4,457 research outputs found
Dynamics of short polymer chains in solution
We present numerical and analytical results describing the effect of
hydrodynamic interactions on the dynamics of a short polymer chain in solution.
A molecular dynamics algorithm for the polymer is coupled to a direct
simulation Monte Carlo algorithm for the solvent. We give an explicit
expression for the velocity autocorrelation function of the centre of mass of
the polymer which agrees well with numerical results if Brownian dynamics,
hydrodynamic correlations and sound wave scattering are included
Statistics of precursors to fingering processes
We present an analysis of the statistical properties of hydrodynamic field
fluctuations which reveal the existence of precursors to fingering processes.
These precursors are found to exhibit power law distributions, and these power
laws are shown to follow from spatial -Gaussian structures which are
solutions to the generalized non-linear diffusion equation.Comment: 7 pages incl. 5 figs; tp appear in Europhysics Letter
The XMM Newton and INTEGRAL observations of the supergiant fast X-ray transient IGR J16328-4726
The accretion mechanism producing the short flares observed from the
Supergiant Fast X-ray Transients (SFXT) is still highly debated and forms a
major part in our attempts to place these X-ray binaries in the wider context
of the High Mass X-ray Binaries.
We report on a 216 ks INTEGRAL observation of the SFXT IGR J16328-4726
(August 24-27, 2014) simultaneous with two fixed-time observations with XMM
Newton (33ks and 20ks) performed around the putative periastron passage, in
order to investigate the accretion regime and the wind properties during this
orbital phase. During these observations, the source has shown luminosity
variations, from 4x10^{34} erg/s to 10^{36} erg/s, linked to spectral
properties changes. The soft X-ray continuum is well modeled by a power law
with a photon index varying from 1.2 up to 1.7 and with high values of the
column density in the range 2-4x10^{23}/cm^2. We report on the presence of iron
lines at 6.8-7.1 keV suggesting that the X-ray flux is produced by accretion of
matter from the companion wind characterized by density and temperature
inhomogeneities
Questioning the validity of non-extensive thermodynamics for classical Hamiltonian systems
We examine the non-extensive approach to the statistical mechanics of
Hamiltonian systems with where is the classical kinetic energy. Our
analysis starts from the basics of the formalism by applying the standard
variational method for maximizing the entropy subject to the average energy and
normalization constraints. The analytical results show (i) that the
non-extensive thermodynamics formalism should be called into question to
explain experimental results described by extended exponential distributions
exhibiting long tails, i.e. -exponentials with , and (ii) that in the
thermodynamic limit the theory is only consistent in the range
where the distribution has finite support, thus implying that configurations
with e.g. energy above some limit have zero probability, which is at variance
with the physics of systems in contact with a heat reservoir. We also discuss
the (-dependent) thermodynamic temperature and the generalized specific
heat.Comment: To appear in EuroPhysics Letter
-Scale Decoupling of the Mechanical Relaxation and Diverging Shear Wave Propagation Lengthscale in Triphenylphosphite
We have performed depolarized Impulsive Stimulated Scattering experiments to
observe shear acoustic phonons in supercooled triphenylphosphite (TPP) from
10 - 500 MHz. These measurements, in tandem with previously performed
longitudinal and shear measurements, permit further analyses of the relaxation
dynamics of TPP within the framework of the mode coupling theory (MCT). Our
results provide evidence of coupling between the shear and
longitudinal degrees of freedom up to a decoupling temperature = 231 K. A
lower bound length scale of shear wave propagation in liquids verified the
exponent predicted by theory in the vicinity of the decoupling temperature
A 16 x 16 CMOS amperometric microelectrode array for simultaneous electrochemical measurements
There is a requirement for an electrochemical sensor technology capable of making multivariate measurements in environmental, healthcare, and manufacturing applications. Here, we present a new device that is highly parallelized with an excellent bandwidth. For the first time, electrochemical cross-talk for a chip-based sensor is defined and characterized. The new CMOS electrochemical sensor chip is capable of simultaneously taking multiple, independent electroanalytical measurements. The chip is structured as an electrochemical cell microarray, comprised of a microelectrode array connected to embedded self-contained potentiostats. Speed and sensitivity are essential in dynamic variable electrochemical systems. Owing to the parallel function of the system, rapid data collection is possible while maintaining an appropriately low-scan rate. By performing multiple, simultaneous cyclic voltammetry scans in each of the electrochemical cells on the chip surface, we are able to show (with a cell-to-cell pitch of 456 μm) that the signal cross-talk is only 12% between nearest neighbors in a ferrocene rich solution. The system opens up the possibility to use multiple independently controlled electrochemical sensors on a single chip for applications in DNA sensing, medical diagnostics, environmental sensing, the food industry, neuronal sensing, and drug discovery
Critical dynamics of ballistic and Brownian particles in a heterogeneous environment
The dynamic properties of a classical tracer particle in a random, disordered
medium are investigated close to the localization transition. For Lorentz
models obeying Newtonian and diffusive motion at the microscale, we have
performed large-scale computer simulations, demonstrating that universality
holds at long times in the immediate vicinity of the transition. The scaling
function describing the crossover from anomalous transport to diffusive motion
is found to vary extremely slowly and spans at least 5 decades in time. To
extract the scaling function, one has to allow for the leading universal
corrections to scaling. Our findings suggest that apparent power laws with
varying exponents generically occur and dominate experimentally accessible time
windows as soon as the heterogeneities cover a decade in length scale. We
extract the divergent length scales, quantify the spatial heterogeneities in
terms of the non-Gaussian parameter, and corroborate our results by a thorough
finite-size analysis.Comment: 14 page
Concentration and mass dependence of transport coefficients and correlation functions in binary mixtures with high mass-asymmetry
Correlation functions and transport coefficients of self-diffusion and shear
viscosity of a binary Lennard-Jones mixture with components differing only in
their particle mass are studied up to high values of the mass ratio ,
including the limiting case , for different mole fractions .
Within a large range of and the product of the diffusion coefficient
of the heavy species and the total shear viscosity of the mixture
is found to remain constant, obeying a generalized Stokes-Einstein
relation. At high liquid density, large mass ratios lead to a pronounced cage
effect that is observable in the mean square displacement, the velocity
autocorrelation function and the van Hove correlation function
A generalized Chudley-Elliott vibration-jump model in activated atom surface diffusion
Here the authors provide a generalized Chudley-Elliott expression for
activated atom surface diffusion which takes into account the coupling between
both low-frequency vibrational motion (namely, the frustrated translational
modes) and diffusion. This expression is derived within the Gaussian
approximation framework for the intermediate scattering function at low
coverage. Moreover, inelastic contributions (arising from creation and
annihilation processes) to the full width at half maximum of the quasi-elastic
peak are also obtained.Comment: (5 pages, 2 figures; revised version
- …