A Robust Algorithm for Contact Angle and Interface Detection of Water and Argon Droplets

Contact angle estimation of a liquid droplet on a solid surface is of great importance to understand its wetting properties, capillarity, and surface interaction energy. While studying the droplet on a surface using molecular dynamics (MD) simulations, the main challenge lies in separating the liquid and vapor phases, and estimating the interface, curvature, and contact angle. Further, during transient MD simulations, large amount of data processing is required for calculating the time evolution of contact angle. This is a tedious effort to do manually or with image processing algorithms and can affect the contact angle results. In this work we propose an improved version of contact angle estimation algorithm. This algorithm segregates the droplet molecules from the vapor molecules using Mahalanobis distance technique, following which the density is smeared onto a 2D grid using various interpolation functions. The smearing is done by rotating the droplet multiple times at different angles to attain more interfacial data points. The liquid-vapor interface data is estimated from the grid using density filtering. With the interface data, a circle is accurately fitted using Landau method. The equation of this circle is solved for obtaining the contact angle. We apply this algorithm to study the hydrophilic and hydrophobic contact angles for argon and water droplets on a platinum surface with varying interaction potential parameters leading to different wettability conditions, as well as contact angle evolution of water droplet on platinum surface.