Piezoelectric-synchronized step emulsification for facile generation of microdroplet library with fluid-independent drop size regulation regime

Microdroplets play a crucial role in various applications based on microfluidics, including biomedicine, materials synthesis, and analytical chemistry. However, there is still an urgent need for a microdroplet preparation method that is simple in design, precise in droplet size control and meanwhile independent of the properties of the fluid. Here, we present a piezoelectric-synchronized step emulsification method. The drop formation process can be synchronized by the introduced periodic vibration via the piezoelectric element, which occurs in a specific frequency range determined by a dimensionless parameter. The competing between the step emulsification regime and the piezoelectric-triggered local flow rate fluctuation leads to various microscopic flow behavior. A novel instantaneous jetting regime is found to be responsible for the extended achievable drop size range. By combining the piezoelectric step emulsification and the XY stage control, we achieve facile production of microdroplet and hydrogel microsphere library in well plates, which is promising to be integrated as a low-cost drop-generating technique into bioassay instruments.