Chemically tuning attractive and repulsive interactions between solubilizing oil droplets

Solubilization is a spontaneous transport process occurring in surfactant-stabilized emulsions that can lead to Marangoni flow and droplet motility. Conventionally, active droplets exhibit self-propulsion and pairwise repulsion due to solubilization processes and/or solubilization products raising the droplet’s interfacial tension. Here, we report emulsions with the opposite behavior, wherein solubilization and/or its products decrease the interfacial tension and cause droplets to attract. We systematically characterize the influence of oil structure, nonionic surfactant structure, and surfactant concentration on the interfacial tensions and Marangoni flows of solubilizing oil-in-water drops. Three regimes corresponding to droplet “attraction”, “repulsion” or “inactivity” are identified and the chemical trends leading to these behaviors are discussed. Notably, droplet inactivity, wherein no convective flow is observed, can still occur even when the droplet is solubilizing at appreciable rates. Droplets that are inactive can be induced to become active through doping of the surfactant continuous phase with another oil type. We believe these studies contribute to a new fundamental understanding of solubilization processes in emulsions and provide guidance as to how chemical parameters can be used to influence the dynamics and chemotactic interactions between active droplets.