Sticking Together. Steven Abbott

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.

3.1 LESSONS FROM THE GECKO

      That little tale conceals a rather advanced master class in adhesion science. Let's review it from a human perspective:

       You now know that everything is attracted to everything else with a rather modest force named after van der Waals.

       This van der Waals (vdW) force varies rather little between materials and gives us, for materials we are likely to encounter, surface energy values in the range of ∼20 mJ m−2 for “low surface energy” materials like silicones or Teflon through ∼30 for plastics like polythene, though ∼40 for many other plastics and for glass, then up to ∼50 for real-world “metal” surfaces which are generally covered with some sort of oxide. This immediately tells us that for the sort of strong adhesion most of us need, surface energy is irrelevant. If the whole range of practical materials changes in surface energy by a factor of 2.5, it cannot possibly explain how adhesion can vary by factors of 1000 between different surfaces and adhesives.

       If (and this is a big if) you can get near-perfect contact between two surfaces, then the effective adhesion against a simple pulling force can be rather large. The classic example is when two beautifully machined metal surfaces are put into contact. They are impossible to pull apart, but rather easy to slide apart. Their adhesion, measured in one manner, is large, yet their general purpose practical adhesion is small. You wouldn't stick an aircraft together via pure surface contact!

       If (and this is a big if) you create two beautifully machined metal surfaces out in space where there is no oxygen to corrode the surfaces and if you can push them together then you don't just get strong adhesion, you get a solid piece of metal. Because metals are ductile, slight mismatches between the surfaces will flow out. The atoms that met across the surface have no way to know that they were from different surfaces, so the result is a perfect piece of metal. If, back on Earth, you can smash two pieces of metal together with sufficient force to push the contaminated surfaces out of the way, leaving pure metal-to-metal contact, then you have a “cold weld” which, in theory, is indistinguishable from a pure piece of metal. Although we have merely created pure surface contact, I think it is fair to say that we now have metal-to-metal bonds because there is a continuous electronic structure. This is not the same as having pure surface contact between molecules (such as polymers), where there is no continuous electronic structure.

       If (and this is a big if) you can place a drop of sodium hydroxide solution onto one super-flat piece of glass then place another piece in near-perfect contact, the solution etches the surfaces of both pieces of glass, gradually forming a gel which then solidifies. This is Hydroxide Catalyzed Bonding and is wonderful if you have the time and patience to practice the technique in a super-clean environment and need, for example, to send some Gravity Probe optics into space. Is this gecko-style? Yes, because it's a trick for producing perfect contact between two smooth surfaces, no because it is using something a bit like an adhesive to do it.

       The gecko gains its high levels of surface contact via its multi-level compliance: legs, toes, lamellae, setae, spatulas. Human attempts to achieve gecko-style adhesion are usually far less compliant (they might have 2 or 3 levels, not 5); they are therefore far less capable of dealing with the real world.

       This pure surface adhesion is very easily disrupted by the most humble piece of dirt. The gecko's first step onto the glass was not successful – the foot had brought up bits of dirt from the wall it had just climbed. The gecko has no magic for getting good adhesion between its foot and a smooth surface when there is a small bit of dust in the way. The magic is in what happened when the gecko tried again. The adhesion of the dirt to the glass turns out to be greater than the adhesion to the foot; lifting the foot and trying again provided a self-cleaning action, allowing good adhesion on the next attempt.

       Although it is relatively easy to measure vdW forces and surface energies it happens to be very difficult to take these numbers and calculate the load that a gecko spatula or toe could hold. There are many logical steps involved and they will not be covered in this book.

       Finally, gecko adhesion is easily disrupted by a little flick of its heel. That flick induces a crack at the interface. Once that crack starts, there is nothing much to stop it, so the foot comes away easily. The key message in this book is that Adhesion is a Property of the System. If the system is “gecko upside down on glass” then it will stick there till the end of time (yes, dead geckos stick as well as live ones, it has been tested). If the system is “gecko creating a crack along the interface” then the adhesion is very small.

      It turns out that geckos cannot climb on Teflon, though they have no problem doing so on wet Teflon. There are competing explanations for this, with my favourite being that the tips of the spatulas become more compliant when wet, allowing the little extra adhesion necessary for climbing. The chemical acronym for Teflon is PTFE, with the ‘F’ indicating to us that it is a “fluoro” type of polymer. We shall see later that the easily removed “silicone release paper” on adhesive tapes cannot be replaced by “fluoro release paper”, even though the two polymer types give us the same low surface energy; so low adhesion comes about via other mechanisms that we shall discuss later. I have not found any data of tests of geckos on silicone release surfaces but I suspect that they will have trouble climbing them, even when wet.

      While we are at it, it is worth noting that the same applies for adhesion as well as non-adhesion: silicone adhesives are often wonderful, but there aren't many fluoro adhesives. Our “non-stick” frying pans, on the other hand, are covered with fluoropolymers. Again, whether things stick or not clearly depend on many factors other than surface energy.

      One last thing about walking on glass like a gecko. I once had to give a lecture