Active matter ratchets with an external drift

Reichhardt, C.

doi:10.1103/physreve.88.062310

Cited by 63 publications

(61 citation statements)

References 93 publications

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“…It is also possible to obtain ratchet effects for circle swimmers moving over asymmetric substrates (89). Figure 13(a) shows a counterclockwise swimming particle moving with a motor force amplitude of A = 0.57 over an array of L-shaped barriers.…”

Section: Ratchets In Circle Swimmers and Chiral Systemsmentioning

confidence: 99%

Ratchet Effects in Active Matter Systems

Reichhardt

2017

Annu. Rev. Condens. Matter Phys.

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217

169

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Ratchet effects can arise for single or collectively interacting Brownian particles on an asymmetric substrate when a net dc transport is produced by an externally applied ac driving force or by periodically flashing the substrate. Recently, a new class of active ratchet systems has been realized through the use of active matter, which are self-propelled units that can be biological or non-biological in nature. When active materials such as swimming bacteria interact with an asymmetric substrate, a net dc directed motion can arise even without external driving, opening a wealth of possibilities such as sorting, cargo transport, or micromachine construction. We review the current status of active matter ratchets for swimming bacteria, cells, active colloids, and swarming models, focusing on the role of particle-substrate interactions. We describe ratchet reversals produced by collective effects and the use of active ratchets to transport passive particles. We discuss future directions including deformable substrates or particles, the role of different swimming modes, varied particle-particle interactions, and non-dissipative effects.

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Section: Ratchets In Circle Swimmers and Chiral Systemsmentioning

confidence: 99%

Ratchet Effects in Active Matter Systems

Reichhardt

2017

Annu. Rev. Condens. Matter Phys.

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217

169

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“…The full proof, allowing for pairwise interactions, is given in [38]. Interestingly, the equation of state is valid for any wall, including asymmetric ones which, in the spirit of ratchets [5,[40][41][42][43], may induce a net particle current along the wall. Such a current, whose direction can be controlled by the asymmetry of the wall, is accompanied by a shear force exerted by the active particles on the wall, parallel to its general surface.…”

Section: Fig 1c)mentioning

confidence: 99%

Active Particles with Soft and Curved Walls: Equation of State, Ratchets, and Instabilities

et al. 2016

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We study, from first principles, the pressure exerted by an active fluid of spherical particles on general boundaries in two dimensions. We show that, despite the non-uniform pressure along curved walls, an equation of state is recovered upon a proper spatial averaging. This holds even in the presence of pairwise interactions between particles or when asymmetric walls induce ratchet currents, which are accompanied by spontaneous shear stresses on the walls. For flexible obstacles, the pressure inhomogeneities lead to a modulational instability as well as to the spontaneous motion of short semi-flexible filaments. Finally, we relate the force exerted on objects immersed in active baths to the particle flux they generate around them.Active forces have recently attracted much interest in many different contexts [1]. In biology, they play crucial roles on scales ranging from the microscopic, where they control cell shape and motion [2] to the macroscopic, where they play a dominant role in tissue dynamics [3,4]. More generally, active systems offer novel engineering perspectives, beyond those of equilibrium systems. In particular, boundaries have been shown to be efficient tools for manipulating active particles. Examples range from rectification of bacterial densities [5,6] and optimal delivery of passive cargoes [7] to powering of microscopic gears [8,9]. Further progress, however, requires a predictive theoretical framework which is currently lacking for active systems. To this end, simple settings have been at the core of recent active matter research.Understanding the effect of boundaries on active matter starts with the mechanical pressure exerted by an active fluid on its containing vessel. This question has been recently studied extensively for dry systems [10][11][12][13][14][15][16][17], revealing a surprisingly complex physics. For generic active fluids, the mechanical pressure is not a state variable [13]. The lack of equation of state, through a dependence on the wall details, questions the role of the mechanical pressure in any possible thermodynamic description of active systems [18,19]; it also leads to a richer phenomenology than in passive systems by allowing more general mechanical interplays between fluids and their containers.Interestingly, for the canonical model of self-propelled spheres of constant propelling forces, on which neither walls nor other particles exert torques, the pressure acting on a solid flat wall has been shown to admit an equation of state [11,12,19]. While the physics of this model does not clearly differ from other active systems, showing for instance wall accumulation [20] and motility-induced phase separation [21][22][23], the mechanical pressure exerted on a flat wall satisfies an equation of state, even in the presence of pairwise interactions [11,12,19]. One might thus hope that the intuition built on the rheology of equilibrium fluids extends to this case. Derived in a particular setting, the robustness of this equation of state however remains an open question. For...

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“…When the persistence length of active motion becomes comparable to the mean free path, uniquely active effects arise that transcend the thermodynamically allowed behaviors of equilibrium systems, including giant number fluctuations and spontaneous flow [3,14,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Importantly, a sufficient active persistence length is the only requirement for macroscopic manifestations of activity, as revealed by athermal phase separation of nonaligning, repulsive self-propelled particles [31][32][33][34][35][36][37][38][39][40][41].When boundaries and obstacles are patterned on the scale of the active correlation length, they dramatically alter the dynamics of the system, and striking macroscopic properties emerge [42][43][44][45][46][47][48][49]; for example, ratchets and funnels drive spontaneous flow in active fluids [42][43][44][45][46]. This effect has been used to direct bacterial motion [50] and harness bacterial power to propel microscopic gears …”

mentioning

confidence: 99%

“…When boundaries and obstacles are patterned on the scale of the active correlation length, they dramatically alter the dynamics of the system, and striking macroscopic properties emerge [42][43][44][45][46][47][48][49]; for example, ratchets and funnels drive spontaneous flow in active fluids [42][43][44][45][46]. This effect has been used to direct bacterial motion [50] and harness bacterial power to propel microscopic gears [51][52][53].…”

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confidence: 99%

Dynamics of self-propelled particles under strong confinement

2014

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We develop a statistical theory for the dynamics of non-aligning, non-interacting self-propelled particles confined in a convex box in two dimensions. We find that when the size of the box is small compared to the persistence length of a particle's trajectory (strong confinement), the steady-state density is zero in the bulk and proportional to the local curvature on the boundary. Conversely, the theory may be used to construct the box shape that yields any desired density distribution on the boundary. When the curvature variations are small, we also predict the distribution of orientations at the boundary and the exponential decay of pressure as a function of box size recently observed in 3D simulations in a spherical box.Active fluids consisting of self-propelled units are found in biology on scales ranging from the dynamically reconfigurable cell cytoskeleton [1] to swarming bacterial colonies [2,3], healing tissues [4,5], and flocking animals [6]. Experiments have begun to achieve the extraordinary capabilities and emergent properties of these biological systems in nonliving active fluids of self-propelled particles, consisting of chemically [7][8][9][10][11][12] or electrically [13] propelled colloids, or monolayers of vibrated granular particles [14][15][16].In contrast to thermal motion, active motion is correlated over experimentally accessible time and length scales. When the persistence length of active motion becomes comparable to the mean free path, uniquely active effects arise that transcend the thermodynamically allowed behaviors of equilibrium systems, including giant number fluctuations and spontaneous flow [3,14,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Importantly, a sufficient active persistence length is the only requirement for macroscopic manifestations of activity, as revealed by athermal phase separation of nonaligning, repulsive self-propelled particles [31][32][33][34][35][36][37][38][39][40][41].When boundaries and obstacles are patterned on the scale of the active correlation length, they dramatically alter the dynamics of the system, and striking macroscopic properties emerge [42][43][44][45][46][47][48][49]; for example, ratchets and funnels drive spontaneous flow in active fluids [42][43][44][45][46]. This effect has been used to direct bacterial motion [50] and harness bacterial power to propel microscopic gears [51][52][53]. However, optimizing such devices for technological applications requires understanding the interaction of an active fluid with boundaries of arbitrary shape. More generally, any real-world device necessarily includes boundaries, and thus the effects of boundary size and shape are essential design parameters. Although recent studies have explored confinement in simple geometries [43,47,[54][55][56], there is no general theory for the effect of boundary shape.In this Letter, we study the dynamics of non-aligning and non-interacting self-propelled particles confined to two-dimensional convex containers, such as ellipses and polygons. We find...

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Active matter ratchets with an external drift

Cited by 63 publications

References 93 publications

Ratchet Effects in Active Matter Systems

Ratchet Effects in Active Matter Systems

Active Particles with Soft and Curved Walls: Equation of State, Ratchets, and Instabilities

Dynamics of self-propelled particles under strong confinement

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