Mechanics of Adhesion Through Nanolayers of Liquid

The adhesion of a fine probe to a smooth oxide surface covered with a few layers of liquid molecules is considered by molecular modelling and also by experiment. A modified DL_POLY 2.19 computer code was applied to a magnesium oxide pyramidal probe approaching a plane MgO surface, calculating the equilibrium attractive force as a function of the gap between the surfaces. For clean MgO, there was a jump to contact and strong adhesion force. Detachment of the MgO probe could not be achieved and plastic flow of the probe was observed during pull-off. Contamination of the crystal surfaces by other molecules showed two major changes in the mechanism of adhesion. Firstly, the adhesion was much reduced as indicated by the obscuration of the jump to contact and the ease of detachment of the contaminated probe. Secondly, the contaminant molecules formed ordered layers on the MgO surfaces and each layer had to be squeezed out in a stepwise motion. The final layer could not be removed by normal pressure. Experiments using atomic force microscopy showed that these steps could not be detected in water because of the small size of the molecule and the compliance of the probe. But experiments with larger molecules such as polyacrylate and nanoparticles like gold did reveal periodic attractions and repulsions supporting the layering theory.