Simulation of nano-scale flows by molecular dynamics methods

Abstract

The density variation and the driven flow of a model Uquid interacting via 12- 6 Lennard Jones potential in a nano-sized channel has been studied through molecular dynamics simulations. The channel is unbounded in x and y directions through the use of periodic boundary condition and in the z direction it is bounded by walls (lying in the x-y plane) on both sides and the separation between the walls is of the order of nanometers. Two types of wall have been studied - a smooth structureless wall and an atomic wall. The smooth wall is modelled by a Lennard Jones 10-4 potential. The atomic wall is composed of layers having 001 fee structure with wall atoms interacting with each other through the Lennard Jones 12-6 potential of strength six times that of the fluid-fluid interaction strength. Simulations are done for both static and moving wall atoms. In the first case, wall atoms are assumed to have infinite mass and therefore remain static at their original position throughout the simulation. In the second case, wall atoms have finite mass and are allowed to move about their mean position. The density variation across the channel is computed with the static wall. The simulated density profile is compared with Koplik & Banavar (1988) and found to match very well. A Poiseuille flow is simulated in the same system by imposing a constant body force along the x direction on each fluid atom. The flow is simulated over a range of the body force for which thermal equilibrium exists between the wall and the fluid system. The simulated profiles at different body force are fitted with Stokes profiles and viscosity values are estimated from the fitting parameters. The viscosity values obtained from the present simulation lie within the range estimated by KopUk k Banavar (1988). The shear viscosity of the bulk liquid is calculated independently by non-equihbrium molecular dynamics methods which yield viscosity values consistent with the estimates from fitting the velocity profiles.