Exploring order in active turbulence: Geometric rule and pairing order transition in confined bacterial vortices

Ordered collective motion emerges in a group of autonomously motile elements (known as active matter) as their density increases. Microswimmers, such as swimming bacteria, have been extensively studied in physics and biology. A dense suspension of bacteria forms seemingly chaotic turbulence in viscous fluids. Interestingly, this active turbulence driven by bacteria can form a hidden ensemble of many vortices. Understanding the active turbulence in a bacterial suspension can provide physical principles for pattern formation and insight into the instability underlying biological phenomena. This review presents recent findings regarding ordered structures causing active turbulence and discusses a physical approach for controlling active turbulence via geometric confinement. When the active matter is confined in a compartment with a size comparable to the correlation length of the collective motion, vortex-like rotation appears, and the vortex pairing order is indicated by the patterns of interacting vortices. Additionally, we outline the design principle for controlling collective motions via the geometric rule of the vortex pairing, which may advance engineering microdevices driven by a group of active matter. This article is an extended version of the Japanese article, Ordered Structure and Geometric Control of Active Matter in Dense Bacterial Suspensions, published in SEIBUTSU BUTSURI Vol. 60, p. 13-18 (2020).