A deformed droplet may leap from a solid substrate, impelled to detach through the conversion of surface energy into kinetic energy that arises as it relaxes to a sphere. Electrowetting provides a means of preparing a droplet on a substrate for lift-off. When a voltage is applied between a water droplet and a dielectric-coated electrode, the wettability of the substrate increases in a controlled way, leading to the spreading of the droplet. Once the voltage is released, the droplet recoils, due to a sudden excess in surface energy, and droplet detachment may follow. The process of drop detachment and lift-off, prevalent in both biology and micro-engineering, has to date been considered primarily in terms of qualitative scaling arguments for idealized superhydrophobic substrates. We here consider the eletrically-induced ejection of droplets from substrates of finite wettability and analyze the process quantitatively. We compare experiments to numerical simulations and analyze how the energy conversion efficiency is affected by the applied voltage and the intrinsic contact angle of the droplet on the substrate. Our results indicate that the finite wettability of the substrate significantly affects the detachment dynamics, and so provide new rationale for the previously reported large critical radius for drop ejection from micro-textured substrates.

Adhesion and friction during physical contact of solid components in microelectromechanical systems (MEMS) often lead to device failure. Translational stages that are fabricated with traditional silicon MEMS typically face these tribological concerns. This work addresses these concerns by developing a MEMS vertical translation, or focusing, stage that uses electrowetting-on-dielectric (EWOD) as the actuating mechanism. EWOD has the potential to eliminate solid-solid contact by actuating through deformation of liquid droplets placed between the stage and base to achieve stage displacement. Our EWOD stage is capable of linear spatial manipulation with resolution of 10 μm over a maximum range of 130 μm and angular deflection of approximately ±1°, comparable to piezoelectric actuators. We also developed a model that suggests a higher intrinsic contact angle on the EWOD surface can further improve the translational range, which was validated experimentally by comparing different surface coatings. The capability to operate the stage without solid-solid contact offers potential improvements for applications in micro-optics, actuators, and other MEMS devices. KEYWORDS: Electrowetting on Dielectric (EWOD), Microelectromechanical Systems (MEMS), Tribology, Lab-on-a-Chip 2The efficacy and longevity of actuation-based microelectromechanical systems (MEMS) are limited by tribological issues such as friction and adhesion.1 Adhesion typically occurs between small asperities when nominally smooth solid surfaces come into contact, and friction is the result of this adhesion during motion of the surfaces relative to each other. In some cases, the forces from adhesion and friction are comparable to the forces actuating the device, rendering it unusable. In the presence of water vapor, these concerns are amplified; formation of a thin liquid film on solid surfaces and the corresponding capillary and viscous effects that come into play when these surfaces interact lead to stiction, a phenomenon that increases the adhesion between solid asperities and causes small features to stick together due to exceptionally high friction. 1-3These reactive forces at the physical contact of solid components lead to wear and removal of material, reducing device lifetime. 1,4,5

We demonstrate a MEMS translational stage that uses electrowetting-on-dielectric (EWOD) as the actuating mechanism.Our EWOD stage is capable of linear translation with resolution of 10 μm over a maximum range of 130 μm and angular deflection of approximately ±1° while eliminating solid-solid contact. The range and resolution can be readily improved via higher base contact angle and lower contact angle hysteresis as indicated by the detailed modeling accompanying the experimental demonstration.