6,522 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
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
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
Definition Study for Space Shuttle Experiments Involving Large, Steerable Millimeter-Wave Antenna Arrays
The potential uses and techniques for the shuttle spacelab Millimeter Wave Large Aperture Antenna Experiment (MWLAE) are documented. Potential uses are identified: applications to radio astronomy, the sensing of atmospheric turbulence by its effect on water vapor line emissions, and the monitoring of oil spills by multifrequency radiometry. IF combining is preferable to RF combining with respect to signal to noise ratio for communications receiving antennas of the size proposed for MWLAE. A design approach using arrays of subapertures is proposed to reduce the number of phase shifters and mixers for uses which require a filled aperture. Correlation radiometry and a scheme utilizing synchronous Dicke switches and IF combining are proposed as potential solutions
Receiver techniques and detectors for use at millimeter and submillimeter wave lengths Semiannual status report
Calculations of vibrational wave functions and excitation cross sections of carbon dioxide molecules, and infrared laser experiment
Towards a unification of HRT and SCOZA. Analysis of exactly solvable mean-spherical and generalized mean-spherical models
The hierarchical reference theory (HRT) and the self-consistent
Ornstein-Zernike approximation (SCOZA) are two liquid state theories that both
furnish a largely satisfactory description of the critical region as well as
the phase coexistence and equation of state in general. Furthermore, there are
a number of similarities that suggest the possibility of a unification of both
theories. Earlier in this respect we have studied consistency between the
internal energy and free energy routes. As a next step toward this goal we here
consider consistency with the compressibility route too, but we restrict
explicit evaluations to a model whose exact solution is known showing that a
unification works in that case. The model in question is the mean spherical
model (MSM) which we here extend to a generalized MSM (GMSM). For this case, we
show that the correct solutions can be recovered from suitable boundary
conditions through either of SCOZA or HRT alone as well as by the combined
theory. Furthermore, the relation between the HRT-SCOZA equations and those of
SCOZA and HRT becomes transparent.Comment: Minimal correction of some typos found during proof reading. Accepted
for publication in Phys. Rev.
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