Electrowetting-Induced Stiction Switch of a Microstructured Wire Surface for Unidirectional Droplet and Bubble Motion

Directional motion of fluid droplets is attracting considerable research attention. Despite much progress in this field, electrowetting-based stiction switch that drives unidirectional transport of an underwater oil droplet or air bubble on a wire surface is still a challenge to realize. Here, a strategy to collect and unidirectionally transport an underwater oil droplet or air bubble on a porous polystyrene (PS) film-coated copper wire surface through the combination of electrowetting-induced stiction switch and buoyancy is demonstrated. The porous PS film-coated copper wire fabricated by the breath figure method at a relative humidity of 75% can catch and hold an underwater oil droplet or air bubble without applied voltage. With increasing applied voltage, the stiction of the underwater oil droplet or air bubble on the porous PS film surface weakens because water increasingly wets the film surface. The underwater oil droplet or air bubble can move unidirectionally on the porous PS film-coated wire surface upward or downward at a voltage larger than its critical voltage according to whether its density is lower or higher than that of water. This work should aid the development of devices displaying electrowetting-based changes for use in fluid collection, manipulation, unidirectional transfer, release, and removal.