44 research outputs found
Landscape equivalent of the shoving model
It is shown that the shoving model expression for the average relaxation time
of viscous liquids follows largely from a classical "landscape" estimation of
barrier heights from curvature at energy minima. The activation energy involves
both instantaneous bulk and shear moduli, but the bulk modulus contributes less
than 8% to the temperature dependence of the activation energy. This reflects
the fact that the physics of the two models are closely related.Comment: 4 page
Kinetics of Anchoring of Polymer Chains on Substrates with Chemically Active Sites
We consider dynamics of an isolated polymer chain with a chemically active
end-bead on a 2D solid substrate containing immobile, randomly placed
chemically active sites (traps). For a particular situation when the end-bead
can be irreversibly trapped by any of these sites, which results in a complete
anchoring of the whole chain, we calculate the time evolution of the
probability that the initially non-anchored chain remains mobile
until time . We find that for relatively short chains follows at
intermediate times a standard-form 2D Smoluchowski-type decay law , which crosses over at very large times to the
fluctuation-induced dependence , associated with
fluctuations in the spatial distribution of traps. We show next that for long
chains the kinetic behavior is quite different; here the intermediate-time
decay is of the form , which is the
Smoluchowski-type law associated with subdiffusive motion of the end-bead,
while the long-time fluctuation-induced decay is described by the dependence
, stemming out of the interplay between
fluctuations in traps distribution and internal relaxations of the chain.Comment: Latex file, 19 pages, one ps figure, to appear in PR
Rate processes on fractals: Theory, simulations, and experiments
Heterogeneous kinetics are shown to differ drastically from homogeneous kinetics. For the elementary reaction A + A → products we show that the diffusion-limited reaction rate is proportional to t − h[A] 2 or to [A] x , where h=1- d s /2, X=1+ 2/d s =(h-2)(h-1 ), and d s is the effective spectral dimension. We note that for d = d s =1, h =1/2 and X = 3 , for percolating clusters d s = 4/3, h = 1/3 and X = 5/2 , while for “dust” d s h > 1/2 and ∞ > X > 3. Scaling arguments, supercomputer simulations and experiments give a consistent picture. The interplay of energetic and geometric heterogeneity results in fractal-like kinetics and is relevant to excitation fusion experiments in porous membranes, films, and polymeric glasses. However, in isotopic mixed crystals, the geometric fractal nature (percolation clusters) dominates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45150/1/10955_2005_Article_BF01010846.pd
SEIS: Insight’s Seismic Experiment for Internal Structure of Mars
By the end of 2018, 42 years after the landing of the two Viking seismometers
on Mars, InSight will deploy onto Mars’ surface the SEIS (Seismic Experiment for Internal
Structure) instrument; a six-axes seismometer equipped with both a long-period three-axes
Very Broad Band (VBB) instrument and a three-axes short-period (SP) instrument. These
six sensors will cover a broad range of the seismic bandwidth, from 0.01 Hz to 50 Hz,
with possible extension to longer periods. Data will be transmitted in the form of three
continuous VBB components at 2 sample per second (sps), an estimation of the short period
energy content from the SP at 1 sps and a continuous compound VBB/SP vertical axis at
10 sps. The continuous streams will be augmented by requested event data with sample
rates from 20 to 100 sps. SEIS will improve upon the existing resolution of Viking’s Mars
seismic monitoring by a factor of ∼ 2500 at 1 Hz and ∼ 200 000 at 0.1 Hz. An additional
major improvement is that, contrary to Viking, the seismometers will be deployed via a
robotic arm directly onto Mars’ surface and will be protected against temperature and wind
by highly efficient thermal and wind shielding. Based on existing knowledge of Mars, it is
reasonable to infer a moment magnitude detection threshold of Mw ∼ 3 at 40◦ epicentral
distance and a potential to detect several tens of quakes and about five impacts per year. In
this paper, we first describe the science goals of the experiment and the rationale used to
define its requirements. We then provide a detailed description of the hardware, from the
sensors to the deployment system and associated performance, including transfer functions
of the seismic sensors and temperature sensors. We conclude by describing the experiment
ground segment, including data processing services, outreach and education networks and
provide a description of the format to be used for future data distribution
Dextromethorphan as a potential neuroprotective agent with unique mechanisms of action
BACKGROUND: Dextromethorphan (DM) is a widely-used antitussive. DM\u27s complex central nervous system (CNS) pharmacology became of interest when it was discovered to be neuroprotective due to its low-affinity, uncompetitive N-methyl-d-aspartate (NMDA) receptor antagonism. REVIEW SUMMARY: Mounting preclinical evidence has proven that DM has important neuroprotective properties in various CNS injury models, including focal and global ischemia, seizure, and traumatic brain injury paradigms. Many of these protective actions seem functionally related to its inhibitory effects on glutamate-induced neurotoxicity via NMDA receptor antagonist, sigma-1 receptor agonist, and voltage-gated calcium channel antagonist actions. DM\u27s protection of dopamine neurons in parkinsonian models may be due to inhibition of neurodegenerative inflammatory responses. Clinical findings are limited, with preliminary evidence indicating that DM protects against neuronal damage. Negative findings seem to relate to attainment of inadequate DM brain concentrations. Small studies have shown some promise for treatment of perioperative brain injury, amyotrophic lateral sclerosis, and symptoms of methotrexate neurotoxicity. DM safety/tolerability trials in stroke, neurosurgery, and amyotrophic lateral sclerosis patients demonstrated a favorable safety profile. DM\u27s limited clinical benefit is proposed to be associated with its rapid metabolism to dextrorphan, which restricts its central bioavailability and therapeutic utility. Systemic concentrations of DM can be increased via coadministration of low-dose quinidine (Q), which reversibly inhibits its first-pass elimination. Potential drug interactions with DM/Q are discussed. CONCLUSIONS: Given the compelling preclinical evidence for neuroprotective properties of DM, initial clinical neuroprotective findings, and clinical demonstrations that the DM/Q combination is well tolerated, this strategy may hold promise for the treatment of various acute and degenerative neurologic disorders. © 2007 Lippincott Williams & Wilkins, Inc
Performance evaluation of automatic airdust monitors by means of a simulation code
Automatic analysis of airborne polluttants by X ray fluorescenc
The Western Arctic Linkage Experiment (WALE): Overview and synthesis
ABSTRACT: The primary goal of the Western Arctic Linkage Experiment (WALE) was to better understand uncertainties of simulated hydrologic and ecosystem dynamics of the western Arctic in the context of 1) uncertainties in the data available to drive the models and 2) different approaches to simulating regional hydrology and ecosystem dynamics. Analyses of datasets on climate available for driving hydrologic and ecosystem models within the western Arctic during the late twentieth century indicate that there are substantial differences among the mean states of datasets for temperature, precipitation, vapor pressure, and radiation variables. Among the studies that examined temporal trends among the alternative climate datasets, there is not much consensus on trends among the datasets. In contrast, monthly and interannual variations of some variables showed some correlation across the datasets. Th