On the Role of Fluid-Solid Interaction Strength in Anomalous Fluid Diffusion under Nanoscale Confinement

The transport properties of a nanoconfined fluid can differ significantly as compared to that same fluid under bulk conditions. Previous studies have reported that under certain circumstances, the diffusivity of a nanoconfined fluid can be higher than its bulk counterpart. In this work, we discuss the relationship between the diffusivity of nanoconfined water and the magnitude of interactions between water and its confining solid. We perform extensive molecular-dynamics (MD) simulations in which we vary the energy scale of interactions between water and solid, as well as the aspect ratio and size of the system. We show (following meticulous assessment of associated uncertainties) that the diffusivity of confined water can be a remarkably non-monotonic function of the water-solid interaction strength, with extremal values that differ considerably from the bulk diffusivity. To rationalize this finding, we also present results on direction-decomposed diffusivity. This work highlights a subtle finite-size effect that can appear in MD simulations of nanoconfined fluids, and also underscores the importance of careful uncertainty quantification in such studies.