Cooperative motion of active Brownian spheres in three-dimensional dense suspensions

Wysocki, Adam; Winkler, Roland G.; Gompper, Gerhard

doi:10.1209/0295-5075/105/48004

Cited by 259 publications

(338 citation statements)

References 38 publications

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“…This decrease is caused by the reduction in swim pressure due to the particles' tendency to form clusters, reducing the average distance they travel between reorientations, i.e., reducing the moment arm Uτ R . A study of structural properties [25] corroborates with our nonmonotonic pressure profiles. The probability distribution of the local volume fraction of swimmers computed using a Voronoi construction becomes bimodal at low Pe R and finite ϕ, indicating the presence of a dilute and dense phase.…”

supporting

confidence: 87%

Swim Pressure: Stress Generation in Active Matter

2014

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We discover a new contribution to the pressure (or stress) exerted by a suspension of self-propelled bodies. Through their self-motion, all active matter systems generate a unique swim pressure that is entirely athermal in origin. The origin of the swim pressure is based upon the notion that an active body would swim away in space unless confined by boundaries-this confinement pressure is precisely the swim pressure. Here we give the micromechanical basis for the swim stress and use this new perspective to study self-assembly and phase separation in active soft matter. The swim pressure gives rise to a nonequilibrium equation of state for active matter with pressure-volume phase diagrams that resemble a van der Waals loop from equilibrium gas-liquid coexistence. Theoretical predictions are corroborated by Brownian dynamics simulations. Our new swim stress perspective can help analyze and exploit a wide class of active soft matter, from swimming bacteria to catalytic nanobots to molecular motors that activate the cellular cytoskeleton. DOI: 10.1103/PhysRevLett.113.028103 PACS numbers: 47.63.mf, 05.65.+b, 47.63.Gd, 64.75.Xc From flocks of animals and insects to colonies of living bacteria, so-called active matter exhibits intriguing phenomena owing to its constituents' ability to convert chemical fuel into mechanical motion. Active matter systems generate their own internal stress, which drives them far from equilibrium and thus frees them from conventional thermodynamic constraints, and by so doing they can control and direct their own behavior and that of their surrounding environment. This gives rise to fascinating behavior such as spontaneous self-assembly and pattern formation [1-3] but also makes the theoretical understanding of their complex dynamical behaviors a challenging problem in the statistical physics of soft matter.In this Letter we have identified a new principle that all active matter systems display-namely, through their selfmotion they generate an intrinsic swim stress that impacts their dynamic and collective behavior. In contrast to thermodynamic quantities such as the chemical potential and free energy, the mechanical pressure (or stress) is valid out of equilibrium because it comes directly from the micromechanical equations of motion. To motivate this new perspective, we focus on the simplest model of active matter-a suspension of self-propelled spheres of radii a immersed in a continuous Newtonian solvent with viscosity η. The active particles translate with a constant, intrinsic swim velocity U 0 and tumble with a reorientation time τ R . We do not include the effects of hydrodynamic interactions among the particles.The origin of the swim pressure is based upon a simple notion-a self-propelled body would swim away in space unless confined by boundaries. The pressure exerted by the surrounding walls to contain the particle is precisely the swim pressure. This is similar to the kinetic theory of gases, where molecular collisions with the container walls exert a pressure. Although it is...

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confidence: 87%

Swim Pressure: Stress Generation in Active Matter

2014

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“…-We finally investigate the model at very high densities, approaching dynamical arrest from the fluid. Recent work supports the idea that dense assemblies of self-propelled particles can display a dramatic slow down of the dynamics sharing strong analogies with the glassy dynamics of particle systems in contact with a thermal bath [33,35,[41][42][43][44][45]. It was found in particular that activity shifts the glass transition towards higher densities (or lower temperatures), but leaves the main collective properties of the slow relaxation near the transition qualitatively unchanged [33,41,42].…”

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confidence: 61%

From single-particle to collective effective temperatures in an active fluid of self-propelled particles

Levis¹,

Berthier²

2015

EPL

101

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-We present a comprehensive analysis of effective temperatures based on fluctuationdissipation relations in a model of an active fluid composed of self-propelled hard disks. We first investigate the relevance of effective temperatures in the dilute and moderately dense fluids. We find that a unique effective temperature does not in general characterize the non-equilibrium dynamics of the active fluid over this broad range of densities, because fluctuation-dissipation relations yield a lengthscale-dependent effective temperature. By contrast, we find that the approach to a non-equilibrium glass transition at very large densities is accompanied by the emergence of a unique effective temperature shared by fluctuations at all lengthscales. This suggests that an effective thermal dynamics generically emerges at long times in very dense suspensions of active particles due to the collective freezing occurring at non-equilibrium glass transitions.Introduction. -Statistical mechanics provides a unified theoretical description of systems at thermal equilibrium in terms of the probability distribution over phase space, from which thermodynamic quantities such as temperature can be defined [1]. A similar framework is lacking for out-of-equilibrium systems for which the definition of a temperature remains an open issue [2]. Active matter formed by assemblies of living cells [3], bacteria [4], selfpropelled colloids [5][6][7][8] or grains [9,10], is a coherent class of non-equilibrium systems receiving increasing attention, because they raise fundamental issues and for potential applications in soft matter and biophysics [11,12]. A number of recent studies have addressed the question of whether "effective" thermodynamic concepts can be fruitfully applied to describe the phase behaviour and microscopic dynamics of active matter. This question is natural because if some mapping to an equilibrium situation exists, then the whole arsenal of equilibrium statistical mechanics becomes available for further theoretical treatment. In particular the definition of a non-equilibrium temperature [13][14][15][16][17][18], of an active pressure [8,19,20], of activityinduced interactions [8,21] and non-equilibrium free energies [22,23] have been investigated for self-propelled particles.

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“…Owing to the particles' self-motion, active matter can spontaneously phase-separate into dense and dilute regions (Cates et al 2010;Fily & Marchetti 2012;Buttinoni et al 2013;Palacci et al 2013;Stenhammar et al 2013Stenhammar et al , 2014Takatori, Yan & Brady 2014;Wysocki, Winkler & Gompper 2014;Yang, Manning & Marchetti 2014;Takatori & Brady 2015) and can move collectively under an orienting field .…”

Section: Introductionmentioning

confidence: 99%

The force on a boundary in active matter

2015

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We present a general theory for determining the force (and torque) exerted on a boundary (or body) in active matter. The theory extends the description of passive Brownian colloids to self-propelled active particles and applies for all ratios of the thermal energy k B T to the swimmer's activity k s T s = ζ U 2 0 τ R /6, where ζ is the Stokes drag coefficient, U 0 is the swim speed and τ R is the reorientation time of the active particles. The theory, which is valid on all length and time scales, has a natural microscopic length scale over which concentration and orientation distributions are confined near boundaries, but the microscopic length does not appear in the force. The swim pressure emerges naturally and dominates the behaviour when the boundary size is large compared to the swimmer's run length = U 0 τ R . The theory is used to predict the motion of bodies of all sizes immersed in active matter.

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Cooperative motion of active Brownian spheres in three-dimensional dense suspensions

Cited by 259 publications

References 38 publications

Swim Pressure: Stress Generation in Active Matter

Swim Pressure: Stress Generation in Active Matter

From single-particle to collective effective temperatures in an active fluid of self-propelled particles

The force on a boundary in active matter

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