Bubble raft model for indentation with adhesion

Abstract

Indentation hardness tests are now widely used to measure the mechanical properties of solid surfaces^1–3. Recent developments of this technique^4,5 permit the analysis of the outermost 10 nm of materials. Experimental and theoretical questions arise regarding the physical and mechanical processes involved in such small indentations. We describe here an indentation experiment on a microscopic scale, using soap bubbles blown onto a water surface. Bubble rafts provide a simple two-dimensional model for indentation behaviour; as for other materials, their behaviour is governed by two principal attraction–repulsion forces⁶, and by geometrical constraints. A crystalline two-dimensional lattice is obtained by using bubbles of uniform size^7–9, whereas bubbles of two sizes give an amorphous structure^10,11. Indentation can be represented by the contact between a triangular crystalline raft and a rectangular crystalline raft bordered by an amorphous layer. The flow of the materials, which is dependent on both adhesion and the force between the two rafts, can be analysed during the experiment.