Boundary lubrication:  Dynamics of squeeze-out

Abstract

The dynamics of the expulsion of the last liquid monolayer of molecules confined between two surfaces (measured recently for the first time) has been analyzed by solving the two-dimensional Navier-Stokes equation combined with kinetic Monte Carlo simulations. Instabilities in the boundary line of the expelled film were observed. We show that the instabilities produce a rough boundary for all length scales above a critical value and a smooth boundary for shorter lengths. The squeezing out of all but a few trapped islands of liquid is shown to be the result of the pressure gradient in the contact area.

Two-dimensional observation of drainage and layering transitions in confined liquids

“…In our experiments, u c : 5 mm. Recent Monte Carlo simulations showed that the length scale of the boundary line can be reproduced provided that the line energy is incorporated into the model appropriately [6]. The simulations also showed that the increasing roughness eventually leads to the detachment of liquid pockets from the moving boundary line, which become trapped inside the contact area.…”

“…The roughening of the boundary line can be explained in a similar fashion. Beyond a critical length u c , the boundary line is unstable with respect to deformations [14,6]. Any local perturbation of the boundary line produces segments that are closer to the edge of the contact area than others.…”

“…Recently, we presented the first experimental investigation of this process for a single component molecular liquid [4]. In collaboration with Persson et al, we showed that most aspects of the layering transition can be described in terms of a simple two-dimensional hydrodynamic model [5,6].…”

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

“…The elimination of each layer starts with the nucleation of a 2D void, which progressively grows by ejecting atoms into the low pressure regions. 7,[9][10][11]12 We have shown earlier that the void formation is a thermally activated process, which, for 2D liquid-like layers, can be described by concepts borrowed from classical nucleation theory. 9 In many practical situations the nucleation of the layering transition may occur at some ''weak'' point between the surfaces where imperfections, e.g., foreign adsorbates ͑like water or some organic contamination͒, may locally reduce the spreading pressure.…”

Effects of self-affine surface roughness on the friction coefficient of rubbers in the presence of a liquid interlayer

“…Similar effects have been observed for organic liquid films between mica surfaces. 8,9 Under certain conditions thin and uniformly thick fluid films remain trapped at the rubber/substrate interface. [10][11][12] The latter occurs when electrical charge is introduced upon contact of the surfaces leading to their electrical repulsion.…”