Instantaneous fundamental modes and contact angles of droplets from surface atoms

Abstract

Contact angle measurements of sessile or moving droplets are by far the most common way to characterize their wetting properties. The typical route to obtain contact angle estimates from atomistic molecular dynamics simulations requires the calculation of an isochore or the equimolar dividing surface, both of which need sizeable statistics to achieve acceptable accuracy and are less suitable in non-stationary conditions. Here, we propose an algorithm for estimating the contact angle that relies on the identification of interfacial molecules, which can determine the instantaneous location of the liquid surface. We apply this algorithm to calculate the contact angles of water droplets at equilibrium and out of equilibrium on graphite-like substrates, paying particular attention to modeling the presence of excited modes using general ellipses to fit the droplet surface. The algorithm is implemented in a user-friendly way in the Pytim software package.