Effect of Carrier Frequency and Liquid Conductivity on Electrowetting

We observe the effect of AC voltage electrowetting on aqueous salt solutions with different conductivities. Also, we evaluate the validity of the Young--Lippmann equation when applied to both deionized (DI) and conducting water droplets on a hydrophobic Teflon surface. Through analysing rescaled electrowetting responses, we determine that the optimal conductivity for aqueous solutions, where electrowetting curves demonstrate linearity and ideal behaviour, is approximately ≥ 0.1 mS/cm. Additionally, we delve into the notion of "characteristic time" or "dielectric relaxation time", representing how long it takes for charge to reach the surface of the aqueous droplet. This characteristic time is theoretically calculated from Maxwell’s equation and the continuity equation for free charge, and experimental verification is conducted using electrowetting on aqueous drops of varying conductivities on a hydrophobic Teflon surface. We find that the characteristic time becomes negligible for conductivity σ ≥ 0.134 mS/cm, thus establishing an optimal aqueous conductivity of around 0.1 mS/cm for electrowetting studies. Furthermore, we present graphical representations of electrowetting responses with increasing carrier AC voltage frequencies.