Controlled reversal of vortex chirality in populations of colloidal rollers*

Chiral active liquids composed of spinning individual units represent a new class of active materials where both energy and angular momentum is injected at the microscopic level. Spontaneous emergence of particle flocks and global polar states are prime examples of remarkable collective dynamics and self-organization recently observed in active chiral liquids. Formation of the globally correlated polar states in such systems proceeds through an emergence of a macroscopic steadily rotating vortex that spontaneously selects a clockwise or counterclockwise global chiral state. We demonstrate in experiments and simulations that active chiral liquids in a collective vortex state exhibit memory and the subsequent formation of the polar states is not random. Our results provide new fundamental insights into mechanisms of formation of collective polar states in active systems.