Salt-mediated bidirectional propulsion of oil droplets in confined spaces

Salinity gradients frequently arise during hydrocarbon recovery from subsurface reservoirs and can drive interfacial transport phenomena such as diffusiophoresis and Marangoni flows. Here, we investigate the diffusiophoretic migration of surfactant-laden oil emulsions induced by salinity gradients. While oil droplets experience diffusiophoretic motion along the salinity gradient, under high-salinity conditions, we observe an unexpected reversal of droplet migration, preceded by rapid droplet aggregation. By systematically varying monovalent chloride salts and surfactants, we demonstrate that this bidirectional behavior results from the competition between diffusiophoresis and Marangoni propulsion, the latter arising from salting-out-induced micellar solubilization. Our findings provide insights into chemically powered emulsion transport in geologically relevant settings and suggest strategies for enhancing oil recovery in high-salinity environments.