303 research outputs found
Towards a continuum theory of clustering in a freely cooling inelastic gas
We performed molecular dynamics simulations to investigate the clustering
instability of a freely cooling dilute gas of inelastically colliding disks in
a quasi-one-dimensional setting. We observe that, as the gas cools, the shear
stress becomes negligibly small, and the gas flows by inertia only. Finite-time
singularities, intrinsic in such a flow, are arrested only when close-packed
clusters are formed. We observe that the late-time dynamics of this system are
describable by the Burgers equation with vanishing viscosity, and predict the
long-time coarsening behavior.Comment: 7 pages, 5 eps figures, to appear in Europhys. Let
Ins1 Gene Up-Regulated in a ÎČ-Cell Line Derived from Ins2 Knockout Mice
The authors have derived a new ÎČ-cell line (ÎČIns2â/âlacZ) from Ins2â/â mice that carry the lacZ reporter gene under control of the Ins2 promoter. ÎČIns2â/âlacZ cells stained positively using anti-insulin antibody, expressed ÎČ-cellâspecific genes encoding the transcription factor PDX-1, glucokinase, and Glut-2, retained glucose-responsiveness for insulin secretion, and expressed the lacZ gene. Analysis of Ins1 expression by reverse transcriptaseâpolymerase chain reaction (RT-PCR) showed that Ins1 transcripts were significantly raised to compensate for the lack of Ins2 transcripts in ÎČIns2â/âlacZ cells, as compared to those found in ÎČTC1 cells expressing both Ins1/Ins2. Thus, transcriptional up-regulation of the remaining functional insulin gene in Ins2â/â mice could potentially contribute to the ÎČ-cell adaptation exhibited by these mutants, in addition to the increase in ÎČ-cell mass that we previously reported.We have also shown that lacZ expression, as analyzed by determining ÎČ-galactosidase activity, was up-regulated by incubating ÎČIns2â/âlacZ cells with GLP-1 and/or IBMX, 2 known stimulators of insulin gene expression. These cells thus represent a new tool for testing of molecules capable of stimulating Ins2 promoter activit
Fast diffusion of a Lennard-Jones cluster on a crystalline surface
We present a Molecular Dynamics study of large Lennard-Jones clusters
evolving on a crystalline surface. The static and the dynamic properties of the
cluster are described. We find that large clusters can diffuse rapidly, as
experimentally observed. The role of the mismatch between the lattice
parameters of the cluster and the substrate is emphasized to explain the
diffusion of the cluster. This diffusion can be described as a Brownian motion
induced by the vibrationnal coupling to the substrate, a mechanism that has not
been previously considered for cluster diffusion.Comment: latex, 5 pages with figure
Spatial Correlations in Compressible Granular Flows
For a freely evolving granular fluid, the buildup of spatial correlations in
density and flow field is described using fluctuating hydrodynamics. The theory
for incompressible flows is extended to the general, compressible case,
including longitudinal velocity and density fluctuations, and yields
qualitatively different results for long range correlations. The structure
factor of density fluctuations shows a maximum at finite wavenumber, shifting
in time to smaller wavenumbers and corresponding to a growing correlation
length. It agrees well with two-dimensional molecular dynamics simulations.Comment: 12 pages, Latex, 3 figure
Phase Changes in an Inelastic Hard Disk System with a Heat Bath under Weak Gravity for Granular Fluidization
We performed numerical simulations on a two-dimensional inelastic hard disk
system under gravity with a heat bath to study the dynamics of granular
fluidization. Upon increasing the temperature of the heat bath, we found that
three phases, namely, the condensed phase, locally fluidized phase, and
granular turbulent phase, can be distinguished using the maximum packing
fraction and the excitation ratio, or the ratio of the kinetic energy to the
potential energy.It is shown that the system behavior in each phase is very
different from that of an ordinary vibrating bed.Comment: 4 pages, including 5 figure
Shock-Like Dynamics of Inelastic Gases
We provide a simple physical picture which suggests that the asymptotic
dynamics of inelastic gases in one dimension is independent of the degree of
inelasticity. Statistical characteristics, including velocity fluctuations and
the velocity distribution are identical to those of a perfectly inelastic
sticky gas, which in turn is described by the inviscid Burgers equation.
Asymptotic predictions of this continuum theory, including the t^{-2/3}
temperature decay and the development of discontinuities in the velocity
profile, are verified numerically for inelastic gases.Comment: 4 pages, 5 figures, revte
A dc voltage step-up transformer based on a bi-layer \nu=1 quantum Hall system
A bilayer electron system in a strong magnetic field at low temperatures,
with total Landau level filling factor nu =1, can enter a strongly coupled
phase, known as the (111) phase or the quantum Hall pseudospin-ferromagnet. In
this phase there is a large quantized Hall drag resistivity between the layers.
We consider here structures where regions of (111) phase are separated by
regions in which one of the layers is depleted by means of a gate, and various
of the regions are connected together by wired contacts. We note that with
suitable designs, one can create a DC step-up transformer where the output
voltage is larger than the input, and we show how to analyze the current flows
and voltages in such devices
Diffusion of gold nanoclusters on graphite
We present a detailed molecular-dynamics study of the diffusion and
coalescence of large (249-atom) gold clusters on graphite surfaces. The
diffusivity of monoclusters is found to be comparable to that for single
adatoms. Likewise, and even more important, cluster dimers are also found to
diffuse at a rate which is comparable to that for adatoms and monoclusters. As
a consequence, large islands formed by cluster aggregation are also expected to
be mobile. Using kinetic Monte Carlo simulations, and assuming a proper scaling
law for the dependence on size of the diffusivity of large clusters, we find
that islands consisting of as many as 100 monoclusters should exhibit
significant mobility. This result has profound implications for the morphology
of cluster-assembled materials
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