Droplet microswimmers: chemohydrodynamic and collective effects

In this thesis, we investigate the chemohydrodynamics of droplet microswimmers from the individual to the collective scale. A popular experimental realization of a droplet micro-swimmer consists of oil-in-water (alternatively, water-in-oil) emulsions in which the outer phase is a supramicellar surfactant solution that acts as a fuel for droplet propulsion. Through a non-linear coupling of chemical transport and fluid flow, a self-amplifying interfacial Marangoni gradient is created, which leads to droplet self-propulsion.
In the first two chapters of the thesis, we focus on individual droplet dynamics. We investigate droplet micropumps and a new way of immobilizing droplets by heat (“heat arrest”). Droplet micropumps refer to droplets that are immobilized between the top and bottom of a microfluidic cell. We rationalize their multistable and self-throttling dynamics via the competing time scales of slow micellar diffusion, which governs the chemical buildup, and faster molecular diffusion, which powers the underlying transport mechanism. In the case of droplet heat arrest, we examine active droplets in a temperature-sensitive mixture of co-surfactants, where increasing temperature leads to the depletion of available fuel. With increasing temperature, the droplet motion transitions across different modes of mobility, from meandering to straight to eventual arrest. Upon cooling, the droplet arrest is extended with a sudden onset of motion at a lower temperature. In this specific system, we are able to document the fundamental transition from passive dissolution to active self-propulsion.
In the last three chapters of the thesis, we study the collective dynamics of active droplets. The constituent droplets of an active emulsion sediment inside a reservoir, hydrodynamically interact, and form ordered planar hexagonal clusters. We observe both static and rotating clusters. We quantify the convection roll around a droplet cluster and map a system spanning chemical buildup at the bottom boundary. We further study the effect of increasing the droplet number density, which promotes the merging and growth of clusters and eventually leads to the formation of layered clusters. On extending study to cover binary active emulsions, we observe highly diverse aggregation regimes and spontaneous demixing dependent on the surfactant concentration.