The swim force as a body force

Yan, Wen; Brady, John F.

doi:10.1039/c5sm01318f

Cited by 65 publications

(97 citation statements)

References 32 publications

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“…The N-particle Smoluchowski equation for passive Brownian particles including excluded volume and full hydrodynamic interactions is well known, as is the form of the many-body hydrodynamic swim force (Yan & Brady 2015). Reduction to the lowest moments, n and m, is certain to give rise to new phenomena since the swim diffusivity, which enters the flux expressions, can be a decreasing function of the swimmers' concentration .…”

Section: Discussionmentioning

confidence: 99%

“…can all affect the value of the concentration at the surface and thus the force on the boundary. We showed recently (Yan & Brady 2015) that the polar order induced by an orienting field acts like a body force on the active material, and when this 'internal' body force is included in the momentum balance, the force per unit area on the wall plus the integral of the internal body force is equal to the active pressure far from the surface, thus restoring the active pressure as a state function.…”

Section: Discussionmentioning

confidence: 99%

“…The swim pressure is only defined on, and applies for, lengths greater than the run length. And its use to compute forces on boundaries necessitates that the boundary or macroscopic length scale, L, be much larger than the run length (Yan & Brady 2015). What happens when the length scale of interest is not large compared to the run length?…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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The force on a boundary in active matter

2015

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“…Importantly, the existence of an equation of states implies that the value of the pressure does not depend on the microscopic detail of the interaction between the particles and the walls, be this interaction soft or hard, and torque-free or not. This is no longer the case for fluids far from equilibrium like active fluids, whose constituent particles are capable of autonomous dissipative motion like self-propulsion [1], and for which pressure [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], as well as stress [18] and other thermodynamic parameters like chemical potential [19], loose some of their standard thermodynamic properties. It has indeed been shown recently [8,9,16] that the pressure of active fluids is generally not a state function and that the average force exerted on confining walls by the fluid does depend on the detail of the interaction between the walls and the particles.…”

Section: -Introductionmentioning

confidence: 99%

Pressure of a gas of underdamped active dumbbells

Joyeux

Bertin

2016

Phys. Rev. E

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Abstract:The pressure exerted on a wall by a gas at equilibrium does not depend on the shape of the confining potential defining the walls. In contrast, it has been shown recently [A.P. Solon et.al., Nat. Phys. 11, 673 (2015)] that a gas of overdamped active particles exerts on a wall a force that depends on the confining potential, resulting in a net force on an asymmetric wall between two chambers at equal densities. Here, considering a model of underdamped self-propelled dumbbells in two dimensions, we study how the behavior of the pressure depends on the damping coefficient of the dumbbells, thus exploring inertial effects.We find in particular that the force exerted on a moving wall between two chambers at equal density continuously vanishes at low damping coefficient, and exhibits a complex dependence on the damping coefficient at low density, when collisions are scarce. We further show that this behavior of the pressure can to a significant extent be understood in terms of the trajectories of individual particles close to and in contact with the wall. 02.70.Ns (Molecular dynamics and particle methods) (#)

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“…This dynamics encompass the two well-studied models of run-and-tumble particles (RTPs, with α = 0 and D r = 0) [30] and active Brownian particles (ABPs, having α = 0 and D r = 0) [31,32]. There has been much recent progress [24,26,[33][34][35][36][37][38][39][40][41][42][43][44][45][46] in the characterization of forces in this class of systems and we build upon it. Note that the model falls into the class of dry active systems, which do not conserve momentum.…”

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confidence: 99%

Generic Long-Range Interactions Between Passive Bodies in an Active Fluid

Baek

Solon

et al. 2018

Phys. Rev. Lett.

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Because active particles break time-reversal symmetry, a single non-spherical body placed in an active fluid generates currents. We show that when two or more passive bodies are placed in an active fluid these currents lead to long-range interactions. Using a multipole expansion we characterize their leading-order behaviors in terms of single-body properties and show that they decay as a power law with the distance between the bodies, are anisotropic, and do not obey an action-reaction principle. The interactions lead to rich dynamics of the bodies, illustrated by the spontaneous synchronized rotation of pinned non-chiral bodies and the formation of traveling bound pairs. The occurrence of these phenomena depends on tunable properties of the bodies, thus opening new possibilities for self-assembly mediated by active fluids.Active matter is a class of nonequilibrium systems in which energy is converted into systematic motion on a microscopic scale [1]. They have attracted much attention [2,3] due to a host of interesting physical phenomena [4][5][6][7][8][9], their relevance to many biological systems [10][11][12][13], and their potential use for self-assembly applications [14]. They have also been suggested as tools for novel engineering applications -for example, active fluids have been used to power microscopic gears [15][16][17][18][19][20][21]. This results from the fact that, when an asymmetric body is immersed in a fluid with broken time-reversal symmetry, it experiences a net force [22][23][24] which is coupled to the generation of ratchet-like currents [25,26].In this Letter we study passive bodies immersed in an active fluid. We show that the ratchet-like currents generated by each body give rise to forces and torques which decay as a power law with distance, are anisotropic, and do not obey an action-reaction principle. Using a multipole expansion, the leading-order behavior of the interactions can be expressed in terms of single-body quantities that can be measured independently in experiments or numerical simulations. Moreover, by designing the two bodies one can control the amplitude and polarity of the interactions between them. This leads to a host of interesting dynamical phenomena of which we illustrate two: the spontaneous synchronized rotations of pinned rotors and the formation of traveling bound pairs. Our results suggest a new method for self-assembly by embedding passive bodies in an active fluid.We stress that the interactions studied here exist even between non-moving bodies and are therefore distinct from usual hydrodynamic interactions [27]. They are also different from thermal Casimir interactions [28,29], because they do not rely on correlations between the fluid particles and are present even in a dilute active fluid.Model. -We base our study on a common model of an active fluid consisting of N point-like particles, which * y.baek@damtp.cam.ac.uk do not interact among themselves and self-propel at a constant speed v in two dimensions. The position r i and the orientation θ i of acti...

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The swim force as a body force

Cited by 65 publications

References 32 publications

The force on a boundary in active matter

The force on a boundary in active matter

Pressure of a gas of underdamped active dumbbells

Generic Long-Range Interactions Between Passive Bodies in an Active Fluid

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