Phenomenology of squeezing and sliding of molecularly thin Xe, CH4 and C16H34 lubrication films between smooth and rough curved solid surfaces with long-range elasticity

Abstract

The properties of Xe, CH 4 and C 16 H 34 lubricant confined between two approaching solids are investigated by a model that accounts for the curvature and elastic properties of the solid surfaces. We consider both smooth surfaces, and surfaces with short-scale roughness. In most cases we observe well defined molecular layers develop in the lubricant film when the width of the film is of the order of a few atomic diameters, but in some cases atomic scale roughness inhibit the formation of these layers, and the lubricant exhibit liquid-like properties. An external squeezing-pressure induces discontinuous, thermally activated changes in the number n of lubricant layers. We observe that the layering transition tends to nucleate in disordered or imperfect regions in the lubrication film. We also present and discuss results of sliding dynamics for Xe and C 16 H 34 lubrication films.

Squeezing molecular thin alkane lubrication films between curved solid surfaces with long-range elasticity: Layering transitions and wear

“…As can be observed in this figure, the pressure is negative in the begining of the squeezing. This behavior has been observed earlier, 14 and it is due to capillary bridge attraction as the lubricants are in a high-mobility, fluidlike state at this stage. During the remaining part of the squeezing, the lubricant behaves more solidlike and pressure build up precedes the squeeze out of the monolayers.…”

“…Furthermore, computer simulations have shown that molecularly thin layers of branched hydrocarbons give rise to more disordered structures than linear chains, 23,24 and that the pressure to nucleate squeeze-out decreases when disorder occurs in the lubricant film. 14 We will look into this aspect of boundary lubrication in the near future.…”

Probing the properties of confined liquids

“…Other non-equilibrium MD simulations showed that the forces due to layering can be so high that it can elastically deform the solid surfaces [81] and thus that the elasticity of the walls needs to be taken into account to quantitatively reproduce the experimental results. In view of the SFA squeeze-out experiments [6][7][8] also MD simulations have been performed to study the drainage and layering transitions upon confinement [82,83]. In agreement with the experimental observation, this study showed that the increased pressure during approach induces a nucleation of a hole of n-1 layers, which grows and results in the expulsion of the liquid.…”

The effect of surface roughness on the adhesion of solid surfaces for systems with and without liquid lubricant

“…21,22, and 24, but we review it briefly here. We are concerned with the properties of a lubricant film squeezed between the curved surfaces of two elastic solids.…”

“…22 we have studied the adhesion force, and the nϪ1→n layering transition during pull-off, for elastic solids with smooth surfaces. In the present paper, we focus on the influence of surface roughness ͑on two length scales͒ and thin lubricant or contamination films, on the adhesion between two surfaces.…”