Static and Dynamic Electrowetting of an Ionic Liquid in a Solid/Liquid/Liquid System

摘要

Abstract: A droplet of an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, bmim.BF4) is immersednin an immiscible liquid (n-hexadecane) and electrowetted on a flat Teflon AF1600-coated ITO electrode.nThe static contact angle decreases significantly when voltage is applied between the droplet and thenelectrode: from 145° down to 50° (with DC voltage) and 15° (with AC voltage). The electrowetting curvesn(contact angle versus voltage) are similar to the ones obtained in other solid/liquid/vapor and solid/liquidliquid systems: symmetric with respect to zero voltage and correctly described by Young-Lippmann equationnbelow saturation. The reversibility is excellent and contact angle hysteresis is minimal (∼2°). The step sizenused in applying the DC voltage and the polarity of the voltage are unimportant. The saturation contactnangle cannot be predicted with the simple zero-interfacial tension theory. Spreading (after applying a DCnvoltage) and retraction (after switching off the voltage) of the droplet is monitored. The base area of thendroplet varies exponentially during wetting (exponential saturation) and dewetting (exponential decay). Thencharacteristic time is 20 ms for spreading and 35 ms for retraction (such asymmetry is not observed withnwater glycerol mixtures of a similar viscosity). The spreading kinetics (dynamic contact angle versus contactnline speed) can be described by the hydrodynamic model (Voinov’s equation) for small contact angles andnby the molecular-kinetic model (Blake’s equation) for large contact angles. The role of viscous and molecularndissipation follows the scheme outlined by Brochard-Wyart and de Gennes.