Collective motion of self-propelled elements, as seen in bird flocks, fish schools, bacterial swarms, etc., is so ubiquitous. Physicists’ efforts to find their universal properties now constitute a new field in nonequilibrium statistical physics: “active matter physics”. Evidence for such universality has been provided by many theoretical and numerical studies using simple models. However, no experiments so far have been fully convincing in demonstrating this universality. In this seminar, after introducing standard models of collective motion and giving state-of-the-art interpretations on previous experimental studies, I show our experiments on collective dynamics of elongated bacteria swimming in a quasi-two-dimensional fluid layer [1]. Strong confinement and the high aspect ratio of bacteria induce weak nematic alignment upon collision, which gives rise to spontaneous breaking of rotational symmetry and global nematic order at sufficiently high density of bacteria. This homogeneous but fluctuating ordered phase has turned out to have true long-range orientational order and non-trivial giant number fluctuations associated with Nambu-Goldstone modes, which verifies the existence of an active nematic phase predicted to universally emerge in simple standard models. Through our experiments, I will also discuss what makes our system different from previous experiments and what might be crucial for the emergence of such universality in reality.

[1] D. Nishiguchi, K. H. Nagai, H. Chate, and M. Sano, “Long-range nematic order and anomalous fluctuations in suspensions of swimming filamentous bacteria”, Physical Review E, 95, 020601(R) (2017).