Contact-angle hysteresis in solid-on-solid wetting

The spreading of an elastic adhesive sphere on a substrate is calculated using continuum elasticity theory and the van der Waals interaction between the solid surfaces. The deformation and contact area are obtained self-consistently as a function of lend. Hysteresis is demonstrated between the loading and the unloading cycles: the receding contact angle exceeds the advancing one. In addition, for a given deformation (flattening), or fur a given applied load, the receding contact area exceeds the advancing one. This nonequilibrium phenomenon is velocity dependent and in accord with experiments on adhesion and crack propagation.