Polarity-Reversible Droplet Transport at Liquid-Liquid Interfaces

This work investigates electrically driven transport of aqueous NaCl droplets in the ionic liquid [EMIM] [$\text{NTf}_{2}$] within open microfluidic channels. Under low-voltage DC fields, charge redistribution at the liquid–liquid interface creates electrocapillary surface tension gradients that drive Marangoni propulsion without the need for solid dielectrics or liquid metals. Droplet velocity, acceleration, polarity reversal, and onset behavior are quantified using segmentation-based tracking, revealing transient dynamics governed by electrical double-layer charging and strong dependencies on ionic strength and droplet size. Higher NaCl concentrations enhance interfacial capacitance and transport speed, while low concentrations fail to sustain motion. This fully liquid–liquid mechanism enables versatile continuous droplet manipulation in microfluidics.