Disordered hyperuniform state of circularly swimming algae*

Active matter comprises of individual units that convert locally stored energy into mechanical motion. Interactions between active units can lead to a wide range of collective phenomena, which do not exist in equilibrium systems. Here we show that density fluctuations in active system can be greatly suppressed. Our experiments are carried out with marine algae (Symbiodinium voratum). Cells swim in circles at interface and long-ranged nature of hydrodynamic interactions suppresses density fluctuations; disordered hyperuniform state is observed over a wide range of conditions. Emergence of hyperuniformity can be quantitatively reproduced in a numerical model whose main ingredients are hydrodynamic interactions and uncorrelated random cell motion. Our results demonstrate a new form of collective state in active matter and suggest the possibility to use hydrodynamic flow to assemble active matter into hyperuniform states.