Finite conductivity effects and apparent contact angle saturation in AC electrowetting
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0899-N06-01.1
Finite conductivity effects and apparent contact angle saturation in AC electrowetting A. Kumar, M. Pluntke, B. Cross, J.-C. Baret, and F. Mugele University of Twente; Physics of Complex Fluids; PO Box 217; 7500 AE Enschede (The Netherlands) Abstract We measured the electrowetting behavior of aqueous salt solutions. By varying the conductivity and the frequency of the applied AC voltage we determined the range of the validity perfect conductor assumption of the standard electrowetting theory for the case of AC voltage. We show that the contact angle reduction is dramatically reduced at high frequency and low salt concentration due to Ohmic losses with the liquid. A simple RC-equivalent circuit model explains the observations. It is demonstrated that finite conductivity effects are more pronounced for sessile droplets than for droplets confined between to parallel plates. Introduction Electrowetting allows for controlling the contact angle of partially wetting conductive liquids by applying a voltage between the liquid and insulator-covered electrodes on the substrate. In recent years it has raised considerable interest as a tool to control and manipulate liquid droplets at surfaces [1]. Applications range from electrically addressable displays [2], to variable focus optics [3, 4] and to microfluidic systems for biomedical applications [5, 6]. One important requirement for these practical applications was the development of reliable substrates that provide both long live times without degradation and low operation voltage, preferably no higher than a few tens of volt. Processes that can lead to degradation include the permanent adsorption of ions from liquid to the substrate surface, which can be suppressed by using AC instead of DC voltage. If the AC frequency is above the mechanical eigenfrequency of the droplet (or its viscous relaxation time – depending on the viscosity), the droplet “sees” the root-mean-square value of the applied voltage. Beyond a certain conductivity-dependent threshold voltage, however, the liquid ceases to behave like a perfect conductor. As a result, ohmic losses within the droplet occur and the effect on the contact angle is reduced [7]. So far this effect was mainly studied indirectly by monitoring the electrowetting-induced height of rise of liquid columns between parallel electrodes [8-10] and by monitoring the electrowetting-induced filling of grooves on Si surfaces [11]. In this work, we report on systematic direct measurements of finite conductivity effects on the contact angle reduction in AC electrowetting. We show that finite conductivity effects are much less pronounced from droplets confined between two parallel surfaces than for sessile droplets on a single substrate. Experimental details The experiments were performed for two different geometries, as illustrated in Figure 1. The liquids consisted of solutions of NaCl in deionized water (Millipore). The salt concentration was varied between 2×10-5 and 2×10-2 mol/l corresponding to a conductivity range from approx
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