Molecular Analysis on the Dynamic Properties of Water Droplet at Solid-Liquid Interface Based on MD Simulations

Molecular dynamics simulations of single water or water-IPA (Isopropyl-alcohol) mixture droplets on a solid surface were performed. The interaction for solid-water molecules was modeled as the combination of L-J and Coulomb potentials, and the effects of Coulomb interaction were independently investigated by appending arbitrary electric charge as a parameter on the solid surface. The water droplet became more wettable both with positive and negative surface charges as the absolute value of the electric charge increased, and the cosine of the contact angle was roughly a linear function of the absolute value of the electric charge although the correlation was obviously different between positive and negative charge values. Multiple molecular orientations seemed possible as local equilibrium states near the adsorption layer on negatively charged surface, and the mean rotational diffusion was higher there. On the other hand, mean rotational diffusion was reduced in the vicinity of the positively charged surface.