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CHAPTER 3

Sticking Like a Gecko

Congratulations! You have just been promoted to chief science officer of the geckos. And you have been presented with geckodom's biggest challenge yet. How are you going to climb up all those smooth glass structures that humans have created? You know that the smooth surface gives you nothing with which to grip. What are you going to do?

Some of your fellow geckos suggest that you create a glue gun for your lizard feet, but you immediately reject this idea. “The problem with glue isn't how we are going to stick, but how we can unstick ourselves. A glue will be too good – we'll hang there, unable to move”. Translating this into scientific language you say: “What we want is the world's worst adhesive – just strong enough to hold a gecko, yet weak enough to be easily broken when we need to take another step”.

Alone in your well-equipped lab, you look with annoyance at some dust on your equipment. It is everywhere, even on the sides. This gives you an idea. You get out your atomic force microscope (AFM) and attach a bit of dust to the end of the probe ready to measure the force between the dust particle and the surface. What you want to do is push the particle onto the surface and measure the force needed to pull it off, yet to your surprise, as the particle gets close to the surface it jumps into contact (Figure 3.1). It really wants to be on that surface!

Figure 3.1 A bit of dust on the end of an AFM tip is pulled into contact with the surface. This is a surface energy effect.

At first you think this is because the particle is statically charged, but even when you carefully neutralize the static with a deionizer, the particle still jumps into contact. You try many different types of dust and many different types of surfaces and you find that the attractive force does not change much. It seems to be a general property of all materials that they attract each other with a small force. [This general attraction is called the van der Waals force after the scientist who first characterized it properly.]

As a good scientist you want to give this force a value. The force on its own doesn't tell you much because it depends on the area of contact. To make the value universal you decide to look at it in terms of the amount of energy in Joules, J, needed to separate one square metre of surface, and call it surface energy in units of J m⁻². For all surfaces you can find this varies from a low of 0.02 to a high of 0.06, with most being around 0.04 J m⁻² or, for convenience, 40 mJ m⁻².

When you then do the surprisingly complicated calculations about how much of a gecko's weight could be supported if you only had those surface energies, you find a clear answer. If the whole area of your four feet was in contact with the smooth glass, you would never move again – it could support 100 kg. You therefore need only a modest fraction of your feet to be in perfect contact. Examining your feet you realize that they have a complex, multi-level design that allows lots of contact (Figure 3.2). For the simpler feet found on most other animals, the total contact area with smooth glass would be so small that there is no hope of holding on. The problem is that for surface adhesion you need “contact” and this means being within 1 nm of the surface. Any “normal” foot is rough to at least the 1 μm level and usually the 1 mm level, so the total area in contact with the glass would be far too small to provide grip.

Figure 3.2 The hierarchal structure of a gecko foot giving compliance to the surface at every scale from mm down to the sub-nm structures visible in the 1 µm view.

But your feet seem amazingly well designed. You have toes that can make sure that each pad of the foot gets close to the glass, then you have lamellae (5 mm image) on your toe pads that can adjust into broad, good contact, then you have setae (50 µm image) on the lamellae that can adjust into fine scale contact and then the setae have spatulas (1 µm image) that come into intimate nano contact. Indeed, the spatulas are remarkably like the cantilevers used in AFMs to allow nice, controlled contact with any surface. You realize that your whole system is compliant to the surface – able to accommodate to its ups and downs at every relevant scale.

You are a scientist, you have done your calculations; it is time to make your announcement. “My fellow geckos, tomorrow I will show how geckodom can conquer the glass buildings of mankind. Meet me at the local shopping mall and you will be amazed”.

The next day your fellow geckos assemble and you clamber up some easy wall to reach the glass. With confidence in the laws of physics you put one foot onto the glass and expect to feel a solid grip. You test it and a moment of panic arrives – there was almost no adhesion. You try again and to your surprise there is a good, solid adhesion. You try the next foot – your first attempt is a near disaster, then things are fine. You confidently step on with all four feet. The cheers from below are starting to fade because you hit another moment of panic. You can't move. Your calculations warned you that you might have too much adhesion and here you are, totally stuck. To buy some time you make a speech about one giant step for geckokind and then a cramp in one foot creates a muscle spasm – and your foot is immediately freed. You vaguely recall a lecture on fracture mechanics and make a wild guess that somehow sudden motion could overcome adhesion. You place your newly-freed foot a little higher on the glass and then try a “flick” motion with your ankle on the next foot. It comes free easily and you scuttle to the top of the glass wall then, with a gasp from the geckos below, you even walk upside down on the glass canopy.