Electro-coalescence of heterogeneous paired-droplets under AC electric field

Controllable droplet coalescence exhibits unique advantages and intriguing prospect in chemical synthesis and biological engineering. Current researches focusing on the droplets of the same physics are, however, limited in terms of the interaction between different reactants. In this work, the electro-coalescence of heterogeneous paired-droplets is investigated in a microfluidic chip controlled by an AC electric field. The characteristics of merging dynamics are analyzed under different electric conditions and fluid properties, and an on-chip cross-linking reaction is conducted to enable the instantaneous production of hydrogel microspheres. We find that the coalescence of heterogeneous paired-droplets expands the range of start positions and prolongs the merging time compared to homogeneous paired-droplets. The evolution process of interfaces is accelerated with the increasing voltage, which contributes to the mixing of diverse components. Different electrical conductivities lead to distinct internal mechanisms within droplets. The voltage across the droplet is reduced with the increasing conductivity, while the enhanced attraction between free charges plays a complimentary role in interface instability. Lowering the surface tension reduced the required electric conditions for coalescence. Endowed with the non-Newtonian property, the droplet presents a non-linear relationship in the coalescence region, triggering coalescence with filaments at low voltages and showcasing superior performance at high frequencies. Based on above findings, we successfully produce alginate hydrogel microspheres with a wide range of concentrations in high monodispersity, achieving a clean fabrication of pure hydrogel without any additives and no need for subsequent cleaning. These results reveal the electro-hydrodynamics of heterogeneous paired-droplets, promoting the development of droplet coalescence in chemical and material science.