147 research outputs found
Relevance of Metric-Free Interactions in Flocking Phenomena
We show that the collective properties of self-propelled particles aligning
with their "topological" (Voronoi) neighbors are qualitatively different from
those of usual models where metric interaction ranges are used. This relevance
of metric-free interactions, shown in a minimal setting, indicate that
realistic models for the cohesive motion of cells, bird flocks, and fish
schools may have to incorporate them, as suggested by recent observations.Comment: To appear on Physical Review Letter
Rare-event induced binding transition of heteropolymers
Sequence heterogeneity broadens the binding transition of a polymer onto a
linear or planar substrate. This effect is analyzed in a real-space
renormalization group scheme designed to capture the statistics of rare events.
In the strongly disordered regime, binding initiates at an exponentially rare
set of ``good contacts''. Renormalization of the contact potential yields a
Kosterlitz-Thouless type transition in any dimension. This and other
predictions are confirmed by extensive numerical simulations of a directed
polymer interacting with a columnar defect.Comment: 4 pages, 3 figure
Anomalous diffusion due to hindering by mobile obstacles undergoing Brownian motion or Orstein-Ulhenbeck processes
In vivo measurements of the passive movements of biomolecules or vesicles in
cells consistently report ''anomalous diffusion'', where mean-squared
displacements scale as a power law of time with exponent
(subdiffusion). While the detailed mechanisms causing such behaviors are not
always elucidated, movement hindrance by obstacles is often invoked. However,
our understanding of how hindered diffusion leads to subdiffusion is based on
diffusion amidst randomly-located \textit{immobile} obstacles. Here, we have
used Monte-Carlo simulations to investigate transient subdiffusion due to
\textit{mobile} obstacles with various modes of mobility. Our simulations
confirm that the anomalous regimes rapidly disappear when the obstacles move by
Brownian motion. By contrast, mobile obstacles with more confined
displacements, e.g. Orstein-Ulhenbeck motion, are shown to preserve
subdiffusive regimes. The mean-squared displacement of tracked protein displays
convincing power-laws with anomalous exponent that varies with the
density of OU obstacles or the relaxation time-scale of the OU process. In
particular, some of the values we observed are significantly below the
universal value predicted for immobile obstacles in 2d. Therefore, our results
show that subdiffusion due to mobile obstacles with OU-type of motion may
account for the large variation range exhibited by experimental measurements in
living cells and may explain that some experimental estimates are below the
universal value predicted for immobile obstacles.Comment: Physical Review E (2014
Competing ferromagnetic and nematic alignment in self-propelled polar particles
We study a Vicsek-style model of self-propelled particles where ferromagnetic
and nematic alignment compete in both the usual "metric" version and in the
"metric-free" case where a particle interacts with its Voronoi neighbors. We
show that the phase diagram of this out-of-equilibrium XY model is similar to
that of its equilibrium counterpart: the properties of the fully-nematic model,
studied before in [F. Ginelli, F. Peruani, M. Baer, and H. Chat\'e, Phys. Rev.
Lett. 104, 184502 (2010)], are thus robust to the introduction of a modest bias
of interactions towards ferromagnetic alignment. The direct transitions between
polar and nematic ordered phases are shown to be discontinuous in the metric
case, and continuous, belonging to the Ising universality class, in the
metric-free version
Comment on ``Phase Transitions in Systems of Self-Propelled Agents and Related Network Models''
In this comment we show that the transition to collective motion in
Vicsek-like systems with angular noise remain discontinuous for large velocity
values. Thus, the networks studied by Aldana {\et al.} [Phys. Rev. Lett. {\bf
98}, 095702 (2007)] at best constitute a singular, large velocity limit of
these systems.Comment: To appear on Physical Review Letter
From Phase to Micro-Phase Separation in Flocking Models: The Essential Role of Non-Equilibrium Fluctuations
We show that the flocking transition in the Vicsek model is best understood
as a liquid-gas transition, rather than an order-disorder one. The full phase
separation observed in flocking models with Z2 rotational symmetry is, however,
replaced by a microphase separation leading to a smectic arrangement of
traveling ordered bands. Remarkably, continuous deterministic descriptions do
not account for this difference, which is only recovered at the fluctuating
hydrodynamics level. Scalar and vectorial order parameters indeed produce
different types of number fluctuations, which we show to be essential in
selecting the inhomogeneous patterns. This highlights an unexpected role of
fluctuations in the selection of flock shapes.Comment: 5 p., 5 fig.. Supplementary material: 7 movie
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