Swimming Emulsion Droplets*

Active droplets create concentration gradients in their surroundings and therefore modify the local composition of their solvent. These solute-mediated interactions depend on droplet size and solute concentration, as we have shown in the case of di-ethyl phthalate (DEP) droplets in an aqueous solution of sodium dodecyl sulphate (SDS) [P. G. Moerman et al. PRE 96, 032607 (2017)]. Around a DEP droplet, the concentration gradient is initially isotropic but fluctuations can give rise to self-sustained motion in a random direction. These droplets exhibit different motility profiles, from ballistic to sub-diffusive, due to the dynamical interplay between the concentration gradient they generate and their autophoretic motion. We explore a wide range of experimental control parameters, such as the fuel concentration, droplet size, and droplet-droplet interactions, in search of a unifying framework to understand this active system.