3,667 research outputs found
Fluctuating Interfaces in Liquid Crystals
We review and compare recent work on the properties of fluctuating interfaces
between nematic and isotropic liquid-crystalline phases. Molecular dynamics and
Monte Carlo simulations have been carried out for systems of ellipsoids and
hard rods with aspect ratio 15:1, and the fluctuation spectrum of interface
positions (the capillary wave spectrum) has been analyzed. In addition, the
capillary wave spectrum has been calculated analytically within the Landau-de
Gennes theory. The theory predicts that the interfacial fluctuations can be
described in terms of a wave vector dependent interfacial tension, which is
anisotropic at small wavelengths (stiff director regime) and becomes isotropic
at large wavelengths (flexible director regime). After determining the elastic
constants in the nematic phase, theory and simulation can be compared
quantitatively. We obtain good agreement for the stiff director regime. The
crossover to the flexible director regime is expected at wavelengths of the
order of several thousand particle diameters, which was not accessible to our
simulations
Exact Solution of a Jamming Transition: Closed Equations for a Bootstrap Percolation Problem
Jamming, or dynamical arrest, is a transition at which many particles stop
moving in a collective manner. In nature it is brought about by, for example,
increasing the packing density, changing the interactions between particles, or
otherwise restricting the local motion of the elements of the system. The onset
of collectivity occurs because, when one particle is blocked, it may lead to
the blocking of a neighbor. That particle may then block one of its neighbors,
these effects propagating across some typical domain of size named the
dynamical correlation length. When this length diverges, the system becomes
immobile. Even where it is finite but large the dynamics is dramatically
slowed. Such phenomena lead to glasses, gels, and other very long-lived
nonequilibrium solids. The bootstrap percolation models are the simplest
examples describing these spatio-temporal correlations. We have been able to
solve one such model in two dimensions exactly, exhibiting the precise
evolution of the jamming correlations on approach to arrest. We believe that
the nature of these correlations and the method we devise to solve the problem
are quite general. Both should be of considerable help in further developing
this field.Comment: 17 pages, 4 figure
The pair annihilation reaction D + D --> 0 in disordered media and conformal invariance
The raise and peel model describes the stochastic model of a fluctuating
interface separating a substrate covered with clusters of matter of different
sizes, and a rarefied gas of tiles. The stationary state is obtained when
adsorption compensates the desorption of tiles. This model is generalized to an
interface with defects (D). The defects are either adjacent or separated by a
cluster. If a tile hits the end of a cluster with a defect nearby, the defect
hops at the other end of the cluster changing its shape. If a tile hits two
adjacent defects, the defect annihilate and are replaced by a small cluster.
There are no defects in the stationary state.
This model can be seen as describing the reaction D + D -->0, in which the
particles (defects) D hop at long distances changing the medium and annihilate.
Between the hops the medium also changes (tiles hit clusters changing their
shapes). Several properties of this model are presented and some exact results
are obtained using the connection of our model with a conformal invariant
quantum chain.Comment: 8 pages, 12figure
Effects of the Newsâ Findsâ Me Perception in Communication: Social Media Use Implications for News Seeking and Learning About Politics
With social media at the forefront of todayâs media context, citizens may perceive they donât need to actively seek news because they will be exposed to news and remain wellâ informed through their peers and social networks. We label this the â newsâ findsâ me perception,â and test its implications for news seeking and political knowledge: â newsâ findsâ me effects.â U.S. panelâ survey data show that individuals who perceive news will find them are less likely to use traditional news sources and are less knowledgeable about politics over time. Although the newsâ findsâ me perception is positively associated with news exposure on social media, this behavior doesnât facilitate political learning. These results suggest news continues to enhance political knowledge best when actively sought.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137553/1/jcc412185.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137553/2/jcc412185_am.pd
On Closed Einstein-de Sitter Universes
We briefly summarize the idea of cosmological models with compact, flat
spatial sections. It has been suggested that, because of the COBE satellite's
maps of the microwave background, such models cannot be small in the sense of
Ellis, and hence are no longer interesting. Here we use Lehoucq et al.'s method
of cosmic crystallography to show that these models are physically meaningful
even if the size of the spatial sections is of the same order of magnitude as
the radius of the observational horizon.Comment: 8 pages, 1 figure. Revision includes comment on "top-down" and
"bottom-up" pictures of structure formation. Figure is unmodifie
Opinion of Belgian egg farmers on hen welfare and its relationship with housing type
As of 2012, the EU has banned the use of conventional cages (CC) for laying hens, causing a shift in housing systems. This studyâs aim was to gain insight into farmersâ opinions on hen health and welfare in their current housing systems. A survey was sent to 218 Belgian egg farmers, of which 127 (58.3%) responded, with 84 still active as egg farmer. Hen welfare tended to be less important in choosing the housing system for farmers with cage than with non-cage systems. Respondents currently using cage systems were more satisfied with hen health than respondents with non-cage systems. Reported mortality increased with farm size and was higher in furnished cages than in floor housing. Feather pecking, cannibalism, smothering and mortality were perceived to be higher in current housing systems than in CC, but only by respondents who shifted to non-cage systems from previously having had CC. Health- and production-related parameters were scored to be more important for hen welfare as compared to behavior-related parameters. Those without CC in the past rated factors relating to natural behavior to be more important for welfare than those with CC. This difference in opinion based on farmer backgrounds should be taken into account in future research
Some spaces are more equal than others
It has generally been thought that in perturbed
Friedmann-Lemaitre-Robertson-Walker models of the Universe, global topology
should not have any feedback effects on dynamics. However, a weak-field limit
heuristical argument, assuming a finite particle horizon for the transmission
of gravitational signals, shows that a residual acceleration effect can occur.
The nature of this effect differs algebraically between different constant
curvature 3-manifolds. This potentially provides a selection mechanism for the
3-manifold of comoving space.Comment: 4 pages, proceedings of the Grassmannian Conference in Fundamental
Cosmology, Szczecin, 14-19 Sep 2009, to be refereed by Annalen der Physi
Detectability of Cosmic Topology in Generalized Chaplygin Gas Models
If the spatial section of the universe is multiply connected, repeated images
or patterns are expected to be detected observationally. However, due to the
finite distance to the last scattering surface, such pattern repetitions could
be unobservable. This raises the question of whether a given cosmic topology is
detectable, depending on the values of the parameters of the cosmological
model. We study how detectability is affected by the choice of the model itself
for the matter-energy content of the universe, focusing our attention on the
generalized Chaplygin gas (GCG) model for dark matter and dark energy
unification, and investigate how the detectability of cosmic topology depends
on the GCG parameters. We determine to what extent a number of topologies are
detectable for the current observational bounds on these parameters. It emerges
from our results that the choice of GCG as an alternative to the CDM
matter-energy content model has an impact on the detectability of cosmic
topology.Comment: Submitted to A&
Effect of polydispersity in concentrated magnetorheological fluids
Magnetorheological fluids (MRF) are smart materials of increasing interest
due to their great versatility in mechanical and mechatronic systems. As main
rheological features, MRFs must present low viscosity in the absence of a
magnetic field (0.1 - 1.0 Pa.s) and high yield stress (50 - 100 kPa) when
magnetized, in order to optimize the magnetorheological effect. Such
properties, in turn, are directly influenced by the composition, volume
fraction, size, and size distribution (polydispersity) of the particles, the
latter being an important piece in the improvement of these main properties. In
this context, the present work aims to analyze, through experiments and
simulations, the influence of polydispersity on the maximum packing fraction,
on the yield stress under field (on-state), and on the plastic viscosity in the
absence of field (off-state) of concentrated MRF (phi = 48.5 vol.%). Three
blends of carbonyl iron powder in polyalphaolefin oil were prepared. These
blends have the same mode, but different polydispersity indexes, ranging from
0.46 to 1.44. Separate simulations show that the random close packing fraction
increases from about 68% to 80% as the polydispersity index increases over this
range. The on-state yield stress, in turn, is raised from 30 +/- 0.5 kPa to 42
+/- 2 kPa (B ~ 0.57 T) and the off-state plastic viscosity, is reduced from 4.8
Pa.s to 0.5 Pa.s. Widening the size distributions, as is well known in the
literature, increases packing efficiency and reduces the viscosity of
concentrated dispersions, but beyond that, it proved to be a viable way to
increase the magnetorheological effect of concentrated MRF. The Brouwers model,
which considers the void fraction in suspensions of particles with lognormal
distribution, was proposed as a possible hypothesis to explain the increase in
yield stress under magnetic field
- âŚ