Mesoscopic turbulence and local order in Janus particles self-propelling under an ac electric field

Nishiguchi, Daiki; Sano, Masaki

doi:10.1103/physreve.92.052309

Cited by 119 publications

(91 citation statements)

References 46 publications

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“…It was applied to characterize active turbulence in bacterial [169] and active colloidal [13] suspensions. An order parameter for the strength of vorticity in the fluid is the so-called enstrophy, which is the spatial average of ω 2 .…”

Section: Dmentioning

confidence: 99%

Emergent behavior in active colloids

Zöttl

Stark

2016

J. Phys.: Condens. Matter

511

559

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Active colloids are microscopic particles, which self-propel through viscous fluids by converting energy extracted from their environment into directed motion. We first explain how articial microswimmers move forward by generating near-surface flow fields via self-phoresis or the self-induced Marangoni effect. We then discuss generic features of the dynamics of single active colloids in bulk and in confinement, as well as in the presence of gravity, field gradients, and fluid flow. In the third part, we review the emergent collective behavior of active colloidal suspensions focussing on their structural and dynamic properties. After summarizing experimental observations, we give an overview on the progress in modeling collectively moving active colloids. While active Brownian particles are heavily used to study collective dynamics on large scales, more advanced methods are necessary to explore the importance of hydrodynamic and phoretic particle interactions. Finally, the relevant physical approaches to quantify the emergent collective behavior are presented.

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Section: Dmentioning

confidence: 99%

Emergent behavior in active colloids

Zöttl

Stark

2016

J. Phys.: Condens. Matter

511

559

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“…In experiments at low density of cells, or with a larger spacing (∼ 10 µm) between the two surfaces, cells do not align enough to order on large scales ( Since it is very difficult to determine the polarity θ of each bacterium at such large concentration, a direct estimate of the nematic order parameter Q = | e 2iθ | previously used even in experiments [39] is out of reach, and we opted instead for the 'structure tensor' method used previously, e.g., for measuring the orientation of collagen fibers [40]. Specifically, given an intensity-calibrated image f (x, y), one calculates the following tensor over a given region of interest (ROI):…”

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confidence: 99%

Long-range nematic order and anomalous fluctuations in suspensions of swimming filamentous bacteria

et al. 2017

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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 ...

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“…DOI: 10.1103/PhysRevE.94.020601 A universal feature shared by many living systems is the emergence of characteristic length and time scales that arise from the nonequilibrium dynamics of their microscopic constituents. Examples range from circadian oscillations in individual cells [1] to multicellular gene-expression patterns in embryos [2] and vortex structures in microbial suspensions, endothelial tissue, and active colloids [3][4][5][6]. Yet, despite their broad biological relevance, it has proved difficult to predict quantitatively how such emergent scales arise from the underlying chemical or physical parameters.…”

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confidence: 99%

Hydrodynamic length-scale selection in microswimmer suspensions

et al. 2016

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A universal characteristic of mesoscale turbulence in active suspensions is the emergence of a typical vortex length scale, distinctly different from the scale invariance of turbulent high-Reynolds number flows. Collective length-scale selection has been observed in bacterial fluids, endothelial tissue, and active colloids, yet the physical origins of this phenomenon remain elusive. Here, we systematically derive an effective fourth-order field theory from a generic microscopic model that allows us to predict the typical vortex size in microswimmer suspensions. Building on a self-consistent closure condition, the derivation shows that the vortex length scale is determined by the competition between local alignment forces, rotational diffusion, and intermediate-range hydrodynamic interactions. Vortex structures found in simulations of the theory agree with recent measurements in Bacillus subtilis suspensions. Moreover, our approach yields an effective viscosity enhancement (reduction), as reported experimentally for puller (pusher) microorganisms. DOI: 10.1103/PhysRevE.94.020601 A universal feature shared by many living systems is the emergence of characteristic length and time scales that arise from the nonequilibrium dynamics of their microscopic constituents. Examples range from circadian oscillations in individual cells [1] to multicellular gene-expression patterns in embryos [2] and vortex structures in microbial suspensions, endothelial tissue, and active colloids [3][4][5][6]. Yet, despite their broad biological relevance, it has proved difficult to predict quantitatively how such emergent scales arise from the underlying chemical or physical parameters. In the past decade, bacterial and other active suspensions [4,5,7] have emerged as important biophysical model systems that can help bridge the gap between large-scale spatiotemporal pattern formation and microscopic nonequilibrium dynamics [8]. At high densities, bacterial fluids form coherent vortex structures, spanning several cell lengths in diameter [5,7,9] and persisting for several seconds [9] or even minutes [10][11][12]. Although a number of insightful theoretical models have been proposed [13][14][15][16][17][18], a theory connecting microswimmer properties to the experimentally observed vortex patterns has been lacking.Here, we present such a theory by drawing guidance from the recent observation [5,9] that an effective fourth-order continuum model can provide a quantitative phenomenological description of dense bacterial suspensions [19]. This model, which combines the seminal Toner-Tu description of flocking [20] with the Swift-Hohenberg equation from pattern formation [21], describes the effective bacterial velocity field w(t,x) bywhere the bacterial pressure field q(t,x) accounts for incompressibility, ∇ · w = 0. Although a direct fit of Eq. (1) (1) directly from a generic model for polar microswimmers. The derivation specifies each parameter in the continuum theory in terms of microscopic swimmer parameters and yields direct theoretical ...

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Mesoscopic turbulence and local order in Janus particles self-propelling under an ac electric field

Cited by 119 publications

References 46 publications

Emergent behavior in active colloids

Emergent behavior in active colloids

Long-range nematic order and anomalous fluctuations in suspensions of swimming filamentous bacteria

Hydrodynamic length-scale selection in microswimmer suspensions

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