1,787 research outputs found
Flocking Regimes in a Simple Lattice Model
We study a one-dimensional lattice flocking model incorporating all three of
the flocking criteria proposed by Reynolds [Computer Graphics vol.21 4 (1987)]:
alignment, centring and separation. The model generalises that introduced by O.
J. O' Loan and M. R. Evans [J. Phys. A. vol. 32 L99 (1999)]. We motivate the
dynamical rules by microscopic sampling considerations. The model exhibits
various flocking regimes: the alternating flock, the homogeneous flock and
dipole structures. We investigate these regimes numerically and within a
continuum mean-field theory.Comment: 24 pages 7 figure
Emergence of macroscopic directed motion in populations of motile colloids
From the formation of animal flocks to the emergence of coordinate motion in
bacterial swarms, at all scales populations of motile organisms display
coherent collective motion. This consistent behavior strongly contrasts with
the difference in communication abilities between the individuals. Guided by
this universal feature, physicists have proposed that solely alignment rules at
the individual level could account for the emergence of unidirectional motion
at the group level. This hypothesis has been supported by agent-based
simulations. However, more complex collective behaviors have been
systematically found in experiments including the formation of vortices,
fluctuating swarms, clustering and swirling. All these model systems
predominantly rely on actual collisions to display collective motion. As a
result, the potential local alignment rules are entangled with more complex,
often unknown, interactions. The large-scale behavior of the populations
therefore depends on these uncontrolled microscopic couplings. Here, we
demonstrate a new phase of active matter. We reveal that dilute populations of
millions of colloidal rollers self-organize to achieve coherent motion along a
unique direction, with very few density and velocity fluctuations. Identifying
the microscopic interactions between the rollers allows a theoretical
description of this polar-liquid state. Comparison of the theory with
experiment suggests that hydrodynamic interactions promote the emergence of
collective motion either in the form of a single macroscopic flock at low
densities, or in that of a homogenous polar phase at higher densities.
Furthermore, hydrodynamics protects the polar-liquid state from the giant
density fluctuations. Our experiments demonstrate that genuine physical
interactions at the individual level are sufficient to set homogeneous active
populations into stable directed motion
Flowing active liquids in a pipe: Hysteretic response of polar flocks to external fields
We investigate the response of colloidal flocks to external fields. We first
show that individual colloidal rollers align with external flows as would a
classical spin with magnetic fields. Assembling polar active liquids from
colloidal rollers, we experimentally demonstrate their hysteretic response:
confined colloidal flocks can proceed against external flows. We theoretically
explain this collective robustness, using an active hydrodynamic description,
and show how orientational elasticity and confinement protect the direction of
collective motion. Finally, we exploit the intrinsic bistability of confined
active flows to devise self-sustained microfluidic oscillators.Comment: 12 pages, 7 figure; accepted for publication in Physical Review
Boltzmann-Ginzburg-Landau approach for continuous descriptions of generic Vicsek-like models
We describe a generic theoretical framework, denoted as the
Boltzmann-Ginzburg-Landau approach, to derive continuous equations for the
polar and/or nematic order parameters describing the large scale behavior of
assemblies of point-like active particles interacting through polar or nematic
alignment rules. Our study encompasses three main classes of dry active
systems, namely polar particles with 'ferromagnetic' alignment (like the
original Vicsek model), nematic particles with nematic alignment ("active
nematics"), and polar particles with nematic alignment ("self-propelled rods").
The Boltzmann-Ginzburg-Landau approach combines a low-density description in
the form of a Boltzmann equation, with a Ginzburg-Landau-type expansion close
to the instability threshold of the disordered state. We provide the generic
form of the continuous equations obtained for each class, and comment on the
relationships and differences with other approaches.Comment: 30 pages, 3 figures, to appear in Eur. Phys. J. Special Topics, in a
Discussion and Debate issue on active matte
Collective Motion with Anticipation: Flocking, Spinning, and Swarming
We investigate the collective dynamics of self-propelled particles able to
probe and anticipate the orientation of their neighbors. We show that a simple
anticipation strategy hinders the emergence of homogeneous flocking patterns.
Yet, anticipation promotes two other forms of self-organization: collective
spinning and swarming. In the spinning phase, all particles follow synchronous
circular orbits, while in the swarming phase, the population condensates into a
single compact swarm that cruises coherently without requiring any cohesive
interactions. We quantitatively characterize and rationalize these phases of
polar active matter and discuss potential applications to the design of
swarming robots.Comment: 6 pages, 4 figure
Mean-field theory of collective motion due to velocity alignment
We introduce a system of self-propelled agents (active Brownian particles)
with velocity alignment in two spatial dimensions and derive a mean-field
theory from the microscopic dynamics via a nonlinear Fokker-Planck equation and
a moment expansion of the probability distribution function. We analyze the
stationary solutions corresponding to macroscopic collective motion with finite
center of mass velocity (ordered state) and the disordered solution with no
collective motion in the spatially homogeneous system. In particular, we
discuss the impact of two different propulsion functions governing the
individual dynamics. Our results predict a strong impact of the individual
dynamics on the mean field onset of collective motion (continuous vs
discontinuous). In addition to the macroscopic density and velocity field we
consider explicitly the dynamics of an effective temperature of the agent
system, representing a measure of velocity fluctuations around the mean
velocity. We show that the temperature decreases strongly with increasing level
of collective motion despite constant fluctuations on individual level, which
suggests that extreme caution should be taken in deducing individual behavior,
such as, state-dependent individual fluctuations from mean-field measurements
[Yates {\em et al.}, PNAS, 106 (14), 2009].Comment: corrected version, Ecological Complexity (2011) in pres
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