Circular swimming motility and disordered hyperuniform state in an algae system

TL;DR

This study demonstrates that marine algae swimming in circles can suppress density fluctuations and form disordered hyperuniform states due to hydrodynamic interactions, revealing a new collective behavior in active matter.

Contribution

It introduces a novel collective state in active matter where hydrodynamic interactions lead to hyperuniformity, contrasting with typical large fluctuations.

Findings

Density fluctuations are suppressed in algae systems due to effective hydrodynamic repulsions.

Disordered hyperuniform states are observed across various densities.

Numerical models reproduce the emergence of hyperuniformity based on hydrodynamics.

Abstract

Active matter comprises individually driven units that convert locally stored energy into mechanical motion. Interactions between driven units lead to a variety of non-equilibrium collective phenomena in active matter. One of such phenomena is anomalously large density fluctuations, which have been observed in both experiments and theories. Here we show that, on the contrary, density fluctuations in active matter can also be greatly suppressed. Our experiments are carried out with marine algae ($\it{Effrenium\ voratum}$) which swim in circles at the air-liquid interfaces with two different eukaryotic flagella. Cell swimming generates fluid flow which leads to effective repulsions between cells in the far field. Long-range nature of such repulsive interactions suppresses density fluctuations and generates disordered hyperuniform states under a wide range of density conditions. Emergence of hyperuniformity and associated scaling exponent are quantitatively reproduced in a numerical model whose main ingredients are effective 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 for self-assembly in active matter.