Interfacial cavitation nuclei studied by scanning probe microscopy techniques

Abstract

The new microscopy techniques for studying solid surfaces, scanning tunneling microscopy (STM) and atomic force microscopy (AFM), also offer possibilities of studying gaseous nano-voids at solid-water interfaces, i.e. cavitation nuclei. The use of STM presupposes that both the surface studied and that of the STM-tip allow electrons to be transferred to/from the location of tunneling. To detect surface nanovoids by STM it is therefore necessary that when the submerged STMtip during scanning of a surface meets a void, the tunneling barrier is smaller along the cavity surface than if the tip moves on along the drained solid surface below the void. Likewise, the use of AFM for void detection presupposes that the liquid-gas interface of a void can supply a detectable force on the AFMtip. Otherwise, the tip will ignore the void, and only the solid surface below it will be detected. With both techniques it has proved possible to meet the demands for detection of surface nano-voids, and today their existence is well established. However, the results obtained depend on the technique of microscopy chosen, and on how it is applied, which makes the evaluation of such measurements difficult. Therefore, an analysis of the physics related to void detection by the scanning probe microscopy (SPM) techniques is important. The present paper presents this physics on the basis of experimental results obtained with SPM techniques since the early 1990 s.