Frequency-based relationship of electrowetting and dielectrophoretic liquid microactuation

Electrowetting and dielectrophoretic actuation are potentially important microfluidic mechanisms for the transport, dispensing, and manipulation of liquid using simple electrode structures patterned on a substrate. These two mechanisms are, respectively, the low- and high-frequency limits of the electromechanical response of an aqueous liquid to an electric field. The Maxwell stress tensor and an RC circuit model are used to develop a simple predictive model for these electromechanics. The model is tested by measuring electric-field-induced pressure changes within an aqueous droplet trapped between two parallel, disk-shaped electrodes immersed in a bath of immiscible, insulating oil. The experiment is an adaptation of Quincke's original bubble method for measuring the dielectric constant of a liquid. For AC voltages lower than similar to100 V-rms, the pressure data largely conform to the square-law predictions of the model. At higher voltages, this square-law behavior is no longer evident, a result consistent with the well-known contact angle saturation effect. Pressure data obtained with DC electric fields are not consistent with either the lowest frequency data (10 Hz) or with the model.