Electrotaxis of artificial swimmers in microchannels

Classically, electrotaxis refers to the directed motion of biological cells or motile organisms, like paramecium, under an applied electric field. Here, we experimentally demonstrate that self-propelled artificial microswimmers also exhibit autonomous changes in their motility to undergo electrotaxis, by considering active droplets as a model system. When a uniform electric field is applied along a microchannel, these self-propelled droplet microswimmers alter their trajectory to swim along the direction of the applied field thereby exhibiting negative electrotaxis. Previously, we demonstrated that an active droplet also autonomously navigates upstream of an external flow (i.e. exhibits positive rheotaxis) in a microchannel in an oscillatory trajectory. Here, we show that on application of an electric field during rheotaxis, the droplet microswimmer attains a steady trajectory along the microchannel center-line, instead of a stable oscillatory trajectory. Finally, we demonstrate that the essential features of the electrotactic and electro-rheotactic behaviour can be explained using a far-field hydrodynamic model considering the electrical response and hydrodynamic interactions of the active microswimmers. Interestingly, the electro-rheotactic behaviour of the microswimmer can be understood as a Hopf bifurcation from a dynamical system perspective. Such electrical sensing of self-propelled artifical microswimmers can be judiciously manipulated for targeted cargo delivery.