Modifying lubrication in micro-patterned and polymer-grafted soft interfaces

Soft contacts are encountered in a variety of natural and technological applications. Two common strategies to modify interfacial friction include the deliberate micropatterning of surfaces and the grafting of charged polymer brushes onto the surfaces. Nevertheless, the connection between macroscopic friction and surface energies on textured substrates is not well understood. We synthesize model poly(dimethylsiloxane) (PDMS) substrates with microtextures and polyzwitterionic brushes, and use them as tribopairs to investigate the effect on elastohydrodynamic (EHL) and boundary lubrication (BL). Our results indicate that patterned surfaces exhibit micro-EHL to EHL transitions in which the critical friction coefficient and Sommerfeld numbers vary as a function of the surface geometry. The experimental data show excellent agreement with a simple scaling theory we developed from Reynold's equations for 1D lubrication flows. Furthermore, we investigate the BL changes in PDMS tribopairs in which betainized poly(2-dimethylamino ethyl methacrylate) (PDMAEMA) brushes are covalently grafted onto the surface. Our preliminary results suggest that the polymer relaxation physics may play an important role in the lubricating efficiency of the PDMAEMA brushes.