Surface friction of polymer gels

The sliding friction of various kinds of hydrogels on solid surfaces or on gels has been studied and it was found that the frictional behaviors of the hydrogels do not conform to Amonton's law F = muW, which well describes the friction of solids. The frictional force and its dependences on the load are quite different depending on the chemical structures of the gels, surface properties of the opposing surfaces, and the measurement condition. Additionally, the frictional coefficient la of the gel reaches a value as low as similar to 10(-3) which is much lower than those observed in solid materials. Theoretical analysis on the mechanism of the gel friction shows that the gel friction can be explained in terms of interfacial interaction, either attractive or repulsive, between the polymer chain and the solid surface. In the repulsive case, the friction is described in terms of the hydrodynamic lubrication of the solvent layer at the interface, which is formed due to the electrostatic repulsion of the two surfaces. For the attractive case, in addition to the hydrodynamic friction, the force to detach the adsorbing chain from the substrate appears as friction. The theoretical results coincide well with experimental observations that confirm the essential feature of the model. (C) 2001 Elsevier Science Ltd. All rights reserved.