2,320 research outputs found
Thermal collapse of a granular gas under gravity
Free cooling of a gas of inelastically colliding hard spheres represents a
central paradigm of kinetic theory of granular gases. At zero gravity the
temperature of a freely cooling homogeneous granular gas follows a power law in
time. How does gravity, which brings inhomogeneity, affect the cooling? We
combine molecular dynamics simulations, a numerical solution of hydrodynamic
equations and an analytic theory to show that a granular gas cooling under
gravity undergoes thermal collapse: it cools down to zero temperature and
condenses on the bottom of the container in a finite time.Comment: 4 pages, 12 eps figures, to appear in PR
Velocity fluctuations of noisy reaction fronts propagating into a metastable state: testing theory in stochastic simulations
The position of a reaction front, propagating into a metastable state,
fluctuates because of the shot noise of reactions and diffusion. A recent
theory [B. Meerson, P.V. Sasorov, and Y. Kaplan, Phys. Rev. E 84, 011147
(2011)] gave a closed analytic expression for the front diffusion coefficient
in the weak noise limit. Here we test this theory in stochastic simulations
involving reacting and diffusing particles on a one-dimensional lattice. We
also investigate a small noise-induced systematic shift of the front velocity
compared to the prediction from the spatially continuous deterministic
reaction-diffusion equation.Comment: 5 pages, 5 figure
Humanin G (HNG) protects age-related macular degeneration (AMD) transmitochondrial ARPE-19 cybrids from mitochondrial and cellular damage.
Age-related macular degeneration (AMD) ranks third among the leading causes of visual impairment with a blindness prevalence rate of 8.7%. Despite several treatment regimens, such as anti-angiogenic drugs, laser therapy, and vitamin supplementation, being available for wet AMD, to date there are no FDA-approved therapies for dry AMD. Substantial evidence implicates mitochondrial damage and retinal pigment epithelium (RPE) cell death in the pathogenesis of AMD. However, the effects of AMD mitochondria and Humanin G (HNG), a more potent variant of the mitochondrial-derived peptide (MDP) Humanin, on retinal cell survival have not been elucidated. In this study, we characterized mitochondrial and cellular damage in transmitochondrial cybrid cell lines that contain identical nuclei but possess mitochondria from either AMD or age-matched normal (Older-normal (NL)) subjects. AMD cybrids showed (1) reduced levels of cell viability, lower mtDNA copy numbers, and downregulation of mitochondrial replication/transcription genes and antioxidant enzyme genes; and (2) elevated levels of genes related to apoptosis, autophagy and ER-stress along with increased mtDNA fragmentation and higher susceptibility to amyloid-β-induced toxicity compared to NL cybrids. In AMD cybrids, HNG protected the AMD mitochondria, reduced pro-apoptosis gene and protein levels, upregulated gp130 (a component of the HN receptor complex), and increased the protection against amyloid-β-induced damage. In summary, in cybrids, damaged AMD mitochondria mediate cell death that can be reversed by HNG treatment. Our results also provide evidence of Humanin playing a pivotal role in protecting cells with AMD mitochondria. In the future, it may be possible that AMD patient's blood samples containing damaged mitochondria may be useful as biomarkers for this condition. In conclusion, HNG may be a potential therapeutic target for treatment of dry AMD, a debilitating eye disease that currently has no available treatment. Further studies are needed to establish HNG as a viable mitochondria-targeting therapy for dry AMD
Navier-Stokes hydrodynamics of thermal collapse in a freely cooling granular gas
We employ Navier-Stokes granular hydrodynamics to investigate the long-time
behavior of clustering instability in a freely cooling dilute granular gas in
two dimensions. We find that, in circular containers, the homogeneous cooling
state (HCS) of the gas loses its stability via a sub-critical pitchfork
bifurcation. There are no time-independent solutions for the gas density in the
supercritical region, and we present analytical and numerical evidence that the
gas develops thermal collapse unarrested by heat diffusion. To get more
insight, we switch to a simpler geometry of a narrow-sector-shaped container.
Here the HCS loses its stability via a transcritical bifurcation. For some
initial conditions a time-independent inhomogeneous density profile sets in,
qualitatively similar to that previously found in a narrow-channel geometry.
For other initial conditions, however, the dilute gas develops thermal collapse
unarrested by heat diffusion. We determine the dynamic scalings of the flow
close to collapse analytically and verify them in hydrodynamic simulations. The
results of this work imply that, in dimension higher than one, Navier-Stokes
hydrodynamics of a dilute granular gas is prone to finite-time density blowups.
This provides a natural explanation to the formation of densely packed clusters
of particles in a variety of initially dilute granular flows.Comment: 18 pages, 19 figure
Switching between phenotypes and population extinction
Many types of bacteria can survive under stress by switching stochastically
between two different phenotypes: the "normals" who multiply fast, but are
vulnerable to stress, and the "persisters" who hardly multiply, but are
resilient to stress. Previous theoretical studies of such bacterial populations
have focused on the \emph{fitness}: the asymptotic rate of unbounded growth of
the population. Yet for an isolated population of established (and not very
large) size, a more relevant measure may be the population \emph{extinction
risk} due to the interplay of adverse extrinsic variations and intrinsic noise
of birth, death and switching processes. Applying a WKB approximation to the
pertinent master equation of such a two-population system, we quantify the
extinction risk, and find the most likely path to extinction under both
favorable and adverse conditions. Analytical results are obtained both in the
biologically relevant regime when the switching is rare compared with the birth
and death processes, and in the opposite regime of frequent switching. We show
that rare switches are most beneficial in reducing the extinction risk.Comment: 11 pages, 5 figures. Additional discussion paragraph, minor language
improvements; content as published in Phys. Rev.
Asymptotically false-positive-maximizing attack on non-binary Tardos codes
We use a method recently introduced by Simone and Skoric to study accusation
probabilities for non-binary Tardos fingerprinting codes. We generalize the
pre-computation steps in this approach to include a broad class of collusion
attack strategies. We analytically derive properties of a special attack that
asymptotically maximizes false accusation probabilities. We present numerical
results on sufficient code lengths for this attack, and explain the abrupt
transitions that occur in these results
Emergence of stability in a stochastically driven pendulum: beyond the Kapitsa effect
We consider a prototypical nonlinear system which can be stabilized by
multiplicative noise: an underdamped non-linear pendulum with a stochastically
vibrating pivot. A numerical solution of the pertinent Fokker-Planck equation
shows that the upper equilibrium point of the pendulum can become stable even
when the noise is white, and the "Kapitsa pendulum" effect is not at work. The
stabilization occurs in a strong-noise regime where WKB approximation does not
hold.Comment: 4 pages, 7 figure
Scaling and self-similarity in an unforced flow of inviscid fluid trapped inside a viscous fluid in a Hele-Shaw cell
We investigate quasi-two-dimensional relaxation, by surface tension, of a
long straight stripe of inviscid fluid trapped inside a viscous fluid in a
Hele-Shaw cell. Combining analytical and numerical solutions, we describe the
emergence of a self-similar dumbbell shape and find non-trivial dynamic
exponents that characterize scaling behavior of the dumbbell dimensions.Comment: 4 pages, 5 figures, to appear in PR
Use of Intravitreal Triamcinolone in the Treatment of Macular Edema Related to Retinal Vein Occlusion
ObjectiveTo analyze the increasing trend of intravitreal triamcinolone (IVTA) use in the treatment of retinal vein occlusion-related macular edema.MethodsWe performed MEDLINE/PUBMED searches (September 1984 - December 2007) to identify articles containing the keywords macular edema and triamcinolone. Case reports, reviews and abstracts were identified from references in the reviewed literature. This review focuses on literature published during the past 7 years with more than two-thirds of the articles that we reviewed being printed during the past 5 years. These reports analyzed the success of IVTA in the treatment of macular edema over a 12 month course of time.ResultsThe majority of studies suggested promising results for short time periods (4-6 months) after IVTA treatments. However, long term results were not encouraging.ConclusionsThe success of IVTA therapy for short durations has been the impetus for development of sustained release devices to be used in the treatment of macular edema associated with various retinal diseases including edema related to retinal vein occlusion
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