Investigation on Electrocapillary and Wetting Phenomena with Level Set Method

Abstract

Contact angle hysteresis is important in many natural and engineering wetting processes, but predicting it numerically is difficult. A numerical code to simulate contact line problems considering contact angle hysteresis and dynamic contact angle is developed including a new algorithm for the numerical implementation of contact angle hysteresis. This algorithm employs feedback control of contact angle which decelerates contact line speed to make the contact line stationary in the hysteretic range of contact angle. The algorithm requires embedding only a simple additional routine with little modification of a code which considers the dynamic contact angle. The method is non-iterative and explicit, and also has less computational load than other algorithms. OpenFOAM, an open-source computational fluid dynamics code, is adopted as a computational platform. OpenFOAM provides various standard solvers and is advantageous for modifying existing code or adding new algorithms. Conservative level set method is embedded into the OpenFOAM to capture the moving interface of the incompressible two-phase flow without gain or loss of mass. For a drop hanging on a wire, the developed code accurately predicts the theoretical equilibrium contact angle. For the drop impacting on a dry surface, the results of the developed code agree well with experimental results including the intermittent occurrence of the pinning of contact line. The proposed algorithm for the contact angle hysteresis is as accurate as other algorithms used in previous work, but faster. The developed code is applied to the analysis of electrowetting-driven spreading of a drop in dc electrowetting, oscillation and jumping of a drop in ac electrowetting, and capillary ratchet by ac electrowetting. The developed code is expected to be further utilized to understand various wetting-based phenomena and to optimize wetting-based devices.