Shear of polymer brushes

We have measured the normal, lateral and hydrodynamic forces that act between polymer-brush-bearing surfaces immersed in a good-solvent medium, both when stationary and when in motion with respect to each other. At low sliding velocities we find a very marked decrease in the frictional force between the surfaces, an effect interpreted in terms of a fluid interfacial layer resulting from the non-interpenetration of the opposing brush layers even when they are compressed against each other. In a different study we measured the lubrication forces between the brush-bearing surfaces, due to hydrodynamic interactions resulting from normal mutual motion. These reveal that, when the surfaces are far apart, the fluid flowing past them penetrates only very slightly into the brush layers. At high compressions, fluid is forced to flow through the brushes; the magnitude of the hydrodynamic forces may then be compared with predictions based on a model for fluid flow through porous media. This is used to discriminate between scaling and mean-field exponents of the composition-dependent hydrodynamic screening length in the semidilute polymer brushes, and shows the results to be much closer to those expected using scaling exponents.