The validity of the continuum modeling limit in a single pore flows to the molecular scale

J Al Hossain and B Kim, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 25, 24919-24929 (2023).

DOI: 10.1039/d3cp02488a

The discrete characteristics of molecules become dominant in the molecular regime when the surface-to-volume ratio becomes very high. Using the well-established continuum approach is questionable due to this dominant behavior. Due to the lack of perfect modeling of such a small-scale system, the experimentalist must rely on the trial and error method. Here we analyze the water transport mechanism through a nanoporous graphene membrane at the molecular level by adopting the classical molecular dynamics (MD) simulation. The results for SPC/E water molecules were compared with those obtained for liquid argon atoms and continuum Sampson's equation predictions. We find that the effect of local variants such as density layering, interatomic forces, slip velocity, and geometric boundary conditions become exponentially dominant with decreasing nanopore size. Consequently, the continuum assumptions break down at 1.5 nm pore diameter due to neglecting the dominant local properties. Flows through the nanopore can be modeled using the continuum approach if the pore diameter exceeds 1.5 nm. Short- range van der Waals forces become dominant within the continuum limit while the long-range Coulombic force contribution is negligible.

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