Salt-mediated non-monotonic transport of surfactant-laden oil droplets in confined spaces*

We report our observation of a non-monotonic motion of oil droplets covered with various surfactants upon exposure to a single, unidirectional ionic solute gradient. Under the influence of high-salinity water, the oil droplets surrounded by anionic surfactants (sodium dodecyl sulfate, SDS) aggregate and either stay motionless or rapidly migrate against the expected direction of the well-studied diffusiophoresis. Meanwhile, the droplets covered by non-ionic surfactants (Triton X-100) remain emulsified and immobile. Noticeably, the droplets covered in a mixture of these two surfactants exhibit a strong diffusiophoresis migration. In many cases, the droplets that are far from the high salinity source start to swim randomly after a few minutes. These observations suggest the existence of competition during the droplet locomotion under confinement, which is between diffusiophoresis and, in this case, Marangoni propulsion triggered by the salted-out SDS. We further experimentally confirm this investigation by characterizing the dynamic interfacial tension of oil droplets in aqueous salt solutions. Our study offers a potential strategy for enhanced oil recovery in high salinity conditions.