51 research outputs found
Simultaneous Phase Separation and Pattern Formation in Chiral Active Mixtures
Chiral active particles, or self-propelled circle swimmers, from sperm cells
to asymmetric Janus colloids, form a rich set of patterns, which are different
from those seen in linear swimmers. Such patterns have mainly been explored for
identical circle swimmers, while real-world circle swimmers, typically possess
a frequency distribution. Here we show that even the simplest mixture of
(velocity-aligning) circle swimmers with two different frequencies, hosts a
complex world of superstructures: The most remarkable example comprises a
microflock pattern, formed in one species, while the other species phase
separates and forms a macrocluster, coexisting with a gas phase. Here, one
species microphase-separates and selects a characteristic length scale, whereas
the other one macrophase separates and selects a density. A second notable
example, here occurring in an isotropic system, are patterns comprising two
different characteristic length scales, which are controllable via frequency
and swimming speed of the individual particles
The Rotating Vicsek Model: Pattern Formation and Enhanced Flocking in Chiral Active Matter
We generalize the Vicsek model to describe the collective behaviour of polar
circle swimmers with local alignment interactions. While the phase transition
leading to collective motion in 2D (flocking) occurs at the same interaction to
noise ratio as for linear swimmers, as we show, circular motion enhances the
polarization in the ordered phase (enhanced flocking) and induces secondary
instabilities leading to structure formation. Slow rotations result in phase
separation whereas fast rotations generate patterns which consist of phase
synchronized microflocks of controllable self-limited size. Our results defy
the viewpoint that monofrequent rotations form a rather trivial extension of
the Vicsek model and establish a generic route to pattern formation in chiral
active matter with possible applications to control coarsening and to design
rotating microflocks.Comment: Contains a Supplementary Materia
From single-particle to collective effective temperatures in an active fluid of self-propelled particles
We present a comprehensive analysis of effective temperatures based on
fluctuation-dissipation 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.Comment: 6 pages, 3 fig
Micro-flock patterns and macro-clusters in chiral active Brownian disks
Chiral active particles (or self-propelled circle swimmers) feature a rich
collective behavior, comprising rotating macro-clusters and micro-flock
patterns which consist of phase-synchronized rotating clusters with a
characteristic self-limited size. These patterns emerge from the competition of
alignment interactions and rotations suggesting that they might occur
generically in many chiral active matter systems. However, although excluded
volume interactions occur naturally among typical circle swimmers, it is not
yet clear if macro-clusters and micro-flock patterns survive their presence.
The present work shows that both types of pattern do survive but feature
strongly enhance fluctuations regarding the size and shape of the individual
clusters. Despite these fluctuations, we find that the average micro-flock size
still follows the same characteristic scaling law as in the absence of excluded
volume interactions, i.e. micro-flock sizes scale linearly with the
single-swimmer radius
Synchronization in dynamical networks of locally coupled self-propelled oscillators
Systems of mobile physical entities exchanging information with their
neighborhood can be found in many different situations. The understanding of
their emergent cooperative behaviour has become an important issue across
disciplines, requiring a general conceptual framework in order to harvest the
potential of these systems. We study the synchronization of coupled oscillators
in time-evolving networks defined by the positions of self-propelled agents
interacting in real space. In order to understand the impact of mobility in the
synchronization process on general grounds, we introduce a simple model of
self-propelled hard disks performing persistent random walks in 2 space and
carrying an internal Kuramoto phase oscillator. For non-interacting particles,
self-propulsion accelerates synchronization. The competition between agent
mobility and excluded volume interactions gives rise to a richer scenario,
leading to an optimal self-propulsion speed. We identify two extreme dynamic
regimes where synchronization can be understood from theoretical
considerations. A systematic analysis of our model quantifies the departure
from the latter ideal situations and characterizes the different mechanisms
leading the evolution of the system. We show that the synchronization of
locally coupled mobile oscillators generically proceeds through coarsening
verifying dynamic scaling and sharing strong similarities with the phase
ordering dynamics of the 2 XY model following a quench. Our results shed
light into the generic mechanisms leading the synchronization of mobile agents,
providing a efficient way to understand more complex or specific situations
involving time-dependent networks where synchronization, mobility and excluded
volume are at play
Velocity alignment promotes motility-induced phase separation
We study the phase behavior of polar Active Brownian Particles moving in
two-spatial dimensions and interacting through volume exclusion and velocity
alignment. We combine particle-based simulations of the microscopic model with
a simple mean-field kinetic model to understand the impact of velocity
alignment on the motility-induced phase separation of self-propelled disks. We
show that, as the alignment strength is increased, approaching the onset of
collective motion from below, orientational correlations grow, rendering the
diffusive reorientation dynamics slower. As a consequence, the tendency of
particles to aggregate into isotropic clusters is enhanced, favoring the
complete de-mixing of the system into a low and high-density phase.Comment: 7 pages, 5 figure
Emergent States in Systems of Chiral Self-Propelled Rods
We study inherently chiral self-propelled particles, self-rotating at a fixed
frequency, in two dimensions, subjected to nematic alignment interactions and
rotational noise. By means of both, homogeneous and spatially resolved mean
field kinetic theory, we identify various different flocking states. We confirm
the presence of the predicted phases using agent-based simulations, in
particular, an homogeneous nematic phase at low frequencies, followed by a
microflock pattern phase at larger frequencies, characterized by finite-size
nematic clusters. We emphasize that special care has to be taken within the
simulations in order to avoid artifacts, and present a non-standard simulation
technique in order to avoid them.Comment: 10 pages, 4 figure
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