Fluid flow inside slit-shaped nanopores: the role of surface morphology at the molecular scale

Non-equilibrium molecular dynamics (NEMD) simulations of fluid flow have highlighted the peculiarities of nanoscale flows compared to classical fluid mechanics. In particular, boundary conditions can deviate from the no-slip behavior at macroscopic scales due to various factors. In this context, we investigate the influence of surface morphology in slit-shaped nanopores on the fluid flow. We demonstrate that the surface morphology effectively controls the slip length, which approaches zero when the molecular structures of the pore wall and the fluid are matched. Using boundary-driven, energy-conserving NEMD simulations with a pump-like driving mechanism, we examine two types of pore walls--mimicking a crystalline and an amorphous material--that exhibit markedly different surface resistances to flow. The resulting flow velocity profiles are consistent with Hagen-Poiseuille theory for incompressible, Newtonian fluids when adjusted for surface slip. For the two pores, we observe partial slip and no-slip behavior, respectively, which correlate with fluid layering and depletion near the surfaces. However, the confinement of the fluid gives rise to an effective viscosity that varies substantially with the pore width. Analysis of the hydrodynamic permeability shows that the simulated flows are in the Darcy regime. Additionally, the thermal isolation of the flow causes a linear increase in fluid temperature along the flow, which we relate to strong viscous dissipation and heat convection, utilizing conservation laws of fluid mechanics. Our findings underscore the need for molecular-scale modeling to accurately capture the fluid dynamics near boundaries and in nanoporous materials, where macroscopic models may not be applicable.