Electrocapillarity-Induced Hurricane-in-a-Tube Enables the Generation and Patterning of Liquid Metal Droplets

Room-temperature liquid metal droplets (LMDs) are a promising material for various applications in soft robotics, active droplets, and biomedical devices. However, controllable and high-throughput production of LMDs remains challenging due to their high surface tension and density. Here, a novel strategy is presented to produce LMDs by combining electric field-induced electrocapillary flow with an external flow field. The basic mechanism is that the electrocapillary flow induced at the LMD/electrolyte interface forms a vortex ring in the electrolyte, creating a hurricane-like effect in the tube, which in turn causes the liquid metal to deform and eventually pinch off into small droplets. It is demonstrated that droplet size and generation frequency can be controlled precisely by adjusting the applied electric current, flow rate, and surfactant concentration, establishing a relationship between radius and experimental parameters through dimensionless analysis. More importantly, this strategy can handle pendant droplets and facilitate programmable droplet patterning. Leveraging established relationships, flexible control over droplet size and spacing during patterning is attained. Furthermore, an iontronic pressure-sensitive device based on LMDs and hydrogel is developed to showcase the versatility of the approach. This technique opens up new opportunities for fabricating soft circuits, composite materials, and other functional devices with LMDs.