On the friction and adhesion hysteresis between polymer brushes attached to curved surfaces: Rate and solvation effects

Computer simulations of friction between polymer brushes are usually simplified compared to real systems in terms of solvents and geometry. In most simulations, the solvent is only implicit with infinite compressibility and zero inertia. In addition, the model geometries are parallel walls rather than curved or rough as in reality. In this work, we study the effects of these approximations and more generally the relevance of solvation on dissipation in polymer-brush systems by comparing simulations based on different solvation schemes. We find that the rate dependence of the energy loss during the collision of brush-bearing asperities can be different for explicit and implicit solvent. Moreover, the non-Newtonian rate dependences differ noticeably between normal and transverse motion, i.e., between head-on and off-center asperity collisions. Lastly, when the two opposing brushes are made immiscible, the friction is dramatically reduced compared to an undersaturated miscible polymer-brush system, irrespective of the sliding direction.