We have measured the hydrodynamic force between a particle (R ≈ 10 μm) and a smooth, flat plate using Atomic Force Microscopy in Newtonian, concentrated sucrose−water solutions for both hydrophilic solids (hydroxyl-terminated silica) and hydrophobic solids (methyl-terminated silica or graphite). For all cases, the measured force is consistent with Reynolds lubrication theory with the no-slip boundary condition and a constant viscosity. Our error in determining the slip length varies according to the particular experiment, but is about 2 nm. We have restricted our analysis to conditions where Reynolds lubrication is valid, i.e., films that are much greater than the molecular diameter of the fluid. Our experimental method contains two significant improvements over earlier work:  the use of much stiffer cantilever springs and the use of evanescent wave scattering as an independent check of the zero of separation. Our results are consistent with molecular dynamics simulations for thinner films and greater shear rates.

We describe measurement and interpretation of the force acting on a smooth hydrophilic glass particle during rapid (1-100 microm s(-1) approach to, and separation from, a hydrophilic glass plate in viscous concentrated aqueous sucrose solutions (0.001 Pa s

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.

By analysis of the thermally driven oscillation of an atomic force microscope ͑AFM͒ cantilever, we have measured both the damping and static forces acting on a sphere near a flat plate immersed in gas. By varying the proximity of the sphere to the plate, we can continuously vary the Knudsen number ͑Kn͒ at constant pressure, thereby accessing the slip flow, transition, and molecular regimes at a single pressure. We use measurements in the slip-flow regime to determine the combined slip length ͑on both sphere and plate͒ and the tangential momentum accommodation coefficient, . For ambient air at 1 atm between two methylated glass solids, the inverse damping is linear with separation and the combined slip length on both surfaces is 250 nmϮ 100 nm, which corresponds to = 0.77Ϯ 0.24. At small separations ͑KnϾ 0.4͒ the measured inverse damping is no longer linear with separation, and is observed to exhibit reasonable agreement with the Vinogradova formula.