Numerical characterization of transient electrohydrodynamic deformation and coalescence of single-core double emulsion droplets by AC field dielectrophoresis

Considering its good biocompatibility, the water-in-oil-in-water type compound double emulsion (DE) droplet serves as an ideal microreaction vessel upon their on-demand electro-coalescence. By coupling phase field description to fluid dielectrophoresis, transient numerical simulation is conducted to characterize the dynamic feature of surface charge wave and electric stress induced at various phase interfaces of single-core DE drops subjected to external AC electric fields. A novel prolate/prolate electro-deformation mode is discovered by numerical analysis for A/B/A type two-phase droplet when both the conductivity and permittivity of oil shell are much smaller than that of water phase. Based upon this, AC electrostatic attraction between two opposing single-core DE droplets is then investigated in a broad parametric space. Collision of two adjacent oil shells is always followed by the dielectrophoretic integration of corresponding water cores wrapped within them, leading to three different possible modes of inter-droplet electro-coalescence under distinct core/shell radius ratio. Our numerical analysis herein unveils for the first time the hitherto unknown spatial-temporal electrohydrodynamic behavior of single-core DE droplets driven by Maxwell-Wagner structural polarization, during their deformation and coalescence as a consequence of an active interplay among those induced droplet dipoles.