Simple Method to Generate Droplets Spontaneously by a Superhydrophobic Double-Layer Split Nozzle

Given the problems of traditional droplet generation devices, such as the complex structure and processing technology, difficulty in droplet separation, and low transfer accuracy, we propose a low-adhesion superhydrophobic double-layer split nozzle (SDSN). It realizes spontaneous droplet generation by using an interfacial tension force inside the micro-hole to drive the droplet snap-off. It successfully achieves stable and highly consistent droplets on the micrometer-scale circular micro-hole. Droplets with a volume in the range of 0.65-1.75 ± 0.007 μL can be precisely achieved by adjusting the hole size of the SDSN from 100 to 500 μm. The SDSN is prepared by conventional mechanical drilling, chemical etching, and low surface energy modification. Compared with traditional droplet generation devices, no photolithography process is required, and the cost is lower. Moreover, the droplets can be obtained directly without any post-processing, avoiding the problem of separating droplets from another solution. The stability of SDSN is good, and the droplet volume is not affected by the fluctuation of external conditions. The rate of droplet generation can be freely adjusted by adjusting the speed of the electronic microinjection pump without affecting the droplet volume. It enables efficient droplet transfer without liquid residue, which improves the transfer accuracy and helps to save the use of expensive reagents. This simple but effective structure will be of great help to make breakthroughs in next-generation spontaneous droplet generation, liquid transport, and digital microfluidic devices.