Manipulation of aqueous droplets in oil using contactless ultrasound

Acoustic droplet manipulation is widely used in biochemical analysis due to its excellent biocompatibility and non-contact operation. However, conventional methods performed in air suffer from aerosol-induced sample cross-contamination. To address this limitation, we propose a quad-focused acoustic tweezer (QFAT) system composed of four ultrasonic transducers orthogonally arranged with a 45° inclination angle. The interference of multiple acoustic beams in the oil phase generates a low-pressure acoustic potential well, and the resulting acoustic radiation force enables non-contact droplet manipulation. Benefiting from the intrinsic isolation of aqueous droplets by oil phase, the system significantly reduces the risk of aerosol contamination. The QFAT enables stable droplet transport and, when integrated with an automated three-axis translation stage, guides droplets along complex trajectories such as circles and hexagons with high precision. In addition, the system supports parallel collective transport of multiple droplets for orderly multi-step chemical reactions, demonstrating strong potential for parallel biochemical manipulation in oil-phase environments. This work provides a robust and highly flexible non-contact droplet manipulation method with significantly reduced contamination risks for droplet-based biochemical analysis and synthesis.