Effects of viscous heating and wall-fluid interaction energy on rate-dependent slip behavior of simple fluids.

Molecular dynamics simulations are used to investigate the rate and temperature dependence of the slip length in thin liquid films confined by smooth, thermal substrates. In our setup, the heat generated in a force-driven flow is removed by the thermostat applied on several wall layers away from liquid-solid interfaces. We found that for both high and low wall-fluid interaction (WFI) energies, the temperature of the fluid phase rises significantly as the shear rate increases. Surprisingly, with increasing shear rate, the slip length approaches a constant value from above for high WFI energies and from below for low WFI energies. The two distinct trends of the rate-dependent slip length are rationalized by examining S(G_{1}), the height of the main peak of the in-plane structure factor of the first fluid layer (FFL) together with D_{WF}, which is the average distance between the wall and FFL. The results of numerical simulations demonstrate that reduced values of the structure factor, S(G_{1}), correlate with the enhanced slip, while smaller distances D_{WF} indicate that fluid atoms penetrate deeper into the surface potential leading to larger friction and smaller slip. Interestingly, at the lowest WFI energy, the combined effect of the increase of S(G_{1}) and decrease of D_{WF} with increasing shear rate results in a dramatic reduction of the slip length.