Modifying the Boundary Conditions of Lubricant Flow

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0119626Granick This work will provide a critical experimental assessment of the"stick" boundary condition of continuum hydrodynamics. The stick assumption is presently one of the cornerstones of calculation regarding fluid flow in tribology yet recent theories and experiments raise the exciting possibility that surfaces can be engineering to produce the boundary condition of partial slip. The approach will consist of testing the classical Reynolds Equation of continuum hydrodynamics for cases where systematic prior experiments do not yet exist: partially-wetted surfaces (finite contact angle of the fluid on the surface) and variable surface roughness (the roughness will be topographical, chemical, and lubrication additives). For measurement, a modified surface forces apparatus will be operated at spacings large enough (10-200 nm) that intervening Newtonian fluids are believed to behave as a continuum. Nanometer-level oscillatory modulations of film spacing will be performed such that the amplitude and frequency of modulation are varied independently, allowing the mean velocity to vary over a wide range without large change of the film thickness. First, we will put to direct test recent theoretical predictions and molecular dynamics simulations that predict breakdown of stick boundary conditions when low-viscosity fluids fail to wet completely a smooth solid boundary. Secondly, we will extend this work to" patchy" surfaces - surfaces that are chemically and geometrically rough - to test theoretical predictions that" stick" boundary conditions of fluid flow stem from the presence of roughness. Third, we will extend this work to "hairy" surfaces - surfaces that bear surface-attached polymer chains. This is the situation that prevails in oil lubrication owing to the presence of polymer additives. It is the situation that prevails in biolubrication owing to the presence of surface-attached biomacromolecules. The engineering impact will be to point the way to new ways by which energy dissipation can be controlled during lubricant flow. The main applications will pertain not so much to the high-load conditions characteristic of heavy machines, as to the low-load conditions characteristic of emerging nanotechnologies and micromachines.***