Adhesion of Viscoelastic Spherical Solids

Abstract

The development of micro-probe instruments, such as the Atomic Force Microscope, for the measurement of surface energy and other material properties on the micro/nano scale has led to the need for continuum mechanics models of the contact of a spherically tipped probe with the surface of a specimen in the presence of adhesive forces. In the case where the tip and specimen are perfectly elastic solids, such models are well developed through the elastic fracture mechanics principle of equating the strain energy release rate to the work of adhesion. The process of increasing and decreasing the contact load is then fully reversible. When the specimen material is viscoelastic, e.g. polymer or rubber, the situation is complicated by viscoelastic dissipation. The apparent work of adhesion becomes rate dependent and irreversible: more energy is required to separate adhering surfaces than is returned when they come together, an effect known as ‘adhesion hysteresis’. The paper will outline progress in extending the elastic analysis to the viscoelastic situation by modelling the material as a linear viscoelastic solid.