# Tunable active rotational diffusion in swimming droplets

## TL;DR

This paper investigates the motility of active swimming droplets, demonstrating how their speed and reorientation times can be tuned by chemical parameters, providing a versatile platform for studying active matter and phase separation.

## Contribution

It introduces a model for active rotational diffusion in swimming droplets, showing how chemical tuning controls their motility and enabling studies of non-equilibrium collective behaviors.

## Key findings

- Droplet speed $V$ can be tuned from 3-15 diameters/sec by salt concentration.
- Reorientation time $ au$ decreases dramatically with surfactant concentration.
- Effective diffusion constant $D_e$ exceeds typical synthetic or biological swimmers.

## 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 show that the origin of locomotion is the difference in the critical micellar concentration $\Delta$CMC in the front and the back of the droplet. Tuning this parameter by salt controls $V$ from $3-15$ diameters $d/s$. Surfactant concentration has little effect on speed, but leads to a dramatic decrease in $\tau$ over four orders of magnitude. 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).