Propulsion reversal in oscillating-bubble powered micro swimmer

A gaseous bubble trapped in a one-end-open tube generates a microstreaming flow when oscillated by externally excited acoustic waves, which has been proven as an effective propulsion force for underwater micro robots. The propulsion force is known to be a pushing force on the tube since the microstreaming flow is outgoing from the tube exit. This article reports that this microstreaming flow and thus propulsion direction can be reversed when the gas–liquid interface of the bubble is exposed outside of the tube, which is confirmed using two types of tubes: commercially available capillary tubes and microfabricated parylene tubes. This result implies that control of the interface position in the tube relative to the exit is critically important. As such, two methods are incorporated to control the position of the interface and thus the length of trapped bubble: neck structure at the tube exit and plasma treatment of the tube. The neck structures enable the three-phase contact line to pin at the neck, thus providing uniform bubble lengths and warranting consistent oscillation spectra and propulsion behaviors. However, the reversal in microstreaming flow and propulsion does not occur since the bubble interface with the neck structure still stays inside the tube. The plasma treatment makes the three-phase contact line pinned right at the tube exit such that the entire interface is exposed to the outside of the tube. In this case, the reversal in microstreaming and propulsion consistently occurs, which provides an additional option to control the propulsion and steering of micro swimmers powered by acoustically oscillating bubbles.