Characterization of electrowetting processes through force measurements

A new method of characterizing electrowetting is presented. In this method, the electrowetting actuation forces are measured rather than the contact angle. The forces on the liquid are measured by trapping a droplet between a flat nanoindenter tip and the test substrate. When voltage is applied to electrodes in the substrate, lateral and normal forces are exerted on the tip and measured by the nanoindenter transducer. Proper selection of the tip geometry permits direct prediction of the resulting in-plane lateral forces using analytical formulas derived from the Young-Lippmann equation. Experimental results show good agreement with both analytical and numerical predictions. Numerical modeling using SURFACE EVOLVER shows that the lateral forces are relatively insensitive to most alignment errors and that the analytical model is most accurate when the flat tip is close to the substrate. Evaporation of the test liquid can introduce modest errors in long measurements, but compensation methods are presented. As the droplet undergoes almost no movement, the fluid dynamics have minimal impact on the measured forces and transient electrowetting events are readily detected. Experimental results show significant response at frequencies up to 40 Hz. This setup is useful in measuring electrowetting responses at high speeds and in measuring system degradation processes.