W hen a gaseous bubble in liquid is excited by acoustic waves, it oscillates (expands and shrinks) at the wave frequencies and generates strong vortical flows around it, the so-called cavitational microstreaming. This article describes the development of a micropumping principle using cavitational microstreaming. The key idea is to place a capillary tube vertically above an oscillating bubble to collect the upward microstreaming flow. When the bubble is excited at its resonance frequency, it oscillates with surface undulations (surface wave mode) and pumps water through the tube. The performance of this pumping mechanism is experimentally studied using millimeter and microscale bubbles. The flow rate and generated pressure are measured in a variety of conditions. The measured results indicate that the present pump falls into the category of moderate-flow-rate and low-pressure type pumps. The present pump operates without physical connections or electrical wiring to the bubbles, implicating potential applications as implantable micropumps in many lab-on-a-chip type systems.

This letter describes a propulsion principle along with experimental verification of this principle by which an air-to-water interface vertically oscillated by ac electrowetting generates a quasisteady, “streaming” flow that can be utilized to propel water-floating objects. This propulsion does not require any mechanical moving parts. Using a centimeter-sized boat whose outer surfaces were covered with microfabricated electrowetting electrodes, linear, and rotational motions of the boat were achieved up to maximum speeds of 5 mm/s and 20 rpm, respectively. By combining the above two motions, the boat was successfully propelled and steered along a curvilinear pathline. A potential application of this principle is to propel and maneuver various water-floating mini/microrobots and boats used for water/air quality monitoring or surveillance/security purposes.

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This paper describes development and experimental verifications of a novel propulsion and rotation technique for water-floating objects. As opposed to mechanical paddling, this technique electrically controls surface tension forces acting on water-floating objects without any moving parts (so-called electrowetting-on-dielectric, EWOD). As a proof of concept, a mini water-floating boat of centimeter size is fabricated from a thin plastic foil of which outer surfaces are covered with microfabricated EWOD electrodes. Applying a voltage to the electrodes changes the symmetric configuration of surface tension on the boat, resulting in generation of propulsion and rotation in the boat. By energizing the frontal or rear EWOD electrode, linear propulsion of the boat is achieved. The maximum speed is measured to be 5 mm/s. In addition, energizing two diagonal side EWOD electrodes on the boat surface generates rotational motion at the maximum rotational speed of20 rpm. Finally, by combining the linear propulsion and rotational actuation, a curvilinear motion with controllability is demonstrated. This novel propulsion and rotation mechanism is simple yet efficient possibly being applied to propel and maneuver water-floating mini/micro robots and boats.