Title: Swimming droplets undergo active rotational diffusion with tunable velocity and persistance time

Author (Talk): Adrien Izzet, NYU

Abstract:

Here we characterize the motility of athermal swimming droplets within the framework of active rotational diffusion. Just like active colloids, their trajectories can be modeled with a constant velocity $v$ and a slow angular diffusion, but the random changes in direction are not thermally driven. Instead, $v$ is determined by the interfacial tension gradient along the droplet surface, while local micellar fluctuations lead to droplet reorientation with a persistence time $\tau$. We unravel a novel mechanism for locomotion, in which the difference in the critical micellar concentration $\Delta$CMC in the front and the back of the droplet, tuned by salt, controls $v$ from $3-15$diameters (d)/s. Surfactant concentration has no effect on speed, but leads to a dramatic decrease in $\tau$ from 300s-30ms. The corresponding range of the effective diffusion constant $D_e$ extends beyond the realm of synthetic or living swimmers, in which $v$ is limited by fuel consumption and $\tau$ is set by temperature or biological activity, respectively. Our tunable swimmers are ideal candidates for the study of the departure from equilibrium to high levels of activity, on both the single particle level and their collective behavior, including the motility-induced phase separation (MIPS).