The micro-meniscus effect of a thin liquid film on the static friction of rough surface contact

Abstract

A numerical method has been developed to account for the micro-meniscus effect of an ultra-thin liquid film on the static friction of rough surface contact. The classical meniscus theory of a single-sphere contact was modified to include the effect of multiple asperity contacts with a pre-existing ultra-thin liquid film during the contact of two rough surfaces. The effect of micro-meniscus formation was considered in terms of elastic - plastic deformation of rough surfaces under applied load, roughness parameters, thickness of liquid film and meniscus height. In the numerical model, elastic - plastic dry contact of rough surfaces is first analysed. In the next step, a liquid film of known mean thickness is introduced over the deformed rough surfaces. Wetted areas are determined by selecting the areas where asperities of both contacting surfaces touch the liquid. The total projected meniscus area is determined by selecting those area islands of cross cut area at a given mean meniscus height which overlap the wetted area. The meniscus force is then calculated using the meniscus equation derived from first principles of the micro-meniscus theory. Two parameters, meniscus height and liquid film thickness, are required for the analysis, which can be obtained for a given relative humidity. However, for a given liquid-film thickness, meniscus height is a function of liquid-film thickness and needs to be estimated. The present numerical method was applied to data-processing magnetic rigid disk drives to analyse the static friction during contact in the presence of an ultra-thin liquid film on one of the surfaces. Trends in numerical predictions correlate well with the measured effect of liquid films on the static friction. The analysis reveals some new insights into the `stiction' problems of disk drives.