Active Caustics

We show that self-propelled particles (SPPs) with no mechanical inertia form caustics when advected by vortical flow. This seemingly inertial behaviour is a result of the persistence of their motion in the direction of their intrinsic orientation. We establish that SPPs form caustics at a critical distance from the origin of a single point vortex, and in straining regions when advected by 2D Navier-Stokes turbulence. This mechanism for singularly high local concentrations in a macroscopically dilute suspension of zero-Re swimmers has potential implications for communication and sexual reproduction. This work was done in collaboration with Rahul Chajwa, Rajarshi, and Rama Govindarajan; see arXiv:2310.01829.

Event resources