(173g) Spatially Distributed Fluid-Solid Friction in Confinement

Traditional hydrodynamics considers fluid-solid friction to reside at a sharp interface, incorporating this as a boundary condition via a slip length, or as a surface momentum accommodation coefficient, when modeling fluid flow in narrow pores and channels. Here we demonstrate, at the nanoscale, that the location of this slip boundary is ill-defined, since the range of fluid-solid interactions is of the same order of magnitude as the system size, and that instead, friction is distributed over the whole of the pore fluid. Using molecular dynamics simulations for the diffusion of several one-centre and multisite molecules in nanopores with an atomically detailed surface, we obtain position-dependent dynamical fluid-solid friction coefficients. These dynamical friction coefficient profiles are independent of the driving force, and only weakly dependent on fluid density. These are subsequently used in a modified hydrodynamic model, incorporating spatially distributed frictional momentum dissipation, to predict collective- and self-diffusion coefficients, which are found to be in very good agreement with those based on NEMD and EMD simulaton. We also determine theoretically a position-dependent static friction coefficient, based on equilibrium density distributions, which is shown to mimic the behaviour of the dynamical coefficient. These results suggest possibilities for tailoring nanomaterials and surfaces to engineer low friction pathways for fluid flow by appropriately tuning the fluid-solid potential energy landscape.