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Local friction law at a rubber/glass multicontact interface
When rough bodies are pressed together, contact only occurs at localized spots between surface asperities. Friction thus involves the shearing of a myriad of micro-contacts which are distributed over length scales ranging from micrometers down to nanometers. Although widely debated, the manner in which these micro-contacts locally dissipate energy remains obscure. As a prerequisite, one should know how frictional stresses are distributed within the contact interfaces. Unfortunately, most experiments only rely on measurements of friction force and of its dependence on load and velocity which are averaged quantities of local frictional properties We recently proposed a method to measure local friction of rubbers by means of a contact imaging approach [1-3]. Silicon rubber substrates marked on their surface are prepared in order to measure the displacement field induced by the steady state friction of a glass lens. Then, the deconvolution of this displacement field provides a spatially resolved measurement of the actual shear stress distribution at the contact interface. As a result, the local friction law, i.e. the relationship between the local shear stress and normal pressure, is obtained. In this study, this approach is applied to contact interfaces between a smooth silicon substrate and statistically rough glass lenses obtained by mechanical and/or chemical treatments. Experimental results are discussed in the light of recent theoretical contact models for the friction of rough surfaces.