Simulations of Nanometer-Thick Lubricating Films
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MRS BULLETIN/MAY 1993
changes, illustrating them with results from our simulations of thin films of spherical and short-chain molecules.1M8 We begin by examining the effect of an isolated solid wall on the structure and dynamics of an adjoining fluid. Substantial layering and in-plane order are induced within a few to ten molecular diameters of the solid. The degree of in-plane order is found to correlate directly with the effective boundary condition for fluid flow.15 Deviations from the commonly used no-slip boundary condition become substantial at the molecular scale. We next consider the effects of two confining walls on a fluid film. As the film thickness h decreases, the ordering influences of the two walls begin to overlap and interfere. This leads to oscillations in the energy, normal force, and effective viscosity with h.2Ai0 As h decreases further, the entire film may undergo a phase tran-
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Figure 1. Sketch of our simulation geometry which mimics the SFA. Solid walls are held together by a constant normal pressure P j . The top wall is pulled by a spring connected to a stage moving at constant velocity v. Periodic boundary conditions are imposed in the x and y directions.
sition to a crystalline or glassy state. The onset of the glassy phase is characterized by rapidly increasing relaxation times, which result in reduced diffusion constants and enhanced viscosities. The viscosity develops universal power-law scaling with shear rate.'18 Crystalline or glassy films resist small shear stresses. However, when their yield stress is exceeded, they begin to shear in an uneven manner. Rather than sliding smoothly, the confining walls alternately stick together and slip past each other. "Stick-slip" motion of this sort is a common occurrence in both dry and lubricated contacts,26 but has only recently been studied at molecular scales.4'516 '" It accounts for many of the sounds heard in our daily lives, from the squeak of hinges to the music of violins, and leads to greatly enhanced wear. We show that transitions between sticking and sliding are accompanied by phase transitions in thin lubricant films. If the molecules are spherical, static plates induce crystalline order in the film. Stick-slip motion involves periodic shearmelting and recrystallization of the film. For chain-molecules there is a periodic transition between glassy and fluid states. Stick-slip in these systems does not result from a velocity-weakening frictional force like that assumed in many models.26 Instead it reflects the thermodynamic instability of a shearing solid state. The next section provides a brief description of our simulation techniques. An excellent review of molecular dynamics techniques can be found in Allen and Tildesley's book,27 and specific details of our work are given in References 16-18. The following sections consider the changes in behavior near a single wall and in thin films, respectively. Simulation Method and Geometry To mimic the geometry of the SFA, a
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