Characterization of electrowetting processes through force measurements

Abstract

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.

Adhesion Hysteresis from Interdependent Capillary and Electrostatic Forces

“…Finally, at the highest bias, the small reduction in attractive force at the onset of capillary condensation (viz., at D *, Figure ) indicates nonadditivity in the capillary and electrostatic forces, in contrast to the common assumption when modeling pull-off forces measured with AFM to determine the capillary contribution . Together, these observations suggest that other factors, such as the Laplace pressure, the preexistence of nanodroplets within the contact region, ,− , and/or the dielectric nature of the interfaces and intervening fluid, may also need to be considered. Thus, refining our theoretical understanding to fully address the complexity of the nanocapillary geometry and environment will be a salient topic for future research.…”

“…The extension of D C with applied bias is a third indicator that the approach force was predominantly governed by the capillary interaction at the lower biases. Briefly, the local field is expected to increase the range of capillary condensation from surface-charge-enhanced ordering of near-surface water (i.e., leading to contact angle reduction), torque on fluid molecules, a reduction in the surface energy of the solids (viz., electrowetting), and balancing of the electrostatic and condensation energies. ,− To put this on a statistical basis, the percentage increase in the nucleation range is defined as Δ D * ≡ ( D C − D C,0 )/ D C,0 × 100, where D C,0 is the nucleation range at 0 V bias. Values are only shown for the majority (>90%) of the measurements where D * could be easily distinguished.…”

Soft Actuators for Small‐Scale Robotics

“…By surrounding a droplet with a flexible film, controlled and repeatable origami‐reminiscent folding has also been accomplished (Figure A) . Crane et al presented work toward enhanced understanding of electrowetting forces and droplet behavior to assist in the design of object manipulators …”

Electrowetting Effect: Theory, Modeling, and Applications

“…This configuration can be achieved if the droplet is placed over two electrodes and there is no ground connected. Under this condition, if a voltage is applied to one of the electrodes, the system energy is minimized if the droplet has an equal area covering both electrodes (16). This can be useful for creating a centering effect.…”