Active particles in external fields

In external fields such as heat, light or viscosity gradients, active particles perform taxis by aligning along or against the field and propelling along this steady state orientation with a speed that is different from that in the absence of the field. We discuss how the boundary conditions on the particle can crucially alter the dynamics of active particles in viscosity gradients and hence the viscotaxis phenomenon. More generally, the external fields can be used to control the speed and orientation of the active particles, ultimately allowing one to control the accumulation of active matter. In a field that weakly reorients the particles, the particles accumulate in the regions of low speed, but in a field that strongly reorients the particles, they align along or against the field and accumulate downstream or upstream relative to the field. In this talk we discuss these effects of external fields on particles, particularly in the presence of walls and on the wall-bound accumulation of active matter.