Dissipation mechanisms and the crossover from Stokes to Coulomb friction: From toy models to disordered water

Many different mechanisms are responsible for the friction that counteracts the relative sliding motion of two solids in mechanical contact. They range from elastic instabilities via plastic deformation to the flow of non-Newtonian fluids, i.e., boundary lubricants, to name a few. Most mechanisms have in common that they can be characterized as instabilities of certain (collective) degrees of freedom: an external force pushes part of the system to a maximum of the potential energy barrier, from where it quickly slides downhill into the next valley without giving the gained kinetic energy back to the solids' center-of-mass motion. In this contribution, I demonstrate that the velocity dependence of an isolated friction mechanism appears universal, i.e., Eyring like, at first sight. However, even a simple model like Prandtl's model exhibits a rate dependence of friction obeying the Carreau Yasuda equation (CYE). The latter applies to complex fluids like highly viscous polystyrene or camel blood. Particular attention is paid to the origin of the inaccuracy of the CYE at very small shear rates. Original data for the simulated shear-rate dependence of the viscosity of amorphous and liquid TIP-4P water will also be scrutinized.