Abstrakt
The colocalization of density modulations and particle polarization is a characteristic emergent feature of motile active matter in activity gradients. We employ the active-Brownian-particle model to derive precise analytical expressions for the density and polarization profiles of a single Janus-type swimmer in the vicinity of an abrupt activity step.
Our analysis allows for an optional (but not necessary) orientation-dependent propulsion speed, as often employed in force-free particle steering. The results agree well with measurement data for a thermophoretic microswimmer presented in the companion paper [Soker et al., Phys.
Rev. Lett. 126, 228001 (2021)], and they can serve as a template for more complex applications, e.g., to motility-induced phase separation or studies of physical boundaries.
The essential physics behind our formal results is robustly captured and elucidated by a schematic two-species "run-and-tumble" model.