Extraction of the solid-liquid friction coefficient between a water-methanol liquid mixture and a non-polar solid crystal surface by Green-Kubo equations

Abstract

Using equilibrium molecular dynamics (EMD), we analyzed the friction between water-methanol liquid mixtures and a non-polar solid wall. Specifically, we calculated the friction coefficient (FC) from the autocorrelation of the shear force exerted on a liquid from a solid wall using two Green-Kubo (GK) equations proposed by Bocquet and Barrat (BB) [Physical Review E, Vol. 49, (1994), 3079], and Huang and Szulfarska (HS) [Physical Review E, Vol. 89 (2014), 032119], and compared these FC values with those obtained in our previous non-equilibrium molecular dynamics (NEMD) work. Both GK equations reproduced the FC dependence on the methanol concentration in the first adsorption layer XaMeOH observed with NEMD, but the BB method gave a better estimate of the NEMD results with a difference of 20 % at largest. The independent molecular FCs of water and methanol, which were only extractable with the HS method from EMD simulations, corresponded qualitatively well with the NEMD results regarding the dependency on XaMeOH. Though certain discrepancies were observed between the EMD and NEMD results, the EMD had a considerable advantage regarding the computational time required to calculate the FC with a comparable error: the EMD computational time was about 20 times shorter than the NEMD time in the present system.