The lifetime of swimming droplets

Active droplets serve as a model system for studying chemotaxis, solute-mediate interactions, and locomotion of micro-organisms, e.g., algae and bacteria. They are intrinsically isotropic and self-propel by spontaneously breaking symmetry [1], by contrast to Janus particles that swim as a result of built-in design asymmetries. Here we study the motion of swimming droplets made of diethyl phthalate oil (DEP) in varying concentrations of aqueous sodium dodecyl sulphate (SDS) solution above the critical micellar concentration [2]. During the course of a lifetime of a droplet, three motility types are identified - self-avoidance, oscillations, and active Brownian particle (ABP) behavior. Analyzing experimental trajectories allows us to map out the probability distribution of the said motility types as a function of both the SDS concentration and the shrinking droplet size. Comparing the experimental data with distinct theoretical models describing ABP, non-Markovian [3], or true self-avoiding random walks (TSAW) [4] opens the path to understanding the microscopic origin of the locomotion of the droplets.