We study the collective dynamics of elongated swimmers in a very thin fluid layer by devising long, filamentous, non-tumbling bacteria. The strong confinement induces weak nematic alignment upon collision, which, for large enough density of cells, gives rise to global nematic order. This homogeneous but fluctuating phase, observed on the largest experimentally-accessible scale of millimeters, exhibits the properties predicted by standard models for flocking such as the Vicsek-style model of polar particles with nematic alignment: true long-range nematic order and non-trivial giant number fluctuations.PACS numbers: 47.63. Gd, 05.65.+b 87.18.Gh 87.18.Hf Collective motion of self-propelled elements, as seen in bird flocks, fish schools, bacterial swarms, etc., is so ubiquitous that it has driven physicists to search for its possibly universal properties [1][2][3]. If generic and robust features of such active matter systems exist, they should also be present in the emergent phenomena observed in simple models. Evidence for such universality has been provided by many theoretical and numerical studies of dry active matter systems where local alignment competes with noise, following the seminal works by Vicsek et al.[4], Toner, Tu , and Ramaswamy et al. [5][6][7][8][9]. It was notably understood that the transition to orientational order/collective motion, in this context, is best described as a phase-separation between a disordered gas and an ordered 'liquid' separated by a coexistence phase whose nature depends on the symmetries of the system [3,[10][11][12][13][14][15][16]. The homogeneous but highly fluctuating liquid phase is characterized by unique properties often different from those of equilibrium orientationally-ordered phases. In particular, the crucial coupling between the order and the density fields generates anomalously-large number fluctuations from the algebraic correlations of orientation and density [5][6][7][8][9].Such 'giant' number fluctuations (GNF), being relatively easy to measure experimentally, have become the landmark signature of orientationally-ordered active matter. Several experimental studies have indeed searched for GNF using controllable systems simpler than bird flocks and fish schools such as biofilaments driven by molecular motors [17], colloids consuming electric energy [18], shaken granular materials [19][20][21], monolayers of fibroblast cells [22], and common bacteria [23,24]. However, none of these experiments has been fully convincing in demonstrating the presence of bona fide GNF * nishiguchi@daisy.phys.s.u-tokyo.ac.jp as predicted from the works of Toner, Tu, Ramaswamy et al. [3, 7, 8], and observed in Vicsek-style models [10, 11, 13, 14]. These GNF have to be discussed in a fluctuating phase with global long-range orientational order and are distinct from the trivial, non-asymptotic ones present in the case of phase-separation into dense clusters sitting in a disordered sparse gas. In some experiments, only normal number fluctuations were found [17,18]. In others, GNF ...
To elucidate mechanisms of mesoscopic turbulence exhibited by active particles, we experimentally study turbulent states of nonliving self-propelled particles. We realize an experimental system with dense suspensions of asymmetrical colloidal particles (Janus particles) self-propelling on a two-dimensional surface under an ac electric field. Velocity fields of the Janus particles in the crowded situation can be regarded as a sort of turbulence because it contains many vortices and their velocities change abruptly. Correlation functions of their velocity field reveal the coexistence of polar alignment and antiparallel alignment interactions, which is considered to trigger mesoscopic turbulence. Probability distributions of local order parameters for polar and nematic orders indicate the formation of local clusters with particles moving in the same direction. A broad peak in the energy spectrum of the velocity field appears at the spatial scales where the polar alignment and the cluster formation are observed. Energy is injected at the particle scale and conserved quantities such as energy could be cascading toward the larger clusters.
A suspension of swimming bacteria is possibly the simplest realization of active matter, i.e. a class of systems transducing stored energy into mechanical motion. Collective swimming of hydrodynamically interacting bacteria resembles turbulent flow. This seemingly chaotic motion can be rectified by a geometrical confinement. Here we report on self-organization of a concentrated suspension of motile bacteria Bacillus subtilis constrained by two-dimensional (2D) periodic arrays of microscopic vertical pillars. We show that bacteria self-organize into a lattice of hydrodynamically bound vortices with a long-range antiferromagnetic order controlled by the pillars’ spacing. The patterns attain their highest stability and nearly perfect order for the pillar spacing comparable with an intrinsic vortex size of an unconstrained bacterial turbulence. We demonstrate that the emergent antiferromagnetic order can be further manipulated and turned into a ferromagnetic state by introducing chiral pillars. This strategy can be used to control a wide class of active 2D systems.
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