A molecular dynamics study of non-Newtonian flows of simple fluids in confined and unconfined geometries

Various fluid flow phenomena originate in the dynamics of the atoms that constitute the fluid. Studying fluids as a collection of atoms is key to a better understanding of, for example, non-Newtonian fluid flow behavior. Molecular dynamics (MD) is a very suitable tool for the study of fluids on the atomic level. Many MD studies have been devoted to the behavior of homogeneous, unconfined fluids under either simple shear or extensional flows, while a combination of both flow types has not been studied extensively. We use MD simulations and analysis tools for: (1) the study of various properties of a simple homogeneous bulk fluid under several planar velocity fields, (2) the calculation of stresses and viscosity using the transient-time correlation function and, (3) the study of properties of an inhomogeneous fluid confined in a nanochannel. The data suggest that the pressure tensor for a homogeneous, simple, monoatomic fluid under any planar flow field can be expressed in a unified form as a combination of equilibrium properties and non-Newtonian phenomena, such as: strain thinning viscosity, viscoelastic lagging, pressure dilatancy and out-of-flow plane anisotropy. We found consistent trends for these non-Newtonian quantities. Also, interesting trends have been found for equilibrium material properties as a function of density. It is often not possible to directly compare experimental data to results from steady non-equilibrium molecular dynamics (NEMD) simulations. Calculating accurate time-averaged values from these simulations is usually only feasible at deformation rates that are much larger than those accessible in experiments. We have shown that the transient-time correlation function provides a more efficient alternative to direct time-averaging of NEMD data. Furthermore, Non-Newtonian stresses have been studied for a simple monoatomic fluid confined in a nanochannel, where the properties vary across the channel. Data for various densities, temperatures and body forces have provided insight in the dependencies of various quantities.