Molecular dynamics analysis of the velocity slip of a water and methanol liquid mixture

Abstract

The effect of methanol mixing on a nanoscale water flow was examined by using nonequilibrium molecular dynamics simulations of a Couette-type flow between nonpolarized smooth solid surfaces. Water and methanol molecules were uniformly mixed in the bulk, whereas at the solid-liquid interface methanol molecules showed a tendency to be adsorbed on the solid surface. Similar to a macroscale Couette flow, the shear stress exerted on the solid surface was equal to the shear stress in the liquid, showing that the mechanical balance holds in nanoscale. In addition, the shear stress in the liquid bulk was equal to the viscous stress which is a product of viscosity and velocity gradient. When more methanol molecules were adsorbed on the solid surface, the friction coefficient (FC) between solid and liquid was largely reduced with a small amount of methanol and that led to a remarkable decrease of the shear stress. The cause of the FC reduction was investigated in terms of the local rotational diffusion coefficient (RDC) near the solid surface, and it was shown that different from an existing model, the FC and local RDC were not simply inversely proportional to each other in the mixture system because the solid-liquid interfacial state was more complex.