Tunable Persistent Random Walk in Swimming Droplets

Adrien Izzet,Jasna Brujic,Jan Groenewold,Pepijn G Moerman,Preston Gross,Andrew D Hollingsworth,Jérôme Bibette

doi:10.1103/physrevx.10.021035

Abstract

The random motion of oil droplets in water is caused by the flow of surfactants at the interface, a finding that gives rise to a broadly tunable swimming system, akin to microorganisms, and allows us to study their self-organization.

Highlights

We characterize the motility of athermal swimming droplets within the framework of a persistent random walk
V is determined by the interfacial tension gradient along the droplet surface, while reorientation of the surfactant gradient leads to changes in direction with a persistence time τ
We show that the origin of locomotion is the difference in the critical micellar concentration in the front and the back of the droplet, ΔCMC

Summary

Tunable Persistent Random Walk in Swimming Droplets

They consume fuel preferentially on one side of the particle, which results in chemical gradients that drive and direct self-propulsion On short timescales, these swimmers move in a straight line, but they are small enough that thermal diffusion randomizes the swimming direction on long timescales. It is thought that droplet motion itself is sufficient to cause the asymmetry needed to sustain a surfactant gradient, as the droplet encounters empty micelles in the front and leaves filled ones at the back [5,6] This Marangoni effect is at play in all droplet swimmers: direct emulsions made of oil [7] or nematic liquid crystals [8,9,10,11], as well as inverse emulsions [12,13]. Because swimming droplets are typically athermal, ranging in size from 10 to 300 μm, their reorientation cannot be a thermally driven process and

Published by the American Physical Society

Δt τ þ exp

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