A New Mechanism for Lubrication in Liquid Crystals
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A NEW MECHANISM FOR LUBRICATION IN LIQUID CRYSTALS Robijn F. Bruinsma* and Cyrus R. Saf'mya Materials and Physics Departments, and the Materials Research laboratory, University of California, Santa Barbara, CA 93106. ABSTRACT Recent synchrotron X-ray scattering studies on the thermotropic liquid crystal lubricant 8CB under shear flow have shown that for high shear rates the smectic layers are oriented perpendicular to the orientation assumed by conventional solid layered lubricants. This result invalidates existing theories of the lubrication mechanism of these materials. We show that the new orientation is the result of flow deformation of thermal fluctuations. This same mechanism is found to create a "normal stress" lift-force which we propose as the new lubrication mechanism.
Lubricants permit metal surfaces to slide over each other without excessive friction or material wear. During this technologically important process, the lubricating liquid is forced to undergo shear flow. A good lubricant (i) must have a low viscosity for a low friction coefficient and (ii) must produce a large lift-force to prevent contact of the surfaces and wear [1]. In a normal liquid these two criteria are difficult to achieve since the large lift force requires a large viscosity [2]. The behavior of relatively simply liquids, like water, is for modest shear rates well described by the classical ("Newtonian") theory of hydrodynamics [2]. However, many liquid lubricants have a more complex interior structure which can be significantly distorted by shear flow. At the macroscopic level, this distortion manifests itself as a breakdown of Newtonian hydrodynamics requiring the introduction of new stresses in the hydrodynamic equations (so called "normal stress" effects). This normal stress plays a central role in the mechanics of the lubrication process since it can produce a strong lift-force on the boundaries. An important group of lubricants involve layered materials such as graphite, block-copolymers, and surfactant membrane materials. We will focus on smectic A (SmA) liquid crystals [3], which are ordered stacks of liquid layers (Fig.1). They have been shown [4] to Mat. Res. Soc. Symp. Proc. Vol. 290. @1993 Materials Research Society
be powerful lubricants and are used in fine-mechanical applications. They have unusually low friction coefficents which depend much less on load and velocity then conventional oils. To explain the lift-force in thermotropic smectics [5], one assumes that under shear flow smectics assume an orientation with the layers sliding over each other (the "c" orientation with the layer normal along the flow gradient direction, see Fig.1), as is the case for solid layered lubricants such as graphite. Since a smectic is solid-like along the normal to the layers, structural defects must be created for the boundary surfaces to reduce their spacing so we would have a natural explanation for the lift- force. Liquid layered lubricants could, in principle, also assume the "a" orientation with the layer normal along the n
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