Electrowetting of ionic liquids on teflon AF1600 in ambient hexadecane

Abstract

A droplet of ionic liquid is immersed in an immiscible liquid (n-hexadecane) and electrowetted on a flat electrode insulated with Teflon AF1600. Two series of ionic liquids were studied: 1-alkyl-3-methylimidazolium tetrafluoroborates (Rmim.BF4) and 1-butyl-3-methylimidazolium with various anions (bmim.X). When voltage is applied between the droplet and the electrode, the static contact angle decreases from about 150 down to 50 (with DC voltage) and 15 (with AC voltage). The electrowetting curves (contact angle vs. voltage) are symmetric with respect to zero voltage and obey the Young-Lippmann equation below saturation. Reversibility is excellent and contact angle hysteresis is minimal (2). The saturation contact angle cannot be predicted with the zero-interfacial tension theory most probably due to the presence of a hexadecane film between the droplet and the solid surface. Spreading of the ionic liquid (under DC voltage) is about twice as fast as its retraction (when voltage is switched off). The base area of the droplet varies exponentially during wetting (exponential saturation) and dewetting (exponential decay). The characteristic time is related to the viscosity of the ionic liquid. The spreading kinetics (dynamic contact angle vs. contact line speed) can be described by the hydrodynamic model (Voinovs equation) for small contact angles and by the molecularkinetic model (Blakes equation) for large contact angles. The role of viscous and molecular dissipation follows the scheme outlined by Brochard-Wyart and de Gennes.