Wettability effect on nanoconfined water flow

Significance The flow of water confined in nanopores is significantly different from that of bulk water. Moreover, understanding and controlling the flow of the confined water remains an open question, especially concerning whether the flow capacity of the confined water increases or not compared with that of bulk water. Here, combining a theoretical analysis and data from molecular dynamics simulations and experiments in the literature we develop a simple model for the flow of water confined in nanopores. We find that a contact angle and a nanopore dimension may substantially affect the confined water flow. We also quantitatively explain a controversy over an increase or decrease in flow capacity. Understanding and controlling the flow of water confined in nanopores has tremendous implications in theoretical studies and industrial applications. Here, we propose a simple model for the confined water flow based on the concept of effective slip, which is a linear sum of true slip, depending on a contact angle, and apparent slip, caused by a spatial variation of the confined water viscosity as a function of wettability as well as the nanopore dimension. Results from this model show that the flow capacity of confined water is 10−1∼107 times that calculated by the no-slip Hagen–Poiseuille equation for nanopores with various contact angles and dimensions, in agreement with the majority of 53 different study cases from the literature. This work further sheds light on a controversy over an increase or decrease in flow capacity from molecular dynamics simulations and experiments.