Molecular dynamics simulations of shear-induced thermophoresis and non-Newtonian flow in compressible fluids

We use molecular dynamics simulations to study the behavior of a compressible Lennard-Jones fluid in simple shear flow in a two-dimensional nanochannel. The system is equilibrated in the fluid phase close to the triple point at which gas, liquid and solid phases coexist and is subjected to steady shear in Couette geometry. It is observed that at higher shear rates, the system develops a density gradient perpendicular to the direction of flow and exhibits solid-like layering near the boundaries. Both the number of solid-like layers and the number of layers that move with the velocity of the neighboring wall, increase with the shear rate. We argue that the inhomogeneous density profile develops as the consequence of thermophoresis due to the non-uniform temperature profile produced by shear-induced viscous heating in the simulated flow cell. The above phenomena are accompanied by non-Newtonian effects such as nonlinear velocity profiles, inhomogeneous stress distributions and shear rate dependent viscosity which exhibits shear thinning followed by shear thickening as the shear rate is increased. The connection between these phenomena is discussed.