ESPCI web site
Jasna Brujic, NYU
Dynamics of Active Droplets
Most active particles, such as Janus colloids, are anisotropic and their orientation determines the swimming direction. This direction is randomized by thermal fluctuations and their motility is described by passive rotational diffusion. By contrast, oil droplets of DEP spontaneously break symmetry as they dissolve in water, resulting in self-propulsion at a fixed speed, largely independent of surfactant concentration. Droplet trajectories also undergo slow angular diffusion, but they change direction via active interactions with micelles in solution. Increasing their bulk concentration decreases the effective diffusion constant four orders of magnitude. The turning of the droplets may be governed by fluctuations in surrounding micellar concentrations, which in turn polarize the surfactant gradient on the surface that directs droplet motion. This active system allows one to tune the effective temperature of the system without changing the velocity. Such control could be useful for studying active phenomena, such as non-equilibrium phase separation and clustering.