SPREADING DYNAMICS OF AN ELECTRICALLY ACTUATED WATER DROPLET OVER A HYDROPHOBIC SURFACE
The spreading dynamics of a water droplet over a hydrophobic surface under the influence of an externally applied electric field is numerically investigated. The simulations have been carried out in an axisymmetric coordinate system using COMSOL Multiphysics® software. A laminar two-phase flow solver with a phase-field model for interface capturing based on the Cahn-Hilliard equation is used. The Cox dynamic contact angle model is formulated as the wetted wall boundary condition. The fluid motion is initiated by spreading at the three-phase contact line (TPCL) that is altered by forces due to the electric field distributed at the liquid-gas interface. A volumetric electrostatic force is evaluated from the Maxwell's stress tensor and introduced in the Navier-Stokes equations. The Maxwell's stress tensor, in turn, is altered by the drop deformation as well as the velocity distribution. Hence, calculations have been reported for a fully coupled approach wherein the Maxwell's stress tensor is iteratively updated with the instantaneous drop shape and the velocity field. The instantaneous shape, dynamic contact angle, and contact line velocity of the droplet are determined. The drop spreading trends are explained in terms of the distribution of electrostatic force field vectors at the interface, velocity vectors inside the droplet, wall pressure, and wall shear stress. The effect of friction over the contact line motion is investigated, and the oscillatory behavior of the contact radius is validated against experimental data available in the literature.