Sticking Like a Gecko

It has become fashionable to talk about gecko adhesives. By finding out how the gecko manages to climb up smooth windows we discover that most of the time we don't want gecko-style adhesion – it is far too weak and unreliable. After all, the gecko needs to move, so strong adhesion is the last thing it wants. This type of adhesion is OK for wrapping sandwiches and a number of other temporary adhesion tasks, but not good for much else. The gecko's trick is to maximize contact between surfaces; we can achieve the same thing by using a liquid to get perfect contact then turning it into a solid – a basic type of glue. This can provide strong adhesion if measured one way, but shows weak adhesion when challenged differently, so these adhesives aren't too reliable. Along the way we dispel two common myths: that surface energy is important for strong adhesion and that roughening a surface gives stronger adhesion. By the end of this chapter we are almost ready to discuss how to get strong adhesion, but first we need to know how to measure whether our adhesion really is strong.