Pushers, pullers, splitters: molecular processes governing the dynamics of active emulsions.

Active emulsions are a versatile microswimmer model system. We study a system of spherical oil droplets gradually dissolving in aqueous surfactant solutions, which move autonomously via self-sustaining Marangoni gradients at the oil-water interface. Owing to this interfacial driving, the droplets are direct experimental analogues to hydrodynamic squirmer models, with the interfacial velocity profile depending on the kinetics of surfactant micelles taking up oil from the swimming droplet. We have observed both hydrodynamic and chemical fields around the droplets and found that the system's geometry, length scales and solubilisation timescales govern various effects: pusher to puller type hydrodynamics; persistent to unsteady swimming; and, for large squeezed droplets, interfacial instabilities leading to deformation and division states.