Electrowetting behaviour of thermostable liquid over wide temperature range

Abstract

Outdoor electrowetting (EW)-based applications need consistent wetting response in the temperatures ranging from − 40 to 70 °C. Aqueous ethylene glycol is commonly used as thermostable liquid; nevertheless, its EW behaviour over a wide range of temperature is less known. We examine this behaviour for both ac and dc voltages over the temperature range from − 25 to 65 °C. The self-consistent EW responses, i.e. cosine of contact angle versus voltage square, are analysed to illustrate the EW behaviour. There is a systematic increase in EW response with temperature confirmed from the linear dependence of interfacial tension on temperature. Our result corroborates Eötvös phenomenological relation. We suggest the need of correction voltage with changing temperature to maintain uniform EW response over this temperature range. Further with decreasing temperature, the solution viscosity increases more rapidly than the increase in interfacial tension value, and thus the capillary number $(Ca = \mu v/\gamma )$ gets partially regulated. This is seen as a marginal variation in the switching time of the EW contact angle during the OFF voltage state to ON state and vice versa. Finally, the working of a prototype liquid lens at − 25 °C, the lowest operating temperature, is demonstrated.