Effect of bubble interface position on propulsion and its control for oscillating-bubble powered microswimmer

It has been previously reported that a gaseous bubble trapped in a one-end-open tube oscillates in the presence of acoustic wave and generates strong microstreaming flows and thus a propulsion force. The propulsion highly depends on the frequency and the voltage of the external acoustic wave. This paper presents a new discovery that the direction of this propulsion is dependent on the relative location of the bubble interface. The oscillating bubble propels forward when its interface stays deep inside the tube. On the contrary, the bubble propels in a reverse direction when its interface is at the exit of the tube. Learning from this phenomenon, we developed and introduced physical structures (necks) to precisely control the location of the bubble interface. As a result, the length and interface position of the bubble is more controllable, and the bubble oscillation and propulsion becomes more predictable and consistent.