Droplet migration into dead-end channels at high salinity enhanced by micelle gradients of a zwitterionic surfactant

Direct microscopic visualization experiments are reported to quantify the diffusiophoretic migration of oil droplets in high-salinity solutions (c > 0.7 mol/L). Contrary to usual results in the low-salinity limit, we find negligible enhancement in the droplet migration under electrolyte gradients at high salinity. Rationalization of this observation suggests nonelectrolyte gradients as possible alternatives to effectively drive diffusiophoresis beyond the low-salinity limit. In particular, micelle gradients of the zwitterionic surfactant clearly enhance droplet migration into dead-end channels at high salinity. Characterization of such migration-proceeding down the gradient-indicates repulsive interaction between micelles and droplets, which may involve various micelle dynamics at different salinity conditions. Furthermore, we identify a synergistic effect from simultaneous electrolyte and micelle gradients to further enhance droplet migration. These findings, together with the robustness of our experimental system, contribute knowledge or enable more studies to understand and design colloid migration in varieties of systems at elevated salinity.

Automated summary

High-salinity solutions were found to have negligible enhancement on the migration of oil droplets under electrolyte gradients, while non-electrolyte gradients and micelle gradients were effective alternatives. The repulsive interaction between micelles and droplets at high salinity conditions played a significant role in enhancing droplet migration. The synergistic effect of simultaneous electrolyte and micelle gradients further promoted droplet migration.