Molecular dynamics simulations of friction forces between silica nanospheres

In this work, the interfacial friction forces between silica nanospheres are explored using molecular dynamics (MD) simulations through oblique impact. The effects of initial relative impact velocity, interaction path, contacting surfaces (relative orientation), impact angle (initial overlapping degree), and particle size on the interfacial forces are first studied. MD simulations demonstrate that the tangential friction forces depend sensitively on the impact velocity, interaction path and impact angle (initial overlapping degree), especially on the joint effect of interaction path and impact angle, but is less dependent on interacting surfaces and particle size. By increasing the initial overlapping degree, the relation between tangential friction forces and normal forces demonstrates rich and unusual behaviours in the approach and departure processes, ranging from hysteresis, incomplete anti-hysteresis, complete anti-hysteresis to negative friction coefficient, and then recovering to the usual hysteresis. Moreover, the classic Mindlin-Deresiewicz theory is still valid at small overlapping degrees, both qualitatively and quantitatively; with increasing overlapping degrees, the trend of rebounding prevails over relative sliding and the deviation from theory is weakened. The thermal vibration and dislocation of atoms during sliding and compression result in small fluctuations, especially in the departure process. This work offers atomic-scale insights into the friction characteristics between nanoparticles and consolidates our capability of modelling nanoparticle behaviours using advanced computing techniques, such as discrete element method in future.