Electrohydrodynamics of compound liquid thread formation in a flow-focusing microchannel under an electric field

The electrohydrodynamics of compound liquid thread formation under an electric field in a flow-focusing microchannel is numerically studied via the phase-field method and the dielectric model. The influences of electric potential and flow rate on the morphology and size of compound liquid thread are clarified. A regime diagram is provided to distinguish the thread formation regimes (i.e., dripping–threading, jetting–threading, and threading–threading regimes) from the non-thread formation regimes. It is found that the electric force and interfacial tension, both acting in the same direction, suppress the growth of the inner phase. Conversely, these two forces oppose each other in the middle phase, counteracting the capillary instability of the middle phase. Therefore, the larger electric force contributes to the elongation and stability of jets, facilitating the generation of continuous compound thread. Moreover, as the larger electric force hinders the growth of the inner phase front under the dripping regime, the formation time and size of the inner droplet are increased, while, under the jetting regime, a larger electric force has a stronger squeezing action on the neck of the inner phase and thus reduces the pinch-off time and size of the inner droplet. Additionally, the straightened thread is widened with increasing electric capillary number. In particular, to control the preferred regime of jet template for producing the peapod-like microfibers, two scaling laws are correspondingly developed to predict the inner droplet radius and outer thread thickness for the dripping–threading regime, which achieve good predict precision with relative error less than ±18%.