Droplet｜交流电场下异质配对液滴的电聚并

(¹National Key Laboratory of Science and Technology on Aero-Engine Aero-Thermodynamics, Research Institute of Aero-Engine, Beihang University, Beijing, China; ²School of Mechanical and Power Engineering, Nanjing Tech University, Nanjing, China; ³School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Singapore)

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.

Fig.1 Schematic of the electro-coalescence system. (a) Three-dimensional illustrative model of the microchip. (b) The fluidic configuration.

Fig.2 Merging dynamics of WG‒W paired-droplets. (a) Force analysis on electro-coalescence of heterogeneous paired-droplets. (b) Growth of liquid neck and bridge under low voltages and high voltages. The temporal evolution of (c) low voltage and (d) high voltage.

Fig.3 Electro-coalescence of heterogeneous and homogeneous paired-droplets. (a) Flexibility of the start position. (b) Merging time under different voltages. (c) Merging dynamics of W‒W, WG‒WG, and WG‒W.

Fig.4 Effect of electrical conductivity in controlling droplet coalescence. (a) The start position varies when U is 0–4 kV. Comparison of the start position (b) and the coalescence ratio (c) when f ranges from 0 to 10 kHz. (d) The electric circuit diagram of the emulsion. (e) The    under different conductivities and frequencies.

Fig.5 Impact of the surface tension. (a) The start position with different voltages. (b) The coalescence ratio under various frequencies. The scale bar is 100 µm.

Fig.6 Influence of non-Newtonian property with the addition of polyethylene oxide (PEO) 1 M under electric field. The effect of (a) voltages and (b) frequencies on the start position and coalescence ratio. (c) The merging dynamics induced by non-Newtonian filament with and without the electric field. The scale bar is 100 µm.

Fig.7 Fabrication of hydrogel microspheres by electro-coalescence. (a) Start position. (b) (i) Size distribution of 100 hydrogel beads when QA:QB:QC is 60:60:240 µL/h. Optical images of different flow rates: (ii) 60:60:120 µL/h, (iii) 60:60:240 µL/h, and (iv) 60:60:360 µL/h. (c) Fabrication of hydrogels under different concentrations of SA and CC. The scale bar is 100 µm.