(757f) Estimating Interfacial and Line Tensions Between Water, Vapor and Solid Substrates

An accurate estimation of the interfacial properties of water adjacent to solid substrates is crucial in characterizing wetting in important contexts such as superhydrophobicity, electrowetting, and heterogeneous nucleation. When a liquid droplet makes contact with a solid surface, its characteristic shape and the macroscopic properties such as the contact angle (CA) and the contact angle hysteresis (CAH) depend on the solid-liquid and liquid-vapor interfacial tensions and the three-phase line-tension. The conventional methods of measuring CA, which involve simulation of cylindrical or spherical droplets to obtain equilibrium density profiles, are often sensitive to the droplet size and somewhat arbitrary choice of the location of three-phase contact line. The interface potential method provides an elegant way to overcome these challenges, and accurately determine CA. Here we illustrate the use of the INdirect Umbrella Sampling (INDUS) method to facilitate calculation of the interface potential using molecular dynamics simulations. We use INDUS to characterize the surface tension and CA of water on a range of substrates with a range of attractions. While the estimation of CA through various methods results in relatively small discrepancies, there is an astounding lack of consensus in estimates of the three-phase contact line tension. While most simulations and experiments predict a negative value of line tension of water on hydrophobic substrates, estimates can vary by several orders of magnitude. Furthermore, thermodynamic stability requires that analogous to surface tension, line tension must always be positive. Moreover, there is a lack of molecular level understanding of line-tension, and consequently of the phenomena that depend on it such as contact line pinning, CAH and even heterogeneous nucleation. Here, we suggest ways to characterize and interpret this elusive three phase contact line-tension of water with various substrates.