 (24:19):
Yeah, yeah, I believe you're right. Yeah, yeah, strong casts
all right. Well, speaking of star power, and speaking of
daring stunts. The main creature we're going to talk about
here today is going to be the gecko. I'm also
going to get a little bit into Spider Man later,
but Spider Man's not real. The gecko is real, and
the gecko is quite amazing, one of the most famous
(24:42):
sticky animals of all, though to be clear, not all
geckos have this ability. For instance, the oft domesticated leopard gecko.
There's actually one of these living in my house. It's
my son's pet, is a ground dwelling lizard and does
not have this ability. But geckos are found on every
continent except Antarctica various species of the infraorder Gekota. Of these,
(25:08):
some sixty percent have adhesive toepads that enable climbing, although
this ability has apparently been lost and gained many times
over their evolution.
Speaker 3 (25:18):
Now, when you hear that a gecko has adhesive toepads,
first of all, that might make sense because you've probably
seen a gecko climbing up a flat surface like a
you know, a window pane or a sliding door, or
the wall of a building or a tree trunk, or
even climbing on the underside of things. Again, much like
tom cruise kind of doing overhang free climbs, going around
(25:40):
on the ceiling. It is pretty amazing what they're able
to do, and across the range of surfaces across which
they're able to do it. So you might just assume, okay,
well they got you know, toes that feel like glue.
Like if you were to go put your hand on
a gecko's toe, it would be a gross, sticky you know,
almost kind of like a like a sugar seer or
(26:00):
something like that. But no, no, that's not the case.
If you go and look at the toe and feel
the toe of a gecko, the kind that has these
amazing climbing powers and clinging powers, it feels totally smooth.
It is not covered in a sticky adhesive like the
back of duct tape.
Speaker 2 (26:18):
Yeah, it's one of the things that's long fascinated us
about the gecko. And again, geckos are widespread, so they've
been there. They're around for great thinkers of different ages
to potentially ponder them, and in fact, Aristotle mentions them
in the History of Animals, the fourth century BCE text.
If you start looking searching around in that text for
(26:39):
mentions of the gecko, they do come up several times.
But he also writes of the amazing climbing ability of
the quote unquote gecko lizard to quote run up and
down a tree in any way, even with the head downwards.
So even Aristotle thought that this was pretty interesting, though
I don't believe he offers any possible explanation for it,
(27:02):
is just kind of an observation about you know, I
think he's comparing it to another creature. Here.
Speaker 3 (27:06):
The quest to understand how gecko toes cling to all
these different surfaces and allow it to walk straight up
vertical walls of any roughness or smoothness, to cling to
ceilings and overhangs. This goes way way back, and people
have put forward tons of hypotheses over the years. Again,
coming back to our mission, impossible digression was due to
(27:28):
talking about suction cup climbing, and for a long time
a lot of people thought that the geckos did climb
by essentially using suction cup grips. But that's not the case.
Speaker 2 (27:39):
Yeah, At different points they you know, hypotheses included sticky
secretions like we've mentioned, suction cups, also tiny hooks. You know,
these are all decent hypotheses to work with but it
didn't take too long for various naturalists and ultimately scientists
and thinkers in general to realize, well, none of these
are really explaining it. Something else is goinging on, and
(28:01):
it seems like it's something maybe more you know, more
physical certainly than chemical. But once again, like it's our
perspective and our language kind of get in the way
of like simple understanding of what the get go is doing.
And I find my experience with researching this it was
kind of like like the Powers of ten sort of
experiences ZUSA in you know, yes, because you start off
(28:24):
with like a simple question, well, why can gecko's climb walls?
And the answer is, well, because they have special flexible
adhesive ridges on their toes. And if you wanted, you
could stop there and be like, okay, question answered, I
can go home now. But but then you might ask
the follow up question, well, how do their adhesive toes work? Well,
the answer is the toes have special tiny hairs like
(28:46):
even you know, far smaller than what we think of
as hairs called ceta on them. And then you might say, okay,
I got it. I can go home now, that's what's
doing it.
Speaker 3 (28:56):
But wait, yeah, how are the tiny hair What do
the tiny hairs do? Or how are they sticky? How
do they stick well?
Speaker 2 (29:03):
Each of these has even smaller bristles called spatula on them.
Speaker 3 (29:08):
Right, So, one way I've seen this described is all
these tiny little hairs. If you can zoom way way in,
like smaller on a scale smaller than you can see
with visible light. You have to use like a scanning
electron microscope, you can see that each hair like frayse
like a rope that just goes out to like millions
of little fibers that are beyond microscopic. They're down on
(29:30):
the nanoscale. They're super super tiny, frayed out to uncountable ends.
Speaker 2 (29:35):
Yeah, I mean, there's so many that a given gecko
i've read have somewhere in the neighborhood of two billion
of these on the pads of their toes combined.
Speaker 3 (29:45):
Okay, so on their toes they have these ridges. The ridges,
by the way, are called lamelli, and then on the
lamelly they have the setae, and the setae are little hairs,
and those hairs fray out into these little frayed bristly
ends called spatchel. But what does the spacially do. What
are those how do they work?
Speaker 2 (30:04):
Well? The answer here is well, it's complicated, but the
basic idea is that their stickiness seems to depend on
a combination of intermolecular forces.
Speaker 3 (30:16):
Yeah, it does seem like there are maybe multiple forces involved,
though the sources I was looking at zeroed in on
the most agreed upon explanation having to do with what
are known as Van Derval's forces.
Speaker 2 (30:28):
Yes, van der Waals force named for Johannes Diedrich van
der Waals who lived eighteen thirty seven through nineteen twenty three.
There's an excellent two thousand and two article I've been
shouse that I that I One of the sources I
turned to for this section is you can find on
science dot org has the just perfect title how Geckos
(30:50):
stick on de valls at Waals spelling for walls. So yes,
that as of two thousand and two is to gused
in this article like that is the primary hypopsis for
how the gecko's toepads are ultimately sticking to the wall.
But another one that was being looked at at this
(31:12):
time in an article I'm going to side here is
this idea that it could have been water based capillary forces,
so like a thin film of water between pads and
the climbing surface. Quote. Because water molecules are polar, their
electrical charges aren't evenly distributed. They might stick to some
polar molecule in gecko's feet.
Speaker 3 (31:33):
Oh okay. Now on the opposite side of that, what
I've been reading about gecko's feed is that they seem
to be composed largely of water repelling hydrophobic materials. But
that again, I think it's complicated.
Speaker 2 (31:46):
Yes, yeah, Now to come back to vander Walls here,
science writer Eleanor Nelson put together a great explainer on
this whole topic for ted D several years ago. You
can look this up. There's a video on ted ED
about the gecko to search for ted ed gecko and
you can watch it as great visuals to help explain
all this. But she stresses that what we're talking about
(32:10):
here isn't attraction between charged molecules positive or negative, but
rather attractiveness between uncharged molecules that experience patches of negative
and positive charges due to the movement of atoms with
different electro negativities in the same molecule. So we're talking
about flickering patches of weak attraction. It's not strong in
(32:33):
isolation like each of those you know, tiny spatula are
not you know, pulling things across the room or anything
remotely like that. But when you have two billion of
these on a single gecko exploiting the force, it adds up.
In fact, it adds up so much that a gecko
can hang by a single digit, you know, from a
(32:54):
like a ceiling, you know. And it has complete control
over this as well, so you know, because they can
manipulate the angle and surface area of those tow ridges
in just the right way to activate and release that
stickiness on demand.
Speaker 3 (33:08):
Yeah, it is amazing, especially when you notice like the
motor activities that the gecko has to coordinate to orient
these little harris correctly to always cling the right way.
But I wanted to clarify one thing because it took
me a minute to understand what was going on with
the Vandervals forces. The vanderwalls or Vanderval's forces are something
that is different from like the attraction you get in
(33:30):
a polar molecule like water, which is permanently polarized in
terms of a charge, so it's always going to be
able to attract one way or another because it's got
a you know, negative charge on one side, positive on
the other. The Vanderval's forces are, Yeah, they're due to
the fluctuations in the orientation of electrons around an atom
(33:53):
that are just random. So they just randomly change over time,
even when an atom is electrically stable, so this atom
is not polarized, but things just sort of flicker in
and out of existence. A charge attraction going one way
or the other flickers in and out of existence because
of just random fluctuations of where the electrons are lined up.
(34:14):
And that in cases where where atoms are packed close
enough together can actually have an attractive effect, but usually
atoms are not packed close enough together for that to matter.
Speaker 2 (34:28):
Yeah, this is a if you're still foggy on this,
I do recommend that ted Ed video because they have
a nice animation that kind of that illustrates what's going
on here. At least for me, it helped clarify the
whole thing.
Speaker 3 (34:40):
Yeah, and so the reason the spatuely are able to
take advantage of the Vanderwolts force is because they're so
tiny and there's so many of them.
Speaker 2 (34:47):
Forests of them, I've seen it described a right. So
coming back to the two thousand and two time period here,
there was a paper that came out in a two
thousand and two edition of Integrative and Comparative Biology titled
(35:11):
Mechanisms of Adhesion in Geckos. This was by Keller autumn
at All and in this they were exploring the vendor
val and the water based capillary forces explanations to you know, see, okay, well,
let's look at these two possible ideas and see which
one seems to seems more evident when put to the test.
(35:34):
And they used the aid of semiconductors, so they ruled
out water droplet hypothesis as it depends on polar surfaces.
They found in their experiments that geckos could equally climb
a non polar surface. In this experiment they used a
semiconductor called gallium arsenide, which is a compound of gallium
(35:54):
and arsenic. By the way, it is also interesting that
gecko feet can not adhere to teflon. You may have
heard this or seen some headlines about this over the years,
but according to Sarah Chodesh on Popular Science back in
twenty eighteen, teflon disrupts the Vanderval's force effect here that
(36:16):
enables them to climb other or seems to be one of,
if not the primary factors in their ability to climb
other surfaces. Quote. Vanderval's forces depend on being able to
induce a positive and negative end of a molecule or atom.
But the layer of fluorines in Teflon just have this
permanent negative charge to them. The electrons stay in one place.
Speaker 3 (36:37):
Teflon's not having any of it. Yeah.
Speaker 2 (36:40):
So anyway, that twenty and two study backs up the
vandervals explanation. In their own words, their findings at the
time provided quote direct evidence that Vanderval's force is the
mechanism of adhesion in get Go ceta and that water
based capillary forces are not significant. So again not to
say it couldn't be doing something, but it's not the
(37:01):
significant factor.
Speaker 3 (37:02):
Yes, though it's interesting. In uh the chapter in Winkless's
book on gecko toes, she talks about, how you know,
it seems like most researchers in this area kind of agree, Yeah,
it's Vanderval's force. That's that's the explanation or the main explanation.
But she says, they all kind of have this bit
of hesitation when they talk about it, like maybe there's
(37:25):
something else going on too that we haven't fully figured
out yet. Yeah.
Speaker 2 (37:29):
I kept thinking about like the experience of trying like
a fancy cocktail somewhere and you're trying to pick out
the ingredients just on taste. You know, generally you can
you can pick out some of the major flavors. But
are you gonna, you know, absolutely put your money down
that there are only three ingredients in this cocktail and
you're there are not four, they're not five, and so forth.
Speaker 3 (37:52):
That's right. What's that tingling in the back of your mouth?
Could that be an absentthe rense?
Speaker 2 (37:59):
So one sample of this concerning gecko feed This comes
up in a twenty fourteen article that I was looking
at role of contact electrification and electrostatic interactions in gecko
adhesion by A. Zadi at All published in the Journal
of the Royal Society Interface, and this one discusses the
(38:19):
CE driven electrostatic interaction. This refers to the contact electrification phenomenon,
and I understand that this is still controversial. For example,
in a twenty twenty three paper by song at All,
in published in the journal Friction. The author's cite the
controversial aspect of the CE driven explanation, though to be clear,
(38:39):
it's ultimately more about the level of contribution that's controversial,
because again it's like saying, well, yes, I think there's
absinth in this cocktail, but maybe it's just a drop.
It's like three drops of absinthe I'm not saying it's
anything more than that, Whereas there might be someone else arguing, actually,
I think they put an ounce in there, and then
(38:59):
the fighting and controversy. But anyway, in this song at
all paper the way they explain it, it seems like
CE may be part of the situation, but their study
suggests that its impact is low, perhaps contributing only around
three percent of the adhesiveness, and this may also be
(39:21):
why it doesn't help them out with something like teflin,
which again essentially cancels out Vanderval's force, which, as far
as we can tell, seems to be the primary factor
in play here. However, they note that quote long range
electrostatic forces may play other roles in a distance range
where the Vanderwals interaction cannot function, So again there could
(39:43):
be certain situations where it's more important to some degree
than other times.
Speaker 3 (39:47):
But it does seem clear that whatever else is going on,
the main ingredient, the whiskey in the cocktail is the
vandervals interaction. Yes, and another interesting thing here, Rob you
alluded to this earlier, but it's something that you can
notice if you watch a gecko actually climbing, which is
the question of wait, how do they make their toes
(40:10):
stick and then unstick? So they've got this really powerful
sticking force, and in some cases the sticking force it
can hold so much more than the weight of a
gecko's body. There's one estimate in Winkliss's book about a
test showing that a gecko I can't remember, I didn't
write down the number, but it's like a gecko's grip
could hold hundreds of pounds up on the wall, despite
(40:33):
the fact that the gecko itself only weighs like half
a pound. Now, on one hand, you might think, well,
that's kind of overkill, but of course that would be
testing it in sort of ideal conditions in a lab,
whereas in nature a lot of times the surfaces it's
climbing on are going to be dirty, which will reduce
the adhesive potential, might be wet, which would potentially reduce
adhesive potential. Some of their toes might be injured or something.
(40:56):
Some of the lamilly might come off. In nature, you know,
it's better safe than sorry. When you're a gecko, have
a lot of sticking power. But yeah, anyway, the question
is how does it stick and unstick and climb in
different directions? And a lot of this comes down to
how it lifts and attaches its toes to the surface
and how it orients them. So like you, if you
(41:18):
watch a gecko crawling up a wall, it will tend
to sort of like peel its peel its toes up
and then lay them down with all of the toes
facing upward on the wall. But if you see a
gecko climbing down a wall, it will rotate its back
feet so that its toes are facing up. Because the
(41:39):
ceta and the spatually on them, they sort of only
grip effectively when laying in one direction.
Speaker 2 (41:47):
Yeah, one way to think about all this is that again,
our understanding of what's going on when a gecko climbs
has not reached a level of perfection, but their exploitation
of it has reached perfection, like they are the perfect
manipulators of this force. So it's pretty amazing to watch.
Speaker 3 (42:08):
Now, whenever evolution achieves something this ingenious, you will bet
that human engineers come running to see, how are they?
How are they doing that? What's going on there? Could
I make robots that could do something like that?
Speaker 2 (42:21):
Yeah? Yeah, So of course biomimicry has gazed longingly at
the gecko for a while now. There have been various
efforts to create artificial ct with some working prototypes and
robotics and even human climbing suits popping up in headlines.
I think there was a DARPA project that made the
news in twenty fourteen. These have certainly not reached a
(42:44):
level of perfection yet, but you know, there are some
interesting applications out there potentially for this, and they're not
all of them involved like super soldiers climbing the windshield
or robots cleaning you know, high rise buildings, for instance.
It's been put far that they could be used in
low gravity environments. They could even be used, you know,
(43:05):
to collect space debris in orbit, that sort of thing.
Speaker 3 (43:08):
So the amount of weight that can be supported by
a gecko's toes and the range over the range of
different surfaces that they can crawl over is truly astounding
and sort of a superlative in nature. But there are
lots of other animals that can crawl up walls, and
I wonder do many of them make use of similar
mechanisms or forces well.
Speaker 2 (43:29):
Spiders climb in a similar way and also depend on
tiny set that make use of Vanderval's force. Though they're
ultimately like masters of these structures, they also use them
for other purposes as well, like like sensing sense sounds, vibrations,
and air currents. But this does lead us back to
(43:49):
the topic of Spider Man, because, as everyone knows, Spider
Man can do whatever a spider can, and it's this.
Speaker 3 (44:00):
He can liquefy your innards and slurp them out through
his mouth orifice.
Speaker 2 (44:05):
Maybe he can, and he chooses not to, But I
guess that's more of a Isn't there a man There's
a man spid. I know there's a batman and a
man bat there's also like a spider man monster creature,
but I don't know if that's man spider or not
off the top of my head. But it gets it
gets so complicated when you start comparing Spider Man to
spiders because, of course there are all these things that
(44:26):
don't match up. For example, it's easy to think of
Spider Man. What does Spider Man do well? He swings
around on webbing through the through the streets, though he
does so in a way that's not particularly spider esque,
and while in some adaptations of Spider Man he's using
like organic like mutations, like he has like he's you know,
he's been by a radioactive spider and now he can
(44:49):
he can shoot webbing out of his wrists. Most versions
of Spider Man you encounter he has these little gadgets
that he built that shoot some sort of nylon like webbing.
So he's this weird mix of like, Okay, I'm part
Spider but also I have like super crazy sci fi
weapons as well, and all of these combined to make
me able to do whatever a spider kit.
Speaker 3 (45:09):
It's not really satisfying that way, is it. He's one
foot in each world. He's half a Charles B. Griffith
script where yes it was atomic radiation. But he's half
Batman and we already have Batman. He makes gadgets.
Speaker 2 (45:21):
Yeah, I mean, I guess I'm not a Spider Man
purist or anything, but I like the mutant version. I
like the idea that he's shooting it out of his wrist,
even though that doesn't really match up. Like spiders don't
they have spinnerets, they don't have wrist openings or whatever.
So you know, it's still not one to one.
Speaker 3 (45:40):
I guess Spider Man fans, please don't hate me. I'm
just joking around. I don't know much about Spider Man.
Speaker 2 (45:45):
I mean, Spider Man is great. But one of Spider
Man's abilities that is or seems to be universally accepted
as an ability brought on by the radioactive spider bite
is his ability to climb walls and stick to ceilings
with out the aid of his wedding. How does he
do this well? On one hand, we have to admit
(46:06):
like this is a fool's errand to try and make
sense of this. Spider Man climbs walls because he's Spider Man,
and any kind of like scientific explanation to this comes
after the fact they were not thinking long and hard
about the anatomy of a spider when they came up
with this character. As fun as this character is, so
(46:26):
any attempt to scientifically explain everything about him is is.
It can be very fun, but it's not going to
get you to a place of absolute certainty.
Speaker 3 (46:36):
You might as well ask, exactly scientifically, how do the
crabs in attack of the crab monsters eat your brains
and gain your knowledge exactly?
Speaker 2 (46:45):
But like I say, it's a fun exercise, and I've
been enjoying this book marvel Anatomy by Sumeruk and Wallace,
and the authors point out several things about Spider Man's
adhesive abilities and how they match up or don't match
up with the natural world. One of the main ishu
that they keep talking about, though, is Okay, Spider Man
can climb, and perhaps he does this because he has
(47:07):
little CD on his hands at least or I guess,
over his entire body, and it allows him to adhere
to walls. But also he's wearing a full body costume.
Spider Man is almost never naked when he's clinging to
walls and so forth, so it raises questions like would
vander walls work through a pair of spider gloves or
(47:29):
would that weaken it to some degree.
Speaker 3 (47:32):
I don't think it would work, because the whole thing
about Vanderval's force is you've got to be incredibly close,
closer than solids can usually get to each other. That's like,
that's why the gecko's foot works. It's the tiny little
thing that can get in there in the cracks and
crevices in the surface, unlike anything else.
Speaker 2 (47:48):
Well, the author suggests that, well, maybe the ceed poke
through the costume, but that raises all sorts of questions,
like then when he takes the costume off, when he
peels off the sweaty Spider Man costume at the end
of the day, does it just rich them all off
and he has to regrow them or I don't know.
It's a wonder fabric that Iron Man made for him,
so it lets them out.
Speaker 3 (48:08):
I don't know.
Speaker 2 (48:09):
Again, it just raises more questions than anything.
Speaker 3 (48:13):
I think Spider Man should have finger gloves like a bandit.
Speaker 2 (48:17):
The authors here suggest that maybe Spider Man can actually
quote consciously control the intra atomic attraction between molecular boundary layers,
which I suppose sounds reasonable, though I mean this raises
the question about to what extent this still makes sense
as a sixties at comic book. Radiation Advancement of natural
World spider abilities, but I'll take it. I'll take it.
(48:41):
Sounds good to me. Now. They do point out that
another character, Spider Woman. I don't know if you're familiar
with Spider Woman. She was introduced in nineteen seventy seven.
She just kind of like a red outfit. But she
is said to be able to climb via secreted biological
adhesive substances that are on her hands and her feet
(49:04):
that I guess swiftly penetrate tiny pores in the climbing surface. Also,
she wears gloves and booties as well, so this secretion
would have to like rapidly soak through the fabric in
these cases as well or through the booties and then
allow her to stick to the walls. And again this
raises more questions because when you look to the natural world,
(49:25):
you do see adhesive secretions in some organisms, but they
tend to be defensive rather than climbing aids. These are
things that you would extrude when threatened, so that whatever
was trying to eat you might decide, oh, I don't
want any of this.
Speaker 3 (49:39):
Yeah, well, maybe I'm not thinking creatively enough, but it
seems like if you had to secrete enough sticky stuff
to hold you to a wall every time you wanted
to climb. That'd just be a lot of stuff that
sounds like you're secreting, Like do you get dehydrated? Do
you run out of energy doing that? Well?
Speaker 2 (49:57):
I mean, Spider Man is leaving nylon thread all over
the city, all over New York as he's fighting crime,
and I guess somebody has to clean that up. They
probably didn't think about it much in the sixties, and
I guess it was acceptable in the eighties, but nowadays,
like it's what's the what's the environmental impact of this
crime fighting?
Speaker 3 (50:16):
But once again the genius of the gecko. The gecko
climbs without goop.
Speaker 2 (50:20):
Yes, yeah, And the authors here they do include a
bit on one of Spider Man's many enemies. You've perhaps
heard of, the lizard. I think the lizard was in
at least one of the movies. I can't remember which
Spider Man this was, which Spider Man regime, But this
is a man, a scientist who turns into a great,
big green lizard and chases Spider Man around. And in
(50:44):
many of the depictions, the lizard can climb around on
walls and on the ceiling. In this particular book that
I was referring to, they discuss it as more of
a claw thing, which I think makes sense for a
super villain, you know, big nasty claws. Can you know
claw into the concrete? Is the creatures climbing around. But
I've also seen some descriptions that discuss the lizard as
(51:08):
having gecko abilities.
Speaker 3 (51:10):
Oh okay, so either way claw or gecko, that would
be San's goop. That would be a mechanical grip of
some sort or of Vandervohal's force. Yeah.
Speaker 2 (51:20):
But again, I think with a super villain like the Lizard,
it makes more sense that his climbing ability is visibly destructive,
though it raises the question like, where's the love for
the gecko? Then in the creation of Superheroes, I was
looking around looking at the various databases, and it looks
like they're a couple of minor Marvel Gecko characters, or
(51:43):
maybe it's two versions of one character that have existed.
And it looks like DC Comics also has a Gecko
or the Gecko. But I don't get this sense that
these are based on anything, you know, true to the
get go. But comic book fans out there let us
know perhaps you have more detail on these Geckos.
Speaker 3 (52:03):
Who is Gordon Gecko? Is he some kind of gecko supervillain?
Speaker 2 (52:08):
I don't know. Maybe it's part of the shared cinematic universe.
Speaker 3 (52:11):
Okay, I think that's all we got for today.
Speaker 2 (52:13):
All right, We're going to go ahead wrap it up here,
but yeah, write in let us know what you think
about stickiness in general, the examples of stickiness that we've discussed,
and yes, even fictional sticky comic book characters and so forth.
Just a reminder that we're primarily a science podcast here
at Stuff to Blow Your Mind, with core episodes on
Tuesdays and Thursdays, listener mail on Mondays, a short form
(52:35):
monster fact or artifact episode on Wednesdays, and on Fridays.
We set aside most serious concerns. Who just talk about
a weird film on Weird House Cinema.
Speaker 3 (52:42):
Huge thanks to our excellent audio producer JJ Posway. If
you would like to get in touch with us with
feedback on this episode or any other, to suggest a
topic for the future, or just to say hello, you
can email us at contact at Stuff to Blow your
Mind dot com.
Speaker 1 (53:03):
Stuff to Blow Your Mind is production of iHeartRadio for
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