Sticky solids

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Abstract
A solid could not exist if its atoms did not stick together. Stickiness is therefore an essential property of solids. Knowing this, there is a problem explaining why solids do not instantly adhere when touched together, just as two liquid droplets fuse together immediately on contact. Another problem arises when a good joint between solids is broken, because such a joint requires much more energy to break than the atomic bond energies would suggest. These observations are paradoxical; some joints between solids seem far more fragile than expected whereas others are much tougher than theory predicts. The purpose of this paper is to recount and rationalize such difficulties, precisely the same difficulties appreciated by Newton three hundred years ago. Repeating Newton's experiment but with rubber spheres, allows fresh observations to be made and leads to a new theory of sticky solids. This theory provides a baseline for the understanding of some curious and still unexplained adhesion phenomena. The ideas can also be applied to practical problems such as failure of adhesive joints, sticking of emulsion paint, adhesion of dirt, setting of cement, and the sintering and agglomeration of powders.

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