Active particles in external fields

In external fields, active particles perform taxis by moving along or against the field. This directional migration can be taken advantage of in controlling the active matter using external fields. For example, in viscosity gradients, active squirmer type particles rotate to align against the gradients and swim in this steady state orientation with a speed different from that in the absence of the gradients. Here, we discuss how the boundary conditions on the particle can crucially alter the particle dynamics in the viscosity gradients. Unlike the viscosity gradients, more generally, an independent control of the particle speed and orientation is possible using two different fields. In the presence of a field that modulates the speed spatially, particles accumulate in the regions of low speed, whereas in the presence of a field that reorients the particles, they rotate to align along or against the field and accumulate in the downstream or upstream relative to the field. Here, we discuss this accumulation in the presence of walls and how reorienting fields can be used to clean off the accumulation at the walls.