Enhanced and Reversible Contact Angle Modulation of Ionic Liquids in Oil and under AC Electric Field

Abstract

Liquid actuation and manipulation using electrowetting is a rapidly growing field of research and has generated considerable interest in developing technologies such as microfluidic devices, liquid optics and displays. Electrowetting is conventionally carried out with (saline) aqueous solution under DC electric field. However, drawbacks such as evaporation and the inconvenient addition of inorganic salts to enhance the electric conductivity have inevitably limited its application. Therefore, development of new and robust media for electrowetting is highly desirable. Ionic liquids (ILs), a novel class of versatile solvents and soft materials possessing unique physicochemical properties, including negligible vapor pressure, being liquid over a wide temperature range, intrinsic ionic conductivity and acceptable electrochemistry stability, and so forth, have recently been developed as a promising alternative medium for electrowetting by Millefiorini et al. and other groups. In comparison with saline, IL-based electrowetting systems may be run under some extreme conditions such as in vacuum, or at temperatures either above 373 K or below 273 K. Recently, studies of electrowetting of ILs in air and under DC electric fields showed some unconventionality compared to saline such as low electrowetting efficiency (contact angle decrease or modulation) and cation/anion-dependent asymmetric behavior, although for the latter there is no unanimity among the different authors. Some effort has also been devoted to the initial applications of electrowetting of ILs as microreactor or microfluidics. Despite its promising potential, the reported electrowetting of ILs exhibits lower efficiency than that of saline with poor reversibility and a narrow range of contact angle modulation (<488). Herein, we presented greatly improved electrowetting efficiency of ILs using oil as the ambient and under AC electric field. Electrowetting behavior of ILs in this case, in particular at a high frequency of 1 kHz, shows greatly enhanced, sensitive, and reversible contact angle modulation in comparison to that in air or under DC electric field.