417 research outputs found
Modelling chemotaxis of microswimmers: from individual to collective behavior
We discuss recent progress in the theoretical description of chemotaxis by
coupling the diffusion equation of a chemical species to equations describing
the motion of sensing microorganisms. In particular, we discuss models for
autochemotaxis of a single microorganism which senses its own secretion leading
to phenomena such as self-localization and self-avoidance. For two
heterogeneous particles, chemotactic coupling can lead to predator-prey
behavior including chase and escape phenomena, and to the formation of active
molecules, where motility spontaneously emerges when the particles approach
each other. We close this review with some remarks on the collective behavior
of many particles where chemotactic coupling induces patterns involving
clusters, spirals or traveling waves.Comment: to appear as a contribution to the book "Chemical kinetics beyond the
textbook
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
Spatiotemporal Oscillation Patterns in the Collective Relaxation Dynamics of Interacting Particles in Periodic Potentials
We demonstrate the emergence of self-organized structures in the course of
the relaxation of an initially excited, dissipative and finite chain of
interacting particles in a periodic potential towards its many particle
equilibrium configuration. Specifically we observe a transition from an in
phase correlated motion via phase randomized oscillations towards oscillations
with a phase difference between adjacent particles thereby yielding the
growth of long time transient spatiotemporal oscillation patterns. Parameter
modifications allow for designing these patterns, including steady states and
even states that combine in phase and correlated out of phase oscillations
along the chain. The complex relaxation dynamics is based on finite size
effects together with an evolution running from the nonlinear to the linear
regime thereby providing a highly unbalanced population of the center of mass
and relative motion
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
Site-selective particle deposition in periodically driven quantum lattices
We demonstrate that a site-dependent driving of a periodic potential allows
for the controlled manipulation of a quantum particle on length scales of the
lattice spacing. Specifically we observe for distinct driving frequencies a
near depletion of certain sites which is explained by a resonant mixing of the
involved Floquet-Bloch modes occurring at these frequencies. Our results could
be exploited as a scheme for a site-selective loading of e.g. ultracold atoms
into an optical lattices
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Data sets and data quality in software engineering
OBJECTIVE - to assess the extent and types of techniques used to manage quality within software engineering data sets. We consider this a particularly interesting question in the context of initiatives to promote sharing and secondary analysis of data sets.
METHOD - we perform a systematic review of available empirical software engineering studies.
RESULTS - only 23 out of the many hundreds of studies assessed, explicitly considered data quality.
CONCLUSIONS - first, the community needs to consider the quality and appropriateness of the data set being utilised; not all data sets are equal. Second, we need more research into means of identifying, and ideally repairing, noisy cases. Third, it should become routine to use sensitivity analysis to assess conclusion stability with respect to the assumptions that must be made concerning noise levels
Interaction-induced current-reversals in driven lattices
We demonstrate that long-range interactions can cause, as time evolves,
consecutive reversals of directed currents for dilute ensembles of particles in
driven lattices. These current-reversals are based on a general mechanism which
leads to an interaction-induced accumulation of particles in the regular
regions of the underlying single-particle phase space and to a synchronized
single-particle motion as well as an enhanced efficiency of Hamiltonian
ratchets.Comment: 5 pages, 5 figure
Disorder-induced regular dynamics in oscillating lattices
We explore the impact of weak disorder on the dynamics of classical particles
in a periodically oscillating lattice. It is demonstrated that the disorder
induces a hopping process from diffusive to regular motion i.e. we observe the
counterintuitive phenomenon that disorder leads to regular behaviour. If the
disorder is localized in a finite-sized part of the lattice, the described
hopping causes initially diffusive particles to even accumulate in regular
structures of the corresponding phase space. A hallmark of this accumulation is
the emergence of pronounced peaks in the velocity distribution of particles
which should be detectable in state of the art experiments e.g. with cold atoms
in optical lattices
Quench Dynamics of Two Coupled Ionic Zig-Zag Chains
We explore the non-equilibrium dynamics of two coupled zig-zag chains of
trapped ions in a double well potential. Following a quench of the potential
barrier between both wells, the induced coupling between both chains due to the
long-range interaction of the ions leads to their complete melting. The
resulting dynamics is however not exclusively irregular but leads to phases of
motion during which various ordered structures appear with ions arranged in
arcs, lines and crosses. We quantify the emerging order by introducing a
suitable measure and complement our analysis of the ion dynamics using a normal
mode analysis showing a decisive population transfer between only a few
distinguished modes
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