Tunable active rotational diffusion in swimming droplets

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).

7 pages, 8 figures

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• Instability and self-propulsion of active droplets along a wall
• Inclusions, Boundaries and Disorder in Scalar Active Matter
• Rheotaxis of Active Droplets
• Self-propulsion of active droplets without liquid-crystalline order
• Interfacial activity dynamics of confined active droplets
• Universal cover-time distribution of heterogeneous random walks
• Self-avoidant memory effects on enhanced diffusion in a stochastic model of environmentally responsive swimming droplets
• A Reduced Model for a Phoretic Swimmer
• Steady state propulsion of isotropic active colloids along a wall
• Conditions for the propulsion of a colloid surrounded by a mesoscale phase separation
• A numerical framework for phoretic particles
• Active adaptolates: motility-induced percolating structures with an adaptive packing geometry