A Method for Triggering Surface Modes by Bubble Coalescence
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Acoustically driven microbubbles find numerous applications in the biomedical field (targeted drug delivery or blood-brain barrier opening for instance) and in engineering (surface cleaning or sonochemistry). In most cases only spherically oscillating bubbles are considered in these techniques. However, bubbles exhibiting surface modes can lead to additional effects (enhanced micromixing amongst others), and are scarcely investigated (mainly due to positional and growing surface instabilities). We present an experimental setup which allows the triggering and control of surface modes of free bubbles. As opposed to earlier studies which are based on short-time exposure to ultrasound waves or an amplitude modulated pressure field, we obtain bubbles with steady-state surface modes. The experimental procedure consists of the following steps: (i) An acoustic field is induced in a water tank by a transducer. The driving frequency is chosen as to create an acoustic standing wave inside the tank. (ii) A bubble is nucleated by a short focused laser pulse. Our bubbles are all below resonance size and are hence driven towards a pressure antinode due to primary Bjerknes forces. (iii) By nucleating a second bubble, the two bubbles will coalesce due to primary Bjerknes forces and mutual attraction by secondary Bjerknes forces. The coalescence leads to a single, initially deformed bubble. By choosing the appropriate set of acoustic pressure and bubble size, a desired surface mode can be triggered. Additionally, we observed that the trajectory of the coalescing bubbles will define the orientation of the axis of symmetry. In sum, coalescence is a fast method to induce surface modes in free acoustically driven bubbles.