This chapter addresses the issue of how to decouple friction forces from normal forces by focusing on the problem of superlubricity and the problem of controlling the boundary condition for hydrodynamic flow when fluid moves past a solid surface. Force, normalized by the mean radius of curvature of mica cylinders, is plotted against surface separation. As the mica sheets are squeezed together, squalane drains smoothly until oscillatory forces of alternating attraction and repulsion are detected at thickness ≈4 nm. This reflects the tendency of squalane to form layers parallel to the solids-the application of pressure causes the fluid to drain in discrete steps corresponding to the squeezing out of the successive layers. The liquid could ultimately be squeezed to ≈0.9 nm-twice the thickness of the chain backbone. Slip need not necessarily be predicated on having surfaces coated with self-assembled monolayers to render them partially wetted. The no-slip boundary condition switches to partial slip when the fluid contains a small amount of adsorbing surfactant.
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