Arrested on heating: controlling the motility of active droplets by temperature

Abstract

One of the challenges in tailoring the dynamics of active, self-propelling agents lies in arresting and releasing these agents at will. Here, we present an experimental system of active droplets with thermally controllable and reversible states of motion, from unsteady over meandering to persistent to arrested motion. These states depend on the Péclet number of the chemical reaction driving the motion, which we can tune by using a temperature sensitive mixture of surfactants as a fuel medium. We quantify the droplet dynamics by analysing flow and chemical fields for the individual states, comparing them to canonical models for autophoretic particles. In the context of these models, we are able to observe in situ the fundamental first transition between the isotropic, immotile base state and self-propelled motility.

Motility of microscopic swimmers as protocells

“…In addition to light, temperature can also be used to control stop-and-go cycles of motile droplets by affecting their micellar solubilization. 68 In the work by Maass and coworkers, droplets immersed in surfactant water displayed Marangoni propulsion, provided that the surfactant concentration, e.g., TTAB, was substantially larger than critical micellar concentration. When coupled with a cosurfactant (PF127), above a critical temperature, micelles composed of both surfactant and cosurfactant were formed, and TTAB was subtracted from the micellar solubilization process, driving the droplet's propulsion and arresting their swimming.…”