Electro-hydrodynamics of droplet generation in a co-flowing microfluidic device under electric control

Abstract

By coupling phase field method and electrostatic model, a numerical study is performed to investigate the electro-hydrodynamics of droplet generation in a co-flowing microfluidic device under electric control. It is indicated that an axisymmetric electric force normal to the interface changes the competition among the governing forces for the droplet generation. Consequently, four flow regimes during droplet generation are observed: dripping, dripping-jetting transition, jetting and threading with no droplets generated. Increasing the electric force can suppress the role of interfacial tension, which restrains the head-growth of the inner fluid in the dripping regime and the Rayleigh-Plateau instability appearing in the jetting regime. The squeezing, necking and breakup of the inner fluid thread is also enhanced by increasing the electric force, leading to increments in the droplet generation frequency and deformation rate as well as decrement in the droplet size. Moreover, the monodispersity in the dripping regime is best, while that in dripping-jetting transition regime is worst. Depending on the hydrodynamic capillary number (Ca) and electric capillary number (CaE), a droplet generation regime diagram is plotted to quantitatively represent the flow regimes, which indicates that, when the Ca and CaE are low enough (Ca ≤0.20 and CaE≤0.8), only the dripping regime is observed, and the threading regime is only able to appear at sufficiently low Ca (Ca ≤0.125) and high CaE (CaE≥2.68). The current study is useful for the precise electric control of microfluidic droplet template generation involved in advanced materials processing, micro-chemical technology, biomedical engineering, etc.