Methane transport through distorted nanochannels: surface roughness beats tortuosity

MARCOS FEDERICO CASTEZ; EMILIO WINOGRAD; MARIANO MARTÍN; VERONICA M. SANCHEZ

CABA

Conferencia; 27th International Conference on Statitical Physics; 2019

Universidad Católica Argentina

In contrast with macroscopic systems, where the interaction between molecules dominates the flow behaviour, as the size of the system decreases, molecules-surface interactions become a competing mechanism. Down at the nanoscale, flow rates of several orders of magnitude higher than those predicted by the continuum analytical equations (Hagen-Poiseuille, Knudsen diffusion, etc) have been reported. It has been recognized that a main driver of this effect is associated with the smoothness of the cylindrical carbon nanotubes. Extrapolations of these findings to nanoporous media, such as a shale rock matrix, it is not straightforward, due to the distortions of the porous structure, connectivity and mineral composition. In this work we focus on modelling this kind of effects on flow rates considering nanochannels that include tortuosity and surface roughness. We apply molecular dynamics to simulate the flow of methane through organic nanopores of varying geometrical properties. We implement two types of tortuosities, including smooth and abrupt curvature variations, and considering roughness at different length-scales. Molecular-level flows are compared to its continuous counterpart by means of the Lattice-Boltzmann method. Our results indicate that the flow rate not only decreases as the tortuosity increases but, in addition, curvature variations also impact strongly in the flow rate. However, in all considered cases, it is surface roughness which more severely impacts on the flow rates and its properties, becoming a first-order effect which, when present, highly determines the flow rates, reducing the expected enhancement relative to the continuum theory.