Velocity alignment promotes motility-induced phase separation

Sesé-Sansa, Elena; Pagonabarraga, Ignacio; Levis, Demian

doi:10.1209/0295-5075/124/30004

Cited by 78 publications

(75 citation statements)

References 48 publications

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“…More recently MIPS has also seen for circle swimmers 49,50 and more complicated aligning interactions 51,52 . This shows that the occurence of MIPS itself is a very robust and general effect.…”

Section: Collective Effects Of Active Brownian Particles: Mipsmentioning

confidence: 99%

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Inertial effects of self-propelled particles: From active Brownian to active Langevin motion

Löwen

2020

The Journal of Chemical Physics

226

180

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Active particles which are self-propelled by converting energy into mechanical motion represent an expanding research realm in physics and chemistry. For micron-sized particles moving in a liquid ("microswimmers"), most of the basic features have been described by using the model of overdamped active Brownian motion. However, for macroscopic particles or microparticles moving in a gas, inertial effects become relevant such that the dynamics is underdamped. Therefore, recently, active particles with inertia have been described by extending the active Brownian motion model to active Langevin dynamics which include inertia. In this perspective article recent developments of active particles with inertia ("microflyers") are summarized both for single particle properties and for collective effects of many particles. There include: inertial delay effects between particle velocity and self-propulsion direction, tuning of the long-time self-diffusion by the moment of inertia, effects of fictitious forces in non-inertial frames, and the influence of inertia on motility-induced phase separation. Possible future developments and perspectives are also proposed and discussed.Invited perspective article for The Journal of Chemical Physics. arXiv:1910.13953v1 [cond-mat.soft]

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Section: Collective Effects Of Active Brownian Particles: Mipsmentioning

confidence: 99%

“…More studies on MIPS including a non-vanishing moment of inertia and an external torque will be performed. The role of aligning interactions needs to be understood better for inertial systems 87,88 . Active crystallization will be studied where inertia provides a latent heat upon crystallization.…”

Section: Perspectivesmentioning

confidence: 99%

Inertial effects of self-propelled particles: From active Brownian to active Langevin motion

Löwen

2020

The Journal of Chemical Physics

226

180

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“…Precisely this inverse effect was reported earlier in Refs. [17,18]: MIPS is enhanced for self-propelled particles that align through Vicsek interactions. In binary collisions, the Vicsek torques always rotate particles towards the combined center of mass, increasing the duration of collisions, increasing hindrance and thus enhancing MIPS.…”

Section: B Torque-induced Suppression Of Motility-induced Phase Sepamentioning

confidence: 99%

“…Studies that do include torques typically fall into two categories. The first uses particles with Vicsek-like alignment interactions [17,18], which mimic a visual alignment mechanism, such as for birds or fish. The second uses particles with an anisotropic, typically rodlike shape [19][20][21][22][23].…”

Section: Introductionmentioning

confidence: 99%

Interparticle torques suppress motility-induced phase separation for rodlike particles

Damme

Rodenburg

Roij

et al. 2019

The Journal of Chemical Physics

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To study the role of torque in motility-induced phase separation (MIPS), we simulate a system of self-propelled particles whose shape varies smoothly from isotropic (disks/spheres) to weakly elongated (rods). We construct the phase diagrams of 2D active disks, 3D active spheres and 2D/3D active rods of aspect ratio l/σ = 2. A stability analysis of the homogeneous isotropic phase allows us to predict the onset of MIPS based on the effective swimming speed and rotational diffusion of the particles. Both methods find suppression of MIPS as the particle shape is elongated. We propose a suppression mechanism based on the duration of collisions, and argue that this mechanism can explain both the suppression of MIPS found here for rodlike particles and the enhancement of MIPS found for particles with Vicsek interactions.

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“…Such a phenomenon, known as Motilityinduced Phase Separation (MIPS) has been largely investigated [40], starting from the pioneering work of Fily and Marchetti [41]. The coexistence of clustering and velocity ordering has been recently considered, and, even if its role in MIPS is still an open question [42][43][44][45], it has been shown that may induce freezing in dense regimes [46]. The alignment, characterizing Vicsek-like models [47], and the ABP phase-separation are phenomena which are usually thought to be generated by two distinct types of interactions between particles.In the present study, we challenge the widespread idea that explicit alignment interactions are necessary to observe a growing orientational order or -equivalently -that the velocity alignment observed in Vicsek-like models do not appear in purely repulsive, spherical ABP particles.…”

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confidence: 99%

Spontaneous Velocity Alignment in Motility-Induced Phase Separation

2020

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We study a system of purely repulsive spherical self-propelled particles in the minimal set-up inducing Motility-Induced Phase Separation (MIPS). We show that, even if explicit alignment interactions are absent, a growing order in the velocities of the clustered particles accompanies MIPS. Particles arrange into aligned or vortex-like domains. Their sizes increase as the persistence of the self-propulsion grows, an effect that is quantified studying the spatial correlation function of the velocities. We explain the velocity-alignment by unveiling a hidden alignment interaction of the Vicsek-like form, induced by the interplay between steric interactions and self-propulsion. As a consequence, we argue that the MIPS transition cannot be fully understood in terms of a scalar field, the density, since the collective orientation of the velocities should be included in effective coarse-grained descriptions. Fishes [1], birds [2] or insects [3] often display fashinating collective behaviors such as flocking [2, 4] and swarming [5], where all units of a group move coherently producing intriguing dynamical patterns. A different mode of organization of living organisms is clustering, for instance in bacterial colonies [6], such as E. Coli [7], Myxococcus xanthus [8] or Thiovulum majus [9], relevant for histological cultures in several areas of medical and pharmaceutical sciences. Out of the biological realm, the occurrence of stable clusters [10-13], stable chains [14] or vortices [15] in activated colloidal particles, e.g. autophoretic colloids or Janus disks [16,17], offers an interesting challenge for the design of new materials.Even if the microscopic details differ case by case, a few classes of minimal models with common coarsegrained features have been introduced in statistical physics. Units in these models are called "active" or "selfpropelled" particles [18-20] to differentiate them from Brownian colloids which passively obey the forces of the surrounding environment. Propelling forces may be either of mechanical origin (flagella or body deformation), or of thermodynamic nature (diffusiophoresis and selfelectrophoresis) [21,22]. In some simple and effective examples, self-propulsion is modeled as a constant force with stochastic orientation, as in the case of Active Brownian Particles (ABP) [23,24]. Thermal fluctuations play only a marginal role and stochasticity is usually due to the unsteady nature of the swimming force itself.It is well-known that dumbells, rods and, in general, elongated microswimmers display a marked orientational order even in the absence of alignment interactions [25][26][27][28]. Instead, in the literature, it is believed that explicit aligning velocity-interactions are crucial to observe velocity alignment between spherical self-propelled units [29]. This kind of interaction, such as that in the seminal Vicsek model [30], consists in a short-range force that aligns the velocity of a target particle to the average of the neighboring ones. Vicsek interactions lead to long-range polar order...

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Velocity alignment promotes motility-induced phase separation

Cited by 78 publications

References 48 publications

Inertial effects of self-propelled particles: From active Brownian to active Langevin motion

Inertial effects of self-propelled particles: From active Brownian to active Langevin motion

Interparticle torques suppress motility-induced phase separation for rodlike particles

Spontaneous Velocity Alignment in Motility-Induced Phase Separation

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