28 research outputs found
The relaxation dynamics of a simple glass former confined in a pore
We use molecular dynamics computer simulations to investigate the relaxation
dynamics of a binary Lennard-Jones liquid confined in a narrow pore. We find
that the average dynamics is strongly influenced by the confinement in that
time correlation functions are much more stretched than in the bulk. By
investigating the dynamics of the particles as a function of their distance
from the wall, we can show that this stretching is due to a strong dependence
of the relaxation time on this distance, i.e. that the dynamics is spatially
very heterogeneous. In particular we find that the typical relaxation time of
the particles close to the wall is orders of magnitude larger than the one of
particles in the center of the pore.Comment: 9 pages of Latex, 4 figure
Relaxation properties in a lattice gas model with asymmetrical particles
We study the relaxation process in a two-dimensional lattice gas model, where
the interactions come from the excluded volume. In this model particles have
three arms with an asymmetrical shape, which results in geometrical frustration
that inhibits full packing. A dynamical crossover is found at the arm
percolation of the particles, from a dynamical behavior characterized by a
single step relaxation above the transition, to a two-step decay below it.
Relaxation functions of the self-part of density fluctuations are well fitted
by a stretched exponential form, with a exponent decreasing when the
temperature is lowered until the percolation transition is reached, and
constant below it. The structural arrest of the model seems to happen only at
the maximum density of the model, where both the inverse diffusivity and the
relaxation time of density fluctuations diverge with a power law. The dynamical
non linear susceptibility, defined as the fluctuations of the self-overlap
autocorrelation, exhibits a peak at some characteristic time, which seems to
diverge at the maximum density as well.Comment: 7 pages and 9 figure
Computer Simulations of Supercooled Liquids and Glasses
After a brief introduction to the dynamics of supercooled liquids, we discuss
some of the advantages and drawbacks of computer simulations of such systems.
Subsequently we present the results of computer simulations in which the
dynamics of a fragile glass former, a binary Lennard-Jones system, is compared
to the one of a strong glass former, SiO_2. This comparison gives evidence that
the reason for the different temperature dependence of these two types of glass
formers lies in the transport mechanism for the particles in the vicinity of
T_c, the critical temperature of mode-coupling theory. Whereas the one of the
fragile glass former is described very well by the ideal version of
mode-coupling theory, the one for the strong glass former is dominated by
activated processes. In the last part of the article we review some simulations
of glass formers in which the dynamics below the glass transition temperature
was investigated. We show that such simulations might help to establish a
connection between systems with self generated disorder (e.g. structural
glasses) and quenched disorder (e.g. spin glasses).Comment: 37 pages of Latex, 11 figures, to appear as a Topical Review article
in J. Phys.: Condens. Matte
The Influence of Recovery and Training Phases on Body Composition, Peripheral Vascular Function and Immune System of Professional Soccer Players
Professional soccer players have a lengthy playing season, throughout which high levels of physical stress are maintained. The following recuperation period, before starting the next pre-season training phase, is generally considered short but sufficient to allow a decrease in these stress levels and therefore a reduction in the propensity for injury or musculoskeletal tissue damage. We hypothesised that these physical extremes influence the body composition, blood flow, and endothelial/immune function, but that the recuperation may be insufficient to allow a reduction of tissue stress damage. Ten professional football players were examined at the end of the playing season, at the end of the season intermission, and after the next pre-season endurance training. Peripheral blood flow and body composition were assessed using venous occlusion plethysmography and DEXA scanning respectively. In addition, selected inflammatory and immune parameters were analysed from blood samples. Following the recuperation period a significant decrease of lean body mass from 74.4±4.2 kg to 72.2±3.9 kg was observed, but an increase of fat mass from 10.3±5.6 kg to 11.1±5.4 kg, almost completely reversed the changes seen in the pre-season training phase. Remarkably, both resting and post-ischemic blood flow (7.3±3.4 and 26.0±6.3 ml/100 ml/min) respectively, were strongly reduced during the playing and training stress phases, but both parameters increased to normal levels (9.0±2.7 and 33.9±7.6 ml/100 ml/min) during the season intermission. Recovery was also characterized by rising levels of serum creatinine, granulocytes count, total IL-8, serum nitrate, ferritin, and bilirubin. These data suggest a compensated hypo-perfusion of muscle during the playing season, followed by an intramuscular ischemia/reperfusion syndrome during the recovery phase that is associated with muscle protein turnover and inflammatory endothelial reaction, as demonstrated by iNOS and HO-1 activation, as well as IL-8 release. The data provided from this study suggest that the immune system is not able to function fully during periods of high physical stress. The implications of this study are that recuperation should be carefully monitored in athletes who undergo intensive training over extended periods, but that these parameters may also prove useful for determining an individual's risk of tissue stress and possibly their susceptibility to progressive tissue damage or injury
Biological Earth observation with animal sensors.
Space-based tracking technology using low-cost miniature tags is now delivering data on fine-scale animal movement at near-global scale. Linked with remotely sensed environmental data, this offers a biological lens on habitat integrity and connectivity for conservation and human health; a global network of animal sentinels of environmen-tal change
Biological Earth observation with animal sensors
Space-based tracking technology using low-cost miniature tags is now delivering data on fine-scale animal movement at near-global scale. Linked with remotely sensed environmental data, this offers a biological lens on habitat integrity and connectivity for conservation and human health; a global network of animal sentinels of environmen-tal change
THE ELECTRON-PHONON COUPLING CONSTANT OF AMORPHOUS METALS
Extending the free-electron theory of electronic transport in simple amorphous metals, the Eliashberg function α2F(Ï) and the electron-phonon constant λ are calculated for such materials. A formula relating λ to the temperature coefficient of the resistivity at higher temperatures (T >â ΞD/2) is derived. The validity of this formula is tested for amorphous Ga
HOPPING AND STATIC CORRELATIONS OF HIGHER ORDER IN A LATTICE GAS
In interacting lattice gases at intermediate concentrations the hopping rate is determined by the static correlations of relatively high order among the neighbours of the diffusing particle. For a square lattice with nearest-neighbour exclusion the relevant correlation functions are studied by Monte Carlo simulation and analytically by an extended Bethe-Peierls cluster approximation. The results of both methods are in good agreement with each other and allow to test the accuracy of approximation schemes for higher-order correlation functions such as Kirkwood's superposition approximation and the Gaussian factorization
Size dependence of self-diffusion in dense binary liquid of hard spheres
For a very dense binary liquid of hard spheres the dependence of the velocity-autocorrelation function (VAF) of tagged particles and of the self-diffusion coefficient on system size is derived from the coupling of self diffusion to modes of collective flow. Collective flow is described by longitudinal and transverse current-current correlation functions for different wavevectors which are obtained by molecular dynamics simulation. The size dependence of the VAF is reproduced rather accurately for longer times t > 7Ï, where Ï is the mean time of free flight. The result for the size dependence of the self-diffusion coefficient is in satisfactory agreement with a direct calculation, but this result also depends on the VAF's at shorter times