PNNL

Abstract

Hydrodynamics has been built on a continuum field structure to provide a macroscopic description of fluid motion. Many fundamental questions arise when it is applied to confined flows, particularly at the nanoscale, where spatially varying molecular fluctuations lead to the breakdown of conventional hydrodynamics and the ``no-slip'' boundary condition. To gain physical insights into the breakdown of conventional hydrodynamics, we devise a simple but informative model system where we can isolate the important coupling of fluid-wall and bulk fluid interactions on the resulting slip length in the boundary condition as a function of fluid composition and confinement. By combining equilibrium molecular dynamics (MD) with non-equilibrium MD simulations of both Couette and Poiseuille flow, our results demonstrate both consistency between equilibrium and non-equilibrium molecular dynamics for different flows and confinement, demonstrating the robust nature of linear response theory. Furthermore, we demonstrate that the computed molecular informed boundary conditions can be incorporated into the continuum description of the hydrodynamic lubrication force. These results will facilitate the use of molecular-based simulations in the exploration of complex confined systems in nanofluidics, biology, and colloidal science where continuum hydrodynamics is known to break down. This work is supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Chemical Sciences, Geosciences, and Biosciences Division, Chemical Physics and Interfacial Sciences Program, FWP 16249.