Breakup length of AC electrified jets in a microfluidic flow-focusing junction

Abstract

Electroactuation of liquid–liquid interfaces offers promising methods to actively modulate droplet for - mation in droplet-based microfluidic systems. Here, flow- focusing junctions are coupled to electrodes to control droplet production in the well-known jetting regime. In this regime, a convective instability develops leading to drop - let formation at the end of a thin and uniform, long liquid finger. We show that in AC electric fields, the jet length is a function of both the magnitude of the applied voltage and the electrical parameters such as the frequency of the AC field and the conductivity of the dispersed phase. We explain that dependency using a simple transmission line model along the liquid jet. An optimum frequency to maxi - mize the liquid ligament length is experimentally observed. Such length simply cannot be obtained by other means under the same operating conditions, in the absence of the AC signal. At low frequency, we reach a transition from a well-behaved, uniform jet brought about near the optimum frequency to highly unstable liquid structures in which axisymmetry is lost rather abruptly

Droplet group production in an AC electro-flow-focusing microdevice

“…This multi-drop regime is observed for voltage amplitudes between V pp = 500 V and V pp = 700 V and signal frequencies between f = 25 Hz and f = 100 Hz. For moderate values of the voltage and signal frequencies above f = 100 Hz the unstable regime appears and with further increment a jet is observed (Tan et al, 2014;Castro-Hernández et al, 2015. For V pp > 700 V the unstable regime is present for all tested frequencies, while for V pp < 500 V drop diameters in a group are more uniform.…”

“…Clustering of droplets is carried out downstream by size-dependent velocity dispersion in the microchannel. AC electro-flow focusing devices have been previously used to control the production of droplets (Tan et al, 2014) and long jets (Castro-Hernández et al, 2015. In these works, the signal frequency was greater than the production droplet rate.…”

On Equilibrium Shape of Charged Flat Drops

“…One would like to understand the equilibrium shape attained by the drop, either at constant voltage or constant total charge, and either at constant or variable volume. Physically, this problem is interesting because the electric field produced by the charges tends to destabilize the liquid interface [22,24,39,51,53], an effect that is used extensively in a variety of applications such as mass spectrometry [30], electrospinning [6] and microfluidics [15].…”

Self-Aligned Interdigitated Transducers for Acoustofluidics

“…However, the lamb waves generated via the aluminium foil electrodes are complex and have uncontrollable patterns, which greatly limits its wide applications compared to the patterned metallic IDTs. Previous studies show that infusing conductive materials and composites along microfluidic channels have enabled the fabrication of microscale electrodes to manipulate particles, droplets and jets [22][23][24][25][26][27].…”