Polar state memory in active fluids*

Active liquids composed of densely packed spinning units represent a new class of active materials where both energy and angular momentum are injected into the system at the microscopic level. Spontaneous emergence of global polar states such as particle flocks and vortices are prime examples of remarkable collective dynamics and self-organization observed in active liquids. The formation of globally correlated polar states in geometrically confined systems proceeds through the emergence of a macroscopic steadily rotating vortex that spontaneously selects a clockwise or counterclockwise global chiral state. Here, we reveal that a global vortex of active rollers exhibits state memory. The instantaneous inter-particle positional order encodes the information about the chiral state. This information remains stored even if the energy injection is ceased and activity is terminated. When the system is re-energized, the subsequent formation of the collective states is not random. We demonstrate in experiments and simulations a controlled sequence of the emergent vortical states in an ensemble of Quincke rollers. Our work provides new fundamental insights into mechanisms of the spontaneous formation of the collective polar states in active systems. The particle local arrangement and inter-particle interactions can be exploited to systematically command the subsequent polar states of an active liquid through temporal control of the activity. With chirality of the emergent collective states controlled on-demand, active polar liquids offer new possibilities for flow manipulation, transport, and mixing at the microscale.