| Solid friction tends to change with velocity rather slowly, i.e., less than linearly with velocity. It has long been recognized that the origin for this deviation from linear response theory must be due to microscopic hystereses or instabilities. In particular Prandtl showed more than 80 years ago in a simple albeit generic model system, why Coulomb friction is indicative for a system to be out of thermal equilibrium and how thermal noise reduces friction. The precise nature of the instabilities for specific systems is however not always clear. In my talk I will review some of the advances made in identifying relevant mechanisms from chemical bond breaking to instabilities in boundary lubricants. The linear response regime will be discussed as well. As a model system, the interaction of a cell on a thin elastic sheet will be considered. If time permits, the elasticity-mediated interaction between cells will also be discussed, in particular, how the transition from a flat sheet to a wrinkled sheet affects substrate-mediated interactions and the local tribology. |
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