Published: Jan 2008
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THIN FILM LUBRICATION (TFL) DEALS WITH THE lubrication region wherein the film gap is of the order of nano metres or molecular scale, i.e., the clearance is usually between several nanometres to several tens of nanometres [1–3]. To date, it is clear that TFL is distinctive enough to be qualified as a separate lubrication regime (for instance, Refs. [4,5]), leaving many mysteries to be unveiled. In literature, some researchers regarded that the continuum mechanic ceases to be valid to describe the lubrication behavior when clearance decreases down to such a limit. Reasons cited for the inadequacy of continuum methods applied to the lubrication confined between two solid walls in relative motion are that the problem is so complex that any theoretical approach is doomed to failure, and that the film is so thin, being inherently of molecular scale, that modeling the material as a continuum ceases to be valid. Due to the molecular orientation, the lubricant has an underlying microstructure. They turned to molecular dynamic simulation for help, from which macroscopic flow equations are drawn. This is also validated through molecular dynamic simulation by Hu et al. [6,7] and Mark et al. . To date, experimental research had “got a little too far forward on its skis;” however, theoretical approaches have not had such rosy prospects as the experimental ones have. Theoretical modeling of the lubrication features associated with TFL is then urgently necessary. As revealed through experimental works, however, the flow of lubricants in TFL provides a hint that the macroscopic properties, such as the viscosity and the elastic modulus remain to be a measurement of the fluid characteristics. In addition, the transition from EHL to TFL is inherently progressive, wherein no abrupt transform in lubrication states are found. Thus, the continuum theory is validated to some extent. Furthermore, one can arrives at a continuum viewpoint, but in a different way from conventional fluid mechanics, by considering the material to be a continuum one in an ensemble averaged, rather than a spatial averaged, sense.
Beijing Jiaotong University, Beijing,