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We describe molecular dynamics simulations of fluid films confined between two solid walls. The films consist of spherical molecules, or flexible linear chains with up to twenty monomers. When the wall separation is only a few molecular diameters, crystalline or glassy order is induced across the film. The onset of the glassy phase is characterized by rapidly increasing relaxation times. These manifest themselves through changes in the diffusion constant and in the response to shear. The viscosity exhibits the same power-law scaling with shear rate that was observed in recent experiments. Our study suggests that this response is a universal property of lubricants near a glass transition. PACS numbers: 68. l 5.+e, 62. 15.+i, 64.70.Pt', 8 l.40.Pq When fluids are confined between solid walls separated by only a few molecular diameters, their structural and dynamical properties are drastically altered [1-5]. Experiments reveal wall-induced layering of fluid films at wall separations of up to -20 molecular diameters [1,2]. At smaller separations, the viscosity increases by several orders of magnitude [3,4]. In many cases the film develops a yield stress, indicating a transition to a solidlike state [3][4][5]. When sheared slowly, these solidlike films may exhibit oscillatory stick-slip motion [5].Computer simulations of films with simple spherical molecules have played a pivotal role in understanding the origin of these phenomena [6-10]. In particular, they reveal substantial in-plane ordering of molecules [7-9] in addition to the layering noted above [6,7]. This in-plane order plays an essential role in transmitting shear stress [7]. The entire film may crystallize when the wall separation is less than -5-6 molecular spacings, leading to a finite yield stress [8,9]. Stresses greater than this value destroy crystalline order. Stick-slip motion involves periodic shear melting and recrystallization of the film [9].In spite of their successes, simulations with spherical molecules have been unable to reproduce several key features of the experimental data. For example, the calculated relaxation times and viscosities remain near bulk fluid values until the film crystallizes.In experiments, both quantities may increase by many orders of magnitude before a yield stress is observed [3][4][5]. Moreover, recent work [11]indicates that there may be a power-law distribution of relaxation times with a universal exponent.In suSciently thin films and at high shear rates, the measured viscosity decreases with shear rate as y ' -independent of the molecular composition of the film.In this paper, we examine whether these phenomena can be attributed to intramolecular dynamics. %e describe molecular dynamics simulations of films composed of freely jointed, linear-chain molecules confined between two solid walls. As observed in studies of spherical molecules [7-10], confinement decreases the entropy of these films and shifts the bulk phase transitions to higher temperatures and lower pressures. However, films of chain molecules undergo a ...

Molecular dynamics simulations of atomically thin, fluid films confined between two solid plates are described. For a broad range of parameters, a generic stick-slip motion is observed, consistent with the results of recent boundary lubrication experiments. Static plates induce crystalline order in the film. Stick-slip motion involves periodic shear-melting transitions and recrystllization of the film. Uniform motion occurs at high velocities where the film no longer has time to order. These results indicate that the origin of stick-slip motion is thermodynamic instability of the sliding state, rather than a dynamic instability as usually assumed.

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