Drops, contact lines, and electrowetting

In this work, we study the behaviour of drops and contact lines under the influence of electric fields, and how these can answer fundamental and industrial questions. Our focus is on studying the varying balance of the electric field, hysteresis forces and inertia as the speed of a contact line changes. We considered improvements in various applications: windscreen drying, lab-on-a-chip devices, and immersion lithography. We find that AC electrowetting can be used to mobilize drops stuck to a surface. The resulting decrease in pinning for fast drops is described by the decrease of the contact angle hysteresis. We next study drops trapping on electric defects, which serve as model systems for trapping on heterogeneities. Moreover, they can be used as a tool for controlling drop motion. We show the use of a capillary force sensor as a tool to measure the electric force exerted by these defects. We then find the critical trapping condition for drops moving over them at high velocities. We find that water drops move faster than glycerol/water drops, as expected. Glycerol/water drops are trapped when the driving and defect force are balanced, while water drops require a higher defect force. Using a harmonic oscillator model, we find that glycerol drops are overdamped, while water drops are underdamped. Using the same defects, we also studied drops driven by air-jets. For glycerol/water we find a relation between air-jet velocity and critical voltage. We also study drops sliding over electrode gaps which are inclined with respect to the direction of motion. We can steer one drop in one direction, and the next in another by switching the applied voltage. Finally, we examined contact lines at high velocities. We discover that the electric field creates a new regime where the advancing contact angle increases much more rapidly with velocity. We show that this is due to the interplay of an electrically-induced oscillation and the average motion of the contact line. We also find that, for the same velocity, thin drops will have an increase in advancing contact angle that is much larger than for thick drops