Modeling moving contact lines during drop spreading over a textured substrate

Contact line movement during spreading of low viscosity liquid drops has been addressed in the literature using a contact angle model that connects the interfacial slope with the contact line velocity. Here, we present such a contact angle model that is general enough to hold for a variety of surfaces, starting from non-hysteretic superhydrophobic all the way to highly sticky, hysteretic hydrophobic surfaces. The surface-related characteristics of spreading, such as friction and pinning appear as parameters in the contact angle model. The model is simulated along with the conservation equations to yield drop spreading details such as the instantaneous drop height, footprint and timescales, along with possible recoil and capillary waves are derived. Further, these details are compared with experimental data recorded from high-speed imaging experiments where the match is seen to be quite good. The contact angle model is then tested against experiments related to electrowetting over a dielectric and simulations are shown to be in good agreement with experiments. In a second application, the proposed contact angle model is tested via simulations for drop spreading over a surface with microscale pillar arrays. The match against experiments reported in the literature is shown to be quite good. Simulations show that the equilibrium drop shape as well as Cassie-Wenzel transitions depend on the spreading dynamics and cannot be specified in advance. The success of the model is related to correctly resolving multiple surface waves propagating over the gas-liquid interface. It being extended to account for droplet impact on textured surfaces.