October 9, 2018

About those gecko feet.

I mean, look at them:

I got there via "A gecko, seriously, made dozens of mysterious phone calls from a Hawaii marine mammal hospital" (WaPo). And I found the closeup of the gecko foot on the Wikipedia article "Gecko feet" (there's an article just for the feet!).
The interactions between the gecko's feet and the climbing surface are stronger than simple surface area effects. On its feet, the gecko has many microscopic hairs, or setae... that increase the Van der Waals forces between its feet and the surface.
The Van der Waals forces!
The following equation can be used to quantitatively characterize the Van der Waals forces, by approximating the interaction as being between two flat surfaces:



where F is the force of interaction, AH is the Hamaker constant, and D is the distance between the two surfaces. Gecko setae are much more complicated than a flat surface, for each foot has roughly 14,000 setae that each have about 1,000 spatulae. These surface interactions help to smooth out the surface roughness of the wall, which helps improve the gecko to wall surface interaction.
And the gecko to touchscreen surface interaction!

47 comments:

rehajm said...

No results found for #hamakerconstant.

Not surprising.

rhhardin said...

Van der Waals forces only came up for me in physics as a volume correction to the ideal gas law for the space taken up by molecules. We didn't do materials science.

It seems today to be a variety of things. The formula isn't as meaningful as where it comes from.

Ignorance is Bliss said...

...for each foot has roughly 14,000 setae that each have about 1,000 spatulae.

I'll bet the gecko can use the phone to make awesome blog comments. (Assuming those are laslo spatulae.)

Ignorance is Bliss said...

God made the bulk; surfaces were invented by the devil.

-Wolfgang Pauli

rehajm said...

If gecko wants to win the crank call battle he has a few more calls to make...

It drives you absolutely out of your mind...I thought, talk dirty to me. Do something. This silence is driving me crazy.

buwaya said...

In the old days you only had to worry about your toddler making direct dial calls to Botswana.

Robin Eatmon said...

I bet there will be new Geico campaign that will feature these sweet feet!

EDH said...

where F is the force of interaction, AH is the Hamaker constant, and D is the distance between the two surfaces.

D = distance cubed seems to be the only variable in that equation. But wouldn't you think the foot is either touching or it's not?

And...

Behold rare video: the Gekko Haymaker.

Ralph L said...

At least it wasn't a credit card offer.

help to smooth out the surface roughness of the wall,

I thought it was the reverse: the tiny hairs attach to the tiny roughness of the smooth wall.

Gabriel said...

@rhhardin:It seems today to be a variety of things. The formula isn't as meaningful as where it comes from.

Perturbation theory can get you the first term of the van der Waals attraction between two neutral atoms. You can use spin-aligned hydrogen electrons, the derivation is pretty straightforward.

It started out as an empirical correction to the gas law, but like lots of things that start out that way, it can now be derived from quantum theory.

Fun fact about Planck's constant: it shows up in statistical mechanics work done by Gibbs on gases, long before Planck. Gibbs proved it had to be there, but he didn't know what it was. Similarly, Maxwell's equations have special relativity in them, but Maxwell didn't know what it was.

FIDO said...

The real question is could Spiderman scientifically cling to walls at his body mass?

Etienne said...

12 π (Pi) is a stupid number. It's actually 6 τ (Tau).

I mean who thinks in half quantities? Sheesh...

Fernandistein said...

And the gecko to touchscreen surface interaction!

The wonders of gravity.

Gabriel said...

@EDH: But wouldn't you think the foot is either touching or it's not?

No, the atoms in the gecko's feet are not at zero distance from the atoms in the surface. They are at some very tiny distance away. Strictly speaking no matter is "touching" any other matter, in the sense of literally zero separation.

When you say "I'm touching the wall", physically the wall is exerting a force on your hand which you can sense, but there is not literally zero separation between your hand and the wall. The words we use were developed long before we knew much about physics, and you can mentally revise the meaning of "touch" to mean "a microscopically small separation that is indistinguishable to human senses from zero separation" but most people who have studied physics navigate through life just fine without having to do so.

Ralph L said...

That's a really big pinkie.

Ralph L said...

Maxwell's equations have special relativity in them

I took a course that derived Maxwell's Equations, and all I remember is the laughter.

Quaestor said...

There's a connection between the physics of gecko feet and the Democrats on the Senate Judiciary Committee who managed to pull defeat from the jaws of victory last week.

Gabriel said...

@Ralph L.I took a course that derived Maxwell's Equations, and all I remember is the laughter.

Indelible in the hippocampus is that the speed of light is the product of permittivity and permeability, and independent of reference frame.

Quaestor said...

The real question is could Spiderman scientifically cling to walls at his body mass?

Well...perhaps yes.

robother said...

Hmmm. So there must be some evolutionary advantage to 5 fingers (as with 4 legs), that transcends mammalian ecological niches.

Phidippus said...

Spiders use the same technique.

Don't ask me how I know this.

Etienne said...

It's funny it doesn't have a larger penis.

chickenlittle said...

“It's funny it doesn't have a larger penis.”

Post coital escape strategy to discourage sticking around.

EDH said...

...you can mentally revise the meaning of "touch" to mean "a microscopically small separation that is indistinguishable to human senses from zero separation"...

Hmm, interesting. In the example of a gecko climbing a wall, what you describe makes me think of the of the nano-distance=D involved in the "touch" as more of a "force" exerted by the gecko foot against the wall in a vector perpendicular to the wall, thereby increasing the surface area of the foot "against" the wall, creating an increasing "friction" overcoming the gravitational force parallel to the wall, rather than some asymptotically collapsing concept of separation.

But I will defer and internalize that sophisticated concept of distance for the future.

Gabriel said...

@EDH: In the example of a gecko climbing a wall, what you describe makes me think of...

I think a clearer way to put it is this:

Each molecule in the wall is attracted to each molecule in the gecko's foot due to van der Waal forces, and that force depends solely on the microscopic distance between any two of those molecules. That would be true of any two objects in "contact". There's nothing special about walls and gecko feet in that regard--the same kind of thing happens between each molecule of an ice cube and each molecule of the wall, but the ice cube slides down.

What IS special about gecko feet, is that the surface of a gecko's foot is able to bring a very large number of the molecules in its feet into a configuration that maximizes the vector sum of those van der Waals forces, and ice cube doesn't do that. The gecko can arrange its feet so that all those forces can add up to something that balances the force of gravity on the gecko.

Ralph L said...

The gecko can arrange its feet

Look, Ma, three feet!

chickenlittle said...

VdW forces fall off as D-cubed but the objects need not actually be touching. Likewise, planetary attraction falls off as D-squared. No one thinks the planets need actually be touching.

Jack said...

How about those quasi-2D van der Waals ferromagnets, though?

chickenlittle said...

London Dispersion Forces are at play in Britain. That explains the on again off again nature of Brexit

Gabriel said...

van der Waal forces are not fundamental forces in themselves, it's worth remembering. They are a label given to a sum of electrical forces arising from a class of configurations of atoms and molecules.

In everyday life, gravitational and electrical forces fundamentally account just about everything. All the forces you learned about in high school: drag, friction, tension, etc, are really just labels given to the net electrical forces that macroscopic objects exert on each other in different situations.

Rick Turley said...

Insurance telemarketer.

Gabriel said...

Fun fact: magnetic forces are incapable of doing work, because they act at right angles to the motions of charges.

So when a magnet pulls a 10 gram steel ball upward 1 cm against gravity, that work was actually done by the electrical forces between atoms in the magnet and the ball. Extremely difficult to show how it was done, but very easy to determine the correct result: the magnetic forces cannot have done any work, since every magnetic force and every charge involved contributed exactly zero to the total, and yet work was done.

chickenlittle said...

Faraday was delighted, and a little alarmed, with Maxwell's approach to his ideas and the way in which it gave his theory the same status as action at a distance theories: [1] 'I was at first almost frightened when I saw such mathematical force made to bear upon the subject and then wondered to see that the subject stood it so well.' He returned to this theme later in 1857 when he asked Maxwell why mathematical conclusions may 'not be expressed in common language as full, clearly, and definitely as in mathematical formulae...translating them out of their hieroglyphics, that we might also work upon them by experiment'.

Ignorance is Bliss said...

Etienne said...

It's funny it doesn't have a larger penis.

If I had a dime for every time I heard that.

Ignorance is Bliss said...

Gabriel said...

In everyday life, gravitational and electrical forces fundamentally account just about everything.

A lot of people think that, right up until the strong nuclear force cuts out and all their protons and neutrons fall apart. Then don't they feel stupid...

Gabriel said...

@chickenlittle:he asked Maxwell why mathematical conclusions may 'not be expressed in common language as full, clearly, and definitely as in mathematical formulae...translating them out of their hieroglyphics, that we might also work upon them by experiment'.

Maxwell's equations have been the topic of quite a lot of experimentation, and have held up pretty well.

It's certainly a worthy goal to express mathematical conclusions in clear succinct English where possible, but sometimes English is as ill-suited to that as interpretive dance or sheet music. The universe doesn't have any reason it needs to behave in a way that can expressed succinctly in English. Sheet music doesn't express Maxwell's equations very well either.

English was not developed to express quantities and relationships with precision.

Bruce Hayden said...

Geckos aren't the only animal that can cause havoc with electronic devices. Our little minicat loves the fact that he can change what he is looking at, by walking over the screen to one of my iPads. So, I can be writing a comment on Althouse, he sees what I am doing, takes interest, and all of a sudden my comment is full on nonsense, and we are in a completely different application. At which time, I open up the Photo app, turn on slide show, and let him have fun, knowing that he will lose interest much faster when I indulge him, than if I resist (which brings up the question of who is training whom?)

walter said...

"who is training whom?"
Commonly referred to as "cat ownership"

Hey, did these phone calls include polite apologies in a cute Aussie accent?

Clyde said...

Do people in Hawaii say "Aloha" at the beginning of a phone call, or at the end of a phone call, or both?

Clyde said...

I know that when it's a baseball home run call, "Aloha means goodbye!"

EDH said...

Based on the science, I'd have to conclude that the gecko's feet are exceedingly ticklish.

traditionalguy said...

Warning: These feet are another hint of Intelligent Design. No wonder the GEICO Gecko is so talented.

Ignorance is Bliss said...

traditionalguy said...

Warning: These feet are another hint of Intelligent Design.

Well, sure, since there's obviously no evolutionary advantage to being able to walk places that the local predators can't.

MayBee said...

Those feet kind of trigger me.

rehajm said...

Those feet kind of trigger me.

Trypophobia. Aptly named, in this case...

Rabel said...

"After receiving the barrage of bizarre calls, fearing something had happened to one of the seals, Simeone tweeted, she abandoned her lunch plans and raced back to the hospital as quickly as she could."

A normal person would have called another person at the hospital before "racing back" as quickly as she could.

So, a cute story, but bolstered by a lie.

Anthony said...

Sometimes even the WaPo can write a good article.

I recall several years ago they did a story on rabbits interfering with planes taking off (may have been the NYT though) and they published a handy graphic comparing the size characteristics of a rabbit with a 747.