Activity waves and freestanding vortices in populations of subcritical Quincke rollers
Résumé
Significance Active matter is made of units that move or displace others by using energy stored internally or gathered from their environment. In most systems and models considered so far, these self-propelled units are constantly moving. Here we study active matter made of units that do not move when isolated, but can be set into motion by close neighbors. Our subcritical active matter consists of Quincke rollers, that is, colloidal spheres at the bottom of a cell filled with conducting fluid submitted to a vertical electric field. We find spectacular collective self-organized phenomena: activity waves propagating in a quiescent population, and arbitrarily large, steadily rotating vortices forming without confinement or particle chirality.