| Abstract
| - The hydrodynamic force between a spherical glass particle (radius ∼ 10 μm) and a smooth, flat glass plate in Newtonian silicone oil (viscosity, η ∼ 95 mPa s) was measured using the atomic force microscopy (AFM) colloidal probe technique and was compared to Reynolds lubrication theory. When the particle and plate were coated with a hydrophobic silane, the measured forces were consistent with Reynolds lubrication theory without the need to introduce the concept of a slip length. When the particle was hydrophilic, the results were more variable, sometimes being consistent with the no-slip boundary condition and sometimes being better fitted by invoking a constant slip length (up to 33 nm). The hydrophilic system was not well characterized because the hydrophilic solid may have entrained or attracted a layer of water (η = 0.001 Pa s) of unknown thickness, which would lubricate the flow and explain the apparent slip length. In addition, all AFM force measurements suffer from the problem that the solids occasionally have rough patches, steps, or attached nanoparticles, which will affect the force and may cause departure from a theory that does not account for their presence. We conclude that the no-slip boundary condition is valid for solid−liquid interfaces but that some experiments entail conditions, for example, surface roughness or lubricating layers, that may give the appearance of violating the no-slip boundary condition.
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