41 research outputs found
Active dry granular flows: rheology and rigidity transitions
The constitutive relations of a dense granular flow composed of
self-propelling frictional hard particles are investigated by means of DEM
numerical simulations. We show that the rheology, which relates the dynamical
friction and the volume fraction to the inertial number ,
depends on a dimensionless number , which compares the active
force to the confining pressure. Two liquid/solid transitions -- in the Maxwell
rigidity sense -- are observed. As soon as the activity is turned on, the
packing becomes an `active solid' with a mean number of particle contacts
larger than the isostatic value. The quasi-static values of and
decrease with . At a finite value of the activity ,
corresponding to the isostatic condition, a second `active rigidity transition'
is observed beyond which the quasi-static values of the friction vanishes and
the rheology becomes Newtonian. For , we provide
evidence for a highly intermittent dynamics of this 'active fluid'.Comment: 7 pages, 7 figures, final version, accepted for publication in
Europhys. Let
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
Interaction of atoms with twisted light
Twisted photons are particles which carry a nonzero projection of the orbital angular momentum onto their propagation direction. During the last years, the interaction between twisted photons and atoms became an active area of fundamental and applied research. In the present work, we show how the twistedness of Bessel and Laguerre-Gauss photons may affect a number of fundamental light-matter interaction processes in comparison with the results for standard plane-wave radiation. In particular, we perform an analysis of the photoionization of hydrogen molecular ions by twisted photons. It is shown that the oscillations in the angular and energy distributions of photoelectrons are affected by the intensity profile of twisted photons. We also investigate the excitation of atoms by these twisted photons. We demonstrate here that the orbital angular momentum of light leads to the alignment or specific magnetic sublevel population of excited atoms. Apart from these studies, we explore the elastic Rayleigh scattering of twisted photons by hydrogenlike ions. Our results indicate that the twistedness of incident photons may significantly influence the polarization properties of scattered light
Large-scale chaos and fluctuations in active nematics
We show that "dry" active nematics, e.g. collections of shaken elongated
granular particles, exhibit large-scale spatiotemporal chaos made of
interacting dense, ordered, band-like structures in a parameter region
including the linear onset of nematic order. These results are obtained from
the study of the relatively simple and well-known (deterministic) hydrodynamic
equations describing these systems in a dilute limit, and of a self-propelled
particle Vicsek-like model for this class of active matter. In this last case,
revisiting the status of the strong fluctuations and long-range correlations
now considered as landmarks of orientationally-ordered active phases, we show
that the giant number fluctuations observed in the chaotic phase are a trivial
consequence of density segregation. However anomalous density fluctuations are
present in the homogeneous quasi-ordered nematic phase and characterized by a
non-trivial scaling exponent
Continuous theory of active matter systems with metric-free interactions
We derive a hydrodynamic description of metric-free active matter: starting
from self-propelled particles aligning with neighbors defined by "topological"
rules, not metric zones, -a situation advocated recently to be relevant for
bird flocks, fish schools, and crowds- we use a kinetic approach to obtain
well-controlled nonlinear field equations. We show that the density-independent
collision rate per particle characteristic of topological interactions
suppresses the linear instability of the homogeneous ordered phase and the
nonlinear density segregation generically present near threshold in metric
models, in agreement with microscopic simulations.Comment: Submitted to Physical Review Letter
Effects of interparticle friction on the response of 3D cyclically compressed granular material
We numerically study the effect of inter-particle friction coefficient on the
response to cyclical pure shear of spherical particles in three dimensions. We
focus on the rotations and translations of grains and look at the spatial
distribution of these displacements as well as their probability distribution
functions. We find that with increasing friction, the shear band becomes
thinner and more pronounced. At low friction, the amplitude of particle
rotations is homogeneously distributed in the system and is therefore mostly
independent from both the affine and non-affine particle translations. In
contrast, at high friction, the rotations are strongly localized in the shear
zone. This work shows the importance of studying the effects of inter-particle
friction on the response of granular materials to cyclic forcing, both for a
better understanding of how rotations correlate to translations in sheared
granular systems, and due to the relevance of cyclic forcing for most
real-world applications in planetary science and industry
Emergent spatial structures in flocking models: a dynamical system insight
We show that hydrodynamic theories of polar active matter generically possess
inhomogeneous traveling solutions. We introduce a unifying dynamical-system
framework to establish the shape of these intrinsically nonlinear patterns, and
show that they correspond to those hitherto observed in experiments and
numerical simulations: periodic density waves, and solitonic bands, or
polar-liquid droplets both cruising in isotropic phases. We elucidate their
respective multiplicity and mutual relations, as well as their existence
domain