Stable generation of Core-Shell droplets in Coaxial Electrohydrodynamic Printing by Numerical Simulation and Jet Mode Recognition

Core-shell droplets exhibit advantageous physical and chemical properties attributed to their unique structural composition, thereby presenting broader prospects for application in diverse fields including biomedicine, drug delivery, food science, and cosmetics industry. Controllable preparation of monodisperse, small-volume, and effectively encapsulated core-shell droplets is crucial for advancing various applications. In this paper, coaxial electrohydrodynamic (EHD) printing technology is used to generate uniform and stable arrays of water-in-oil core-shell droplets. By using theoretical analysis and numerical simulation to determine the conditions for stable generation of water-in-oil droplets, this study successfully addresses the issue of unstable coaxial charged jets and the generation of satellite droplets by monitoring the jet recognition. The simulation results show that too high voltage and inner axis solution supply speed can lead to the formation of satellite droplets or spray. When Q_out/Q_in≥1, it is less likely to produce satellite droplets. Combining the results of numerical simulation and the jet recognition module, the coaxial jet mode can be effectively monitored. The jet is controlled in a stable printing mode (pulsating droplet mode) and complete in one injection within 5 ms to achieve uniform droplet arrays with diameters around 50 μm and a droplet diameter coefficient of variation (CV) below 6%. This work provides significant help on the injection morphology control of water-in-oil droplets, and provides a feasible technical means for the application of coaxial EHD printing technology in various fields such as bioassay, biosensing, and fluorescence display.