Electric Double Layer and Surface Tension Effects in Droplet Transport: A Combined Experimental–Computational Study

This paper explores the electrically-driven motion of aqueous NaCl droplets in EMIM NTF₂ ionic liquid microchannels, offering a novel alternative to conventional liquid metal-based continuous electrowetting (CEW) systems by utilizing ion-based charge carriers. We systematically investigate the influence of NaCl concentration and droplet size under applied DC voltage on droplet transport. Our experimental observations reveal polarity-reversible displacement driven by CEW, with droplet velocity strongly dependent on ionic strength. At low concentrations, insufficient ion density hindered motion, while pre-application of voltage was essential to enable effective electrohydrodynamic coupling. Complementing these results, we performed COMSOL Multiphysics simulations to model the system and gain mechanistic insights. The simulations highlight the critical role of electric double layer (EDL) formation and surface tension gradients at the droplet–ionic liquid interface, which together drive interfacial stresses leading to droplet propulsion. This combined experimental and computational study advances the understanding of electrokinetic phenomena in ionic liquids and supports the development of next-generation electrically-actuated microfluidic systems.