Shear viscosity calculation of water in nanochannel: molecular dynamics simulation

Shear viscosity is one of the important transport properties which affects different phenomena in nanoconfined water. This study aims to investigate the effect of sub-Angstrom variations of nanochannel size on the shear viscosity of water confined in a silicon wall by employing equilibrium molecular dynamics (EMD) simulations. Simulation results demonstrate that water molecules confined in the slits are layered and for channels width less than 21 A, the number of layers varies from one to six. We show that if the capillary size becomes less than 18.5 A, the sub-Angstrom variations significantly affect the layered structure of the confined water. This causes the anomalous behavior of water viscosity and therefore, the flow resistance of nanoconfined water. According to the previous studies, the shear viscosity is greatly enhanced for subnanometer capillaries so that the shear viscosity increases dramatically by decreasing the channel size; however, we found that shear viscosity obeys an oscillatory behavior and has a complicated behavior which originates from the consistency between the channel size and the space required to embed one layer of water molecules. Results show that five minima and four maxima values for the viscosity are observed for channels width less than 18.5 A. Such unfamiliar behavior of viscosity and, consequently, the flow resistance, friction coefficient and slip length should be taken into account in investigation and design of such nanoconfined water.