Rheotaxis of active droplets

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.

Automated summary

Rheotaxis refers to the phenomenon where microbial organisms and artificial active colloids respond to imposed flow. The experimental evidence of upstream rheotaxis by spherical active droplets is reported for the first time, with the strong flow-gradient at the droplet level being identified as the root cause. This behavior is observed within a finite range of shear rates, independent of bulk flow rate, and there is potential for manipulating it for biomedical applications.