The Friction between Coated Surfaces

Dominic Tildesley

Director CECAM, Lausanne Switzerland

The lubrication of surfaces coated with polymers is an important technological problem and one that is difficult to solve effectively using non-equilibrium molecular dynamics with atomistic models. The timescale of the simulation is such that shearing rates that can be achieved are many orders of magnitude higher than those observed in experiment. This problem can be addressed in part by switching to a mesoscale model, which still preserves the important elements such as the shape and charge of the adsorbed polymers but allows for a longer time step.

In this talk we describe the development of dissipative particle dynamics as an important technique for modelling polymers and surfactants in ionic solvents. In particular we compare the behaviour of charged and neutral polymers of the same length. We will demonstrate that it is possible include chain entanglement and charge in the DPD model without losing the advantages of the mesoscale.  

The compression-dependence of the friction coefficients of neutral and charged polymer brushes at the same surface coverage is studied using dissipative particle dynamics (DPD) at two different shear rates. The chemical potential of the solvent particles is kept constant along the compression curve in order to mimic the experimental conditions. We conclude that the kinetic friction coefficient between charged brushes is higher than that of neutral brushes at low compressions and smaller at high compressions. These differences are small. We also show that it is possible to simulate ultra-low friction coefficients comparable with experiments using the smallest shear rate accessible in a mesoscale simulation. The unexpected behavior of the shear deformation-induced structural heterogeneities in charged polymer brushes calls for further experiments to elucidate this local reorganization of the ions in adsorbed charged polymers.