Electrowetting of Ionic Liquids under High Vacuum Conditions

Abstract

Electrowetting of two imidazolium based ionic liquids, [C4mim][PF6] and [C6mim][NTf2], on different dielectric substrates, SiO2-Si and epoxide-Cu, has been studied at various temperatures and droplet radii R = 1–3 mm. To avoid the effect of adsorbed thin liquid films on the contact angle measurements and to resolve possible decomposition of the ionic liquids at high applied voltages we have performed the electrowetting experiments under ultra high vacuum conditions which is reported here for the first time. The electrowetting curves of both ionic liquids on both substrates exhibit a symmetrical, parabolic shape as a function of the applied dc-voltage. However, in comparison with aqueous and organic electrolytes saturation is reached at a lower variation of the contact angle. At saturation a clear decomposition of the ionic liquids sets in which is observed by the formation of large bubbles. Presumably, this is caused by electrical breakdown and the corresponding discharging. The decomposition products can strongly affect the interfacial tensions of the droplets what explains a pronounced contact angle hysteresis of ∼15°. The measured temperature dependence of the electrowetting curves is essentially determined by that of the liquid/vapor interfacial tension in accordance with the classical Young–Lippmann equation. Interpretation of the droplet size dependence of the electrowetting curves by the modified Young equation yields an apparent line tension of ∼10-5 N, which is comparable in magnitude to organic liquids.