Evolution of adhesion force behavior at the silica-HOPG interface from humidity-independent to humidity-dependent revealed on an AFM

Abstract

ABSTRACT The influence of relative humidity (RH) on adhesion force requires clarification. Adhesion forces between atomic force microscopy (AFM) silica cantilevers and highly oriented pyrolytic graphite (HOPG) were measured to study the influence. The force behavior can evolve from RH-independent to RH-dependent with repeated contacts. Initially, the force is relatively small and RH-independent without a large water bridge. However, it becomes RH-dependent after some time at a high RH and never changes again. The RH-dependent force generally exhibits a stable-increasing-stable behavior with RH due to thin-film flow. With a continually increasing-stable-decreasing RH, the RH-dependent force can usually keep pace with RH, indicating that the adsorption and desorption of water molecules are sensitive to RH. However, the force behavior can show a certain lag and advance (including a sudden change). The reasons were manifold: (1) the water bridge can suddenly form and disappear, (2) the equilibrium time for a film-vapor interface is long at high RHs, and (3) the Laplace pressure can change with a constant volume. The RH-dependent force usually keeps pace with a suddenly changed RH, indicating the fast growth process of a water bridge at a high desired RH. The results may enhance the understanding of adhesion mechanisms.