Dynamics of electrocoalescence-induced microfluidic droplet merging: Influence of the applied electric field

Abstract

Merging droplets in microchannels is an essential task in a variety of microfluidic systems. In this study, the electrocoalescence of droplets dispersed in an otherwise immiscible fluid within a microfluidic device was investigated numerically. The microfluidic device, comprising a flow-focusing junction, was employed to generate droplets in an otherwise immiscible fluid. Subsequently, the generated droplets were directed over a series of microelectrodes. The continuous and dispersed phases were modeled as incompressible Newtonian fluids. The interface between the phases was tracked using a phase-field model. For a constant electric field, three distinct threshold voltages were identified. No droplet merging was observed at voltages less than the first threshold-voltage. The regular merging of three droplets was noted at voltages beyond the second threshold-voltage and less than the third threshold-voltage. The irregular merging of droplets occurs at voltages beyond the third threshold-voltage. The influence of the interfacial tension on the first threshold-voltage was examined for the constant electric field. The interfacial tension considerably modulates the first threshold-voltage. Investigations were also conducted on the mechanism of droplet coalescence under alternating and pulsed direct-current electric fields, along with the effect of frequency on the first threshold-voltage. The first threshold-voltage increases with increasing frequency in both electric fields. Generally, the effects of frequency are small compared with, for example, the influences of interfacial tension on the first threshold-voltage.