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In this paper, novel propulsion of micro/millimeter-sized water-floating objects has been experimentally demonstrated. When an acoustic wave propagates onto air bubbles in a liquid medium, the bubbles oscillate and generate cavitational microstreaming flows; this can be used to propel small water-floating objects. This propulsion concept is simple, but the propulsion can provide sufficient force to propel the water-floating objects without electrical connecting wires and mechanical moving parts. In this study, we prepared open-box-type micro/millimeter-sized objects using a thin Al film. We then experimentally realized linear and rotational motions and two-dimensional maneuvers on the surface of water. The effects of the frequency of the acoustic wave and the applied voltage on the motions are quantified with the bubble oscillation amplitude using high speed images. Such water-floating objects propelled by oscillating microbubbles can be integrated with cameras and sensors and used in environment monitoring systems or surveillance security systems.

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This paper describes a novel tweezing system using cavitational microstreaming flows generated by acoustically oscillating twin bubbles for non-contact micromanipulation. A tweezing system with a single acoustically oscillating bubble attached on the tip of a rod integrated with a three-dimensional traverse system is firstly tested to manipulate a fish egg (1 mm diameter) in an aqueous medium. Although the flow generated from the single oscillating bubble is strong enough to push and move the fish egg, the moving direction of the fish egg is not controllable. To improve the control of the manipulation, identical twin bubbles, which have the same size and resonant frequency, are applied. To obtain the identical bubbles, an electrolysis chip consisting of sharp tip-shaped electrodes is microfabricated, and each bubble is generated from the chip by controlling the applied voltage and time and then transported to the tips of a U-shaped rod. Manipulation of a fish egg (1 mm diameter) and glass beads (100 μm diameter) is experimentally demonstrated using acoustically oscillating twin bubbles. Using a high speed camera, the force generated by the acoustically oscillating bubbles and its direction are analyzed in various acoustic excitation conditions. The results show that the generated force is proportional to the bubble oscillation amplitude, and the direction of the force depends on the distance between a bubble and object. A steel ball (500 μm diameter) is used for the investigation of the force direction. When a bubble (600 μm diameter) is acoustically excited, the steel ball is pulled into the oscillating bubble in the short distance (<3 mm); however, the steel ball is pushed from the oscillating bubble in the long distance.