Chemical Design of Self-Propelled Janus Droplets

The study of active colloidal microswimmers with tunable phoretic and self-organizational behaviors is important for understanding out-of-equilibrium systems and the design of functional, adaptive matter. Solubilizing, self-propelling droplets have emerged as a rich chemical platform for exploration of active behaviors, but isotropic droplets rely on spontaneous symmetry breaking to sustain motion. The introduction of permanent asymmetry, e.g. in the form of a biphasic Janus droplet, has not been explored previously as a comprehensive design strategy for active droplets, despite the widespread use of Janus structures in motile solid particles. Here, we uncover the chemomechanical framework underlying the self-propulsion of active, biphasic Janus oil droplets solubilizing in aqueous surfactant. We elucidate how droplet propulsion is influenced by the degree of oil mixing, droplet shape, and oil solubilization rates for a range of oil combinations. A key finding is that for droplets containing both a mobile (solubilizing) and non-mobile oil, the degree of partitioning of the mobile oil across the Janus droplets’ oil-oil interface plays a pivotal role in determining the droplet speed and swimming direction. As a result, we observe propulsion speeds of Janus droplets more than an order-of-magnitude faster than chasing pairs of single emulsion droplets which lack an oil-oil interface. In addition, spatiotemporal control over droplet swimming speed and orientation is demonstrated through the application of local thermal gradients applied via induced via joule heading and laser spot illumination. We also explore the interactions between collections of Janus droplets including the spontaneous formation of multi-droplet spinning clusters that rotate predictably based on symmetry. Our findings provide key insights as to how the chemistry and structure of multiphase fluids can be harnessed to design microswimmers with programmable active and collective behaviors.