Defying gravity. Gecko pads, tree frog toes and Van der Waals forces.

Hey guys! Happy new year! It is always refreshing having a few free days. And I have seen in this a more than excellent opportunity for bringing you a new entry. Plus I have had several ideas flitting around my mind from a time ago.
So, gecko pads! This is probably one of the most known examples of adhesive “spider-man” like extremities. And I sure understand why. If you don’t find the peculiar capability of the geckos to walk up a vertical wall or window as something remarkable, I hope you will after discovering the science behind it.


Gecko sticky feets and intermolecular forces

Geckos are not the only ones with impressive vertical motion abilities. There are others, like spiders, insects and even frogs… And the principles behind their abilities are the same for many of them. But geckos are quite big creatures, and this makes the example even more dramatic. By the way, if you never had the opportunity to see a gecko climbing and its sticky toes behave in real action, here you have a brief slow motion video. Ok, ok. Alhough I admit it is probably not the more splendid gecko run, pay attention to how its pads attach him to the glass surface. And also how he retracts the fingers when taking a new step. It reminds me the mechanics of a sucker, but the physic laws behind are very different in fact. By the way, the glass panel is angled 30º in that video. Is the camera arrangement what gives you a different idea.

So, where is the trick hidden? Wait and see. In the figure below we can observe the magnification of a gecko’s pad under one of its toes. There it is a hierarchical structure that goes from the macroscopic ridges that you can detect with the naked eye, to a small hair-like nanometric fibrils. These fibrils called seta (plural for setae) are in fact way thinner than any hair, and are grouped up in small bundles. In addition, at the very end of every setae there is a wider structure called spatulae. Some may be tempted and come to a hast conclusion: this fibrils may readily get inside the small imperfections of the wall, and thus sustain the animals’ weight. Good thinking but no, sorry. It seems that the explanation for this unreal grip are the Van der Waals forces that arise between this microstructure and the wall.

Macro and SEM photos of the hierarchical microstructure of a gecko’s toe pad. Image from: Marc A. Mayers et al. “Biological materials: A materials science approach” [2010].
Why can’t we see these small “hairs”. As I said, these fibrils are quite small. How small? Tiny. Nanometric in fact. The broad ends, called spatula, are around 50 or 100 nanometers (nm) wide. To give you and idea of the size, 100 nm equals 0.1 micrometer (μm). And the limit of human vision is considered to be of about 100 μm more or less (0.1 milimeters). So you would need to group up more than 1.000 of these fibrils together just to barely start seeing something.

Van der whaat forces? I mean, I just know about THE force. And now is when physics appear (ominous music plays on the background: “Hello darkness my old friend…”). Several mechanisms have been proposed for the sticky effect seen when a gecko uses its magic. He doesn’t seem to care too much about it, but we do. And the latest and more accepted theory implies that the intermolecular forces called after the Dutch scientist Van der Waals are the best explanation. This forces are nothing but electrostatic interactions between dipoles naturally present or induced in the molecules from which organic matter (for example, but not exclusively) is made. If you want a more descriptive mental image, think about the effect after rubbing a plastic film against your hair. You have something similar there. So, these attractive forces between the geckos’ fibrils and the wall/glass/rock/leaf/something, sustain the roughly 100 grams this animal usually weights. Sometimes even significantly more.

More contact surface, more forces in action. That not only explains the spatulae widen shape, but also the flexible fibrilar structure that should adapt easily to the nanometric imperfections of any apparently smooth surface.

Microstructures can and do change the macroscopic reality, arising very unexpected results sometimes. We have talked before about very colored morpho butterflies and how they play with light using their wings and the nanometric tree-shaped structures on them. (Read more here)

I am not the frog he is talking about down there. I am just a cute intruder

A severe case of haxagonal frog toes

SEM images of the Scinax pereca frog’s toes microstructure. Image from: Marc A. Mayers et al. “Biological materials: A materials science approach” [2010].
Geckos are not the only ones with amazing aptitudes regarding the vertical challenges that nature and man create around them. As the tittle of this post suggest, there are frogs with similar and peculiar microstructures at their toes. No need to say that, in a jungle of moist and slippery leaves, an extra of grip is never a bad idea. The Brazilian tree frog Scinax perereca knows this very well.

The purpose and functioning of all these small fibrils called seta at the frog’s toes is, I hope, well clear after the gecko pads analysis. But I can’t avoid being amazed again and again by the peculiar hexagonal super-structure formed by the fibrils. I mean, just look at the photo!

It seems this rifts between hexagons serve as canals for the oozy fluids the frog excrete. This adds a slimy enhancement for the already sticky grip created by the fibrils.

Why hexagons? I’m not sure. But I have an idea! Do you know why bees build its honeycombs using hexagonal polyhedra? It happens that the most efficient way of filling a 2D surface using a regular polygon is exactly that one: using hexagons. In other words, hexagon is the regular 2D-stackable polygon that enclose more area with the smaller perimeter. Less perimeter, less wax used. It is really “that” simple (more here). So this is just the opposite situation: what polyhedron would you use if you needed to create a perimeter of channels loosing the less possible grip surface…

It isn’t dreadfully satisfying?

I don’t know about you, but me: know I feel like a concept in my mind has been correctly packet, stored, and fits perfectly in a previously empty hole. And it feels good. Thank you racional learning. And thank you for reading. See you in the next post, right? Take care!















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