Continuous and tunable droplet splitting using standing-wave acoustofluidics

Droplet splitting plays an important role in droplet microfluidics by providing precise control over droplet size, which is essential for applications such as single-cell analysis, biochemical reactions, and the fabrication of micro- and nanosized material. Conventional methods of droplet splitting using obstructions or junctions in the microchannel have a clear limitation that the split ratio for a particular device remains fixed, while existing active splitting methods are constrained by low flow rates, the need for complex systems, or limitations to specific droplet types. In this study, we demonstrate that droplet splitting can be achieved simply using a one-dimensional standing-wave field excited within a microchannel. The mechanism of droplet splitting is investigated using theoretical analysis, numerical simulations, and high-speed imaging. It is found that splitting occurs due to the opposing acoustic radiation pressure acting on the two sides of the droplet, when the droplet with a negative contrast factor is placed near the pressure node. The entire splitting process can be characterized by necking, full-stretch, and splitting regimes, and it is completed in approximately 1 ms or less, demonstrating the capability to perform in-flow droplet splitting at high throughput. Continuous droplet splitting is successfully performed at a flow rate of 161 μL min^-1 with an equal split ratio, and at flow rates between 33.1 and 45.1 μL min^-1 with unequal split ratios ranging from 0.27 to 0.7. Selective and controllable cross-phase particle manipulation is achieved through droplet splitting and subsequent acoustic actuation, thereby extending the capabilities of droplet microfluidics in microreactions and drug delivery.