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

TL;DR

This study uses molecular dynamics simulations to explore how surface morphology influences fluid slip and flow behavior in slit-shaped nanopores, revealing the importance of molecular-scale effects on nanoscale fluid dynamics.

Contribution

It demonstrates that surface morphology controls slip length in nanopores, showing different flow regimes for crystalline and amorphous walls, and highlights the limitations of macroscopic models at the nanoscale.

Findings

Surface morphology affects slip length and flow regimes.

Flow profiles align with Poiseuille theory when adjusted for slip.

Effective viscosity decreases with confinement and flow conditions.

Abstract

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. For fluid flow in slit-shaped nanopores, we demonstrate that surface morphology provides an efficient control on the slip length, which approaches zero when matching the molecular structures of the pore wall and the fluid. 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 Poiseuille theory for incompressible, Newtonian fluids when adjusted for surface slip. For the two pores, we observe partial slip and no-slip behavior, respectively. The hydrodynamic permeability corroborates that the simulated flows are in the Darcy regime. However, the confinement of the fluid gives rise to an effective viscosity below its bulk value; wide pores exhibit a crossover between boundary and bulk-like flows. 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. Noting that the investigated fluid model does not form droplets, our findings challenge the universality of previously reported correlations between slippage, solvophobicity, and a depletion zone. Furthermore, they 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.