Reconfigurable electrokinetic transport in charge-regulated Janus droplets.

Abstract

Janus droplets enable advanced electrokinetic transport due to their intrinsic interfacial asymmetry, yet most existing systems rely on permanently engineered surface heterogeneity that limits dynamic and reversible control over electrohydrodynamic behavior. Here, we propose a zwitterionically functionalized Janus droplet whose interfacial charge can be programmably regulated through solution pH, enabling chemically tunable induced-charge electrophoretic (ICEP) dynamics under an applied electric field. The pH-responsive zwitterionic coating modulates the balance between electroosmotic slip and electrophoretic motion, leading to reversible transitions between vortex-dominated momentum-dissipative states and streamlined high-mobility transport regimes. Systematic numerical simulations reveal that the extent of zwitterionic coverage, grafting density, droplet size, and electric-field polarity collectively govern ICEP vortex formation and propulsion characteristics. Under acidic conditions, the positively charged interface promotes strong counter-rotating ICEP vortices that enhance viscous dissipation and reduce droplet velocity, whereas under basic conditions the reversal of interfacial charge aligns electroosmotic and electrophoretic transport, suppressing vortices and enhancing propulsion. Reversal of the electric-field polarity enables dynamic switching of vortex formation across pH conditions, providing an additional mechanism for externally controlled transport modulation. To rationalize these behaviors, we further develop a scaling-based analytical framework that classifies the electrohydrodynamic response into wall-dominated, vortex-dominated, and slip-dominated regimes, yielding predictive transition criteria that agree well with numerical observations. The present study establishes charge-regulated zwitterionic functionalization as a viable strategy for reconfigurable electrokinetic transport and adaptive flow manipulation in microfluidic systems, with potential applications in controllable mixing, separation, and programmable droplet transport.