Sticking Together. Steven Abbott

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       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.