Rheotaxis of Active Droplets

TL;DR

This study provides the first experimental evidence of upstream rheotaxis in spherical active droplets, showing how flow gradients and elastic stresses influence their movement, with potential biomedical applications.

Contribution

It demonstrates the occurrence of upstream rheotaxis in spherical active droplets and explores the mechanisms and potential applications of this behavior.

Findings

Rheotaxis occurs only within a specific shear rate range.

Flow-induced micelle distortion propels droplets against flow.

Nematic droplets show elastic stress-induced oscillations during rheotaxis.

Abstract

Rheotaxis is a well-known phenomenon among microbial organisms and artificial active colloids, wherein the swimmers respond to an imposed flow. We report the first experimental evidence of upstream rheotaxis by spherical active droplets. It is shown that the presence of a nearby wall and the resulting strong flow-gradient at the droplet level is at the root of this phenomenon. Experiments with optical cells of different heights reveal that rheotaxis is observed only for a finite range of shear rates, independent of the bulk flow-rate. We conjecture that the flow induced distortion of an otherwise isotropic distribution of filled/empty micelles around the droplet propels it against the flow. We also show that nematic droplets exhibit elastic stress-induced oscillations during their rheotactic flight. A promising potential of manipulating the rheotactic behavior to trap as well as shuttle droplets between target locations is demonstrated, paving way to potentially significant advancement in bio-medical applications.