Interrogation of Droplet Configuration During Electrowetting via Impedance Spectroscopy

Several parameters associated with the configuration of a microliter aqueous droplet are determined by the electrical impedance measurements conducted during electrowetting. In each case, the wetted area of a dielectric-coated electrode is first determined from the impedance data, and the known dielectric thickness and permittivity. The wetted area data is then employed in a series of identification problems. In the simplest of these, we demonstrate that the contact angle can be accurately found as a function of applied bias voltage. Thus, the Lippmann-Young (LY) curve of an electrowetting system can be determined from the impedance data alone without the use of any optical measurements. In another test, we show that the volume of a microliter droplet can be estimated from electrical impedance measurements with an error of <;1%. A similar algorithm was employed to determine the surface tension of the interface between the droplet and the surrounding oil to an accuracy of 4%. In still another identification experiment, the impedance data from multiple bias voltages were used as a batch to identify several unknown configuration parameters simultaneously (volume, interfacial tension, and zero-voltage contact angle). It is shown that identification accuracy is degraded in this case, because there exists a direction within the parameter space of low error sensitivity. Determining multiple parameters together, therefore, requires that the ideal LY relation fits the electrowetting behavior well over the range of voltages employed and that the dielectric parameters are accurately known.