AmericanChemicalSociety.com

The problem of the moving contact line between two immiscible fluids on a smooth surface was considered using molecular dynamics (MD) and continuum simulations. In MD simulations a finite slip is allowed by choosing incommensurate wall-fluid densities and weak wall-fluid interaction energies. The shear stresses and velocity fields are extracted carefully in the bulk fluid region as well as near the moving contact line. In agreement with previous studies, we found slowly decaying partial slip region away from the contact line. In steady-state shear flows we extract the friction coefficient along the liquid-solid interface, the local slip length, and the dynamic contact angle. The MD results show that both dynamic contact angle and slip velocity near the contact line increase with increasing the capillary number (Ca). Also, at high Ca the break up of fluid-fluid interface is observed. The slip boundary conditions near the moving contact line extracted from MD simulations were then used in the continuum solution of the Navier-Stokes equation in the same geometry to reproduce velocity profiles and the shape of the fluid-fluid interface. Anoosheh Niavarani presented these results in the 2010 APS March Meeting in Portland, OR. These results were included in the last chapter of her Ph.D. thesis. The Ph.D. defense date is set at the end of October 2010.

In a related project, molecular dynamics and continuum simulations were carried out to investigate the influence of shear rate and surface roughness on slip flow of a Newtonian fluid. For weak wall-fluid interaction energy, the nonlinear shear-rate dependence of the intrinsic slip length in the flow over an atomically flat surface was computed by MD simulations. We described laminar flow away from a curved boundary by means of the effective slip length defined with respect to the mean height of the surface roughness. Both the magnitude of the effective slip length and the slope of its rate-dependence were significantly reduced in the presence of periodic surface roughness.  We then numerically solved the Navier-Stokes equation for the flow over the rough surface using the rate-dependent intrinsic slip length as a local boundary condition. Continuum simulations reproduced the behavior of the effective slip length obtained from MD simulations at low shear rates. The slight discrepancy between MD and continuum results at high shear rates was explained by examination of the local velocity profiles and the pressure distribution along the wavy surface. We found that in the region where the curved boundary faces the mainstream flow, the local slip is suppressed due to the increase in pressure. The results of the comparative analysis [Phys. Rev. E 81, 011606 (2010)] can potentially lead to the development of an efficient algorithm for modeling rate-dependent slip flows over rough surfaces.

In addition, we examined the shear response of nanoscale lubricant films using non-equilibrium MD simulations. It was found that at low shear rates the velocity profiles are curved near the wall due to the formation of a highly viscous interfacial layer and the slip length (a measure of slippage at the interface) is negative and almost rate-independent. With increasing shear rate, the gradual transition to steady slip flow is associated with the reduction of the fluid viscosity near the wall. We found that in a wide range of fluid densities, the friction coefficient undergoes a universal transition from a constant value to the power law decay as a function of the slip velocity. The numerical analysis indicates that the rate behavior of the slip length correlates well with the inverse relaxation time of the polymer chains in the interfacial layer. The paper [Phys. Rev. E 80, 031608 (2009)] was featured both in the October 5, 2009 issue of Virtual Journal of Nanoscale Science and Technology and in the October 1, 2009 issue of Virtual Journal of Biological Physics Research.

Finally, the molecular mechanism of slip at the interface between polymer melts and weakly attractive smooth surfaces was investigated using MD simulations. In agreement with our previous studies on slip flow of shear-thinning fluids, it was shown that the slip length passes through a local minimum at low shear rates and then increases rapidly at higher shear rates.  We found that at sufficiently high shear rates, the slip flow over atomically flat crystalline surfaces is anisotropic. It is demonstrated numerically that the friction coefficient at the liquid-solid interface (the ratio of viscosity and slip length) undergoes a transition from a constant value to the power-law decay as a function of the slip velocity. The characteristic velocity of the transition correlates well with the diffusion velocity of fluid monomers in the first fluid layer near the solid wall at equilibrium.  We also show that in the linear regime, the friction coefficient is well described by a function of a single variable, which is a product of the magnitude of surface-induced peak in the structure factor and the contact density of the adjacent fluid layer.  The universal relationship between the friction coefficient and induced fluid structure holds for a number of material parameters of the interface: fluid density, chain length, wall-fluid interaction energy, wall density, lattice type and orientation, thermal or solid walls. The manuscript is currently under review in Physical Review E but the preprint is available on cond-mat archive.