Non-contact ultrasonic manipulation for targeted micro-droplet merge by using a flat–sharp cantilever

This paper demonstrates a method for non-contact manipulation by acoustic flow to achieve selective merging of different volumes of micro-droplets. An ultrasonic cantilever with a sharp and flat tip, which worked at 84 kHz, was used to generate an acoustic streaming field in the silicone oil for non-contact manipulating individual micro-droplets. The finite element method (FEM) was used to calculate the acoustic pressure distribution around the tip of the ultrasonic cantilever and the distribution pattern of acoustic flow, respectively. To validate the calculated results, the pattern of the acoustic streaming was investigated by tracing the movement path of the polystyrene (PS) micro-particles. Subsequently, the driving voltage interval and the non-contact manipulation ability of the ultrasonic cantilever without cavitation bubbles were measured, respectively. Then, the factors affecting the manipulation ability of the ultrasonic cantilever: micro-droplet volume, vertical distance between the tip of the ultrasonic cantilever and droplets, the angle of intersection and distance between the droplet and the cantilever in the horizontal direction were experimented and discussed. Based on the above study, the automatic merging of two target droplets is achieved for a pile of micro-droplets randomly placed on the polydimethylsiloxanes (PDMS).