We investigate the dynamics of the droplet pair coalescence and its stability under a direct current (DC) electric field strength by using simulation and theoretical analyses. We conduct a parametric study to investigate the effects of electric capillary number (Cae=ϵoϵoutEo2Ro/γ, the ratio of the electrical Maxwell stress to interfacial capillary stress), droplet size ratios (R/Ro, where R and Ro are the radii of the coalescing drops, respectively), and the droplet interfacial separation distance (S) on the coalescence process. We show that unequal-sized droplets undergo unique dynamics owing to the generation of velocity gradient between the coalescing droplets. Moreover, using theoretical analysis, we delineate the stable and unstable regimes of a coalesced droplet under an electric field. Results show that if the semimajor axis of the coalesced droplet becomes greater than 1.5 times, it continuously stretches and becomes unstable. We believe that the study will be useful for essential physical insights pertaining to the coalescence process and valuable for their applications in various areas of biological engineering, chemical sciences, material sciences, and lab on a chip.
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