Morphological evolution of electrified droplet: From fingering instability to Coulomb explosion

Alternating current (AC) electric fields have the potential to induce complex morphological transitions in liquid–liquid systems. In this paper, the instability evolution of the charged methanol droplets in oleic acid under non-uniform AC electric field is investigated by using high-speed imaging. It is found that charged droplets experienced various morphological transitions induced by the AC electric field. For low electric Bond number (BoE), a finger-shaped instability is initiated from the droplet, followed with finger growth and subsequent branching. With the increase in electric field strength, novel morphologies of stretched bottom membrane and radially ejected annulus-shaped film emerge while an explosive Coulomb breakup is found at extremely high field strengths. Throughout the range of instabilities, a transition from disordered to highly ordered polygonal droplet contours is found to be driven by the bifurcation effect in the internal electrohydrodynamics (EHD) flow. Moreover, the growth rate and characteristic wavelength of the instabilities exhibit saturation at high BoE. This nonlinear effect is attributed to the dominance of charge convection, which limits the local surface charge density at the finger tips. Overall, the non-uniform AC electric field provides a method for regulating multi-scale fluid interface morphology in high-viscosity-ratio fluids.