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On the non-equilibrium nature of the nanopore fluid

We examine the contributions to the shear stress from fluid–fluid and fluid–solid interaction in a confined fluid, using methane in a cylindrical silica nanopore as a model system. The shear stress arising from fluid–fluid interactions rises to a maximum in the vicinity of the pore wall, and falls steeply to zero over a finite distance beyond the location of the minimum of the fluid–solid interaction. A frictional stress arising from solid–fluid interactions correspondingly increases with increase in radial location in this region, and leads to a non-equilibrium behaviour. At low density, this frictional stress is accurately predicted by an extension of the Oscillator model, while at high density the shear stress approaches the uniform density limit of the momentum balance. At any radial location, the streaming velocities of molecules moving towards the wall and those moving away from it can be significantly different, confirming the non-equilibrium nature. Peaks in streaming velocity profiles are observed near the pore wall, demonstrating the non-Newtonian behaviour of the confined fluid. It is shown that the commonly used concept of slip length is ambiguous because of the arbitrariness in defining the position of the surface in the presence of a finite region of friction.