853 research outputs found
Long wavelength structural anomalies in jammed systems
The structural properties of static, jammed packings of monodisperse spheres
in the vicinity of the jamming transition are investigated using large-scale
computer simulations. At small wavenumber , we argue that the anomalous
behavior in the static structure factor, , is consequential of an
excess of low-frequency, collective excitations seen in the vibrational
spectrum. This anomalous feature becomes more pronounced closest to the jamming
transition, such that at the transition point. We introduce an
appropriate dispersion relation that accounts for these phenomena that leads us
to relate these structural features to characteristic length scales associated
with the low-frequency vibrational modes of these systems. When the particles
are frictional, this anomalous behavior is suppressed providing yet more
evidence that jamming transitions of frictional spheres lie at lower packing
fractions that that for frictionless spheres. These results suggest that the
mechanical properties of jammed and glassy media may therefore be inferred from
measurements of both the static and dynamical structure factors.Comment: 8 pages, 6 figure captions. Completely revised version to appear in
Phys. Rev.
Association between urinary sodium, creatinine, albumin, and long term survival in chronic kidney disease
Dietary sodium intake is associated with hypertension and cardiovascular risk in the general population. In patients with chronic kidney disease, sodium intake has been associated with progressive renal disease, but not independently of proteinuria. We studied the relationship between urinary sodium excretion and urinary sodium:creatinine ratio and mortality or requirement for renal replacement therapy in chronic kidney disease. Adults attending a renal clinic who had at least one 24-hour urinary sodium measurement were identified. 24-hour urinary sodium measures were collected and urinary sodium:creatinine ratio calculated. Time to renal replacement therapy or death was recorded. 423 patients were identified with mean estimated glomerular filtration rate of 48ml/min/1.73m<sup>2</sup>. 90 patients required renal replacement therapy and 102 patients died. Mean slope decline in estimated glomerular filtration rate was -2.8ml/min/1.73m<sup>2</sup>/year. Median follow-up was 8.5 years. Patients who died or required renal replacement therapy had significantly higher urinary sodium excretion and urinary sodium:creatinine but the association with these parameters and poor outcome was not independent of renal function, age and albuminuria. When stratified by albuminuria, urinary sodium:creatinine was a significant cumulative additional risk for mortality, even in patients with low level albuminuria. There was no association between low urinary sodium and risk, as observed in some studies. This study demonstrates an association between urinary sodium excretion and mortality in chronic kidney disease, with a cumulative relationship between sodium excretion, albuminuria and reduced survival. These data support reducing dietary sodium intake in chronic kidney disease but further study is required to determine the target sodium intake
Charge Oscillations in Debye-Hueckel Theory
The recent generalized Debye-Hueckel (GDH) theory is applied to the
calculation of the charge-charge correlation function G_{ZZ}(r). The resulting
expression satisfies both (i) the charge neutrality condition and (ii) the
Stillinger-Lovett second-moment condition for all T and rho_N, the overall ion
density, and (iii) exhibits charge oscillations for densities above a "Kirkwood
line" in the (rho_N,T) plane. This corrects the normally assumed DH
correlations, and, when combined with the GDH analysis of the density
correlations, leaves the GDH theory as the only complete description of ionic
correlation functions, as judged by (i)-(iii), (iv) exact low-density (rho_N,T)
variation, and (v) reasonable behavior near criticality.Comment: 6 pages, EuroPhys.sty (now available on archive), 1 eps figur
Description beyond the mean field approximation of an electrolyte confined between two planar metallic electrodes
We study an electrolyte confined in a slab of width composed of two
grounded metallic parallel electrodes. We develop a description of this system
in a low coupling regime beyond the mean field (Poisson--Boltzmann)
approximation. There are two ways to model the metallic boundaries: as ideal
conductors in which the electric potential is zero and it does not fluctuate,
or as good conductors in which the average electric potential is zero but the
thermal fluctuations of the potential are not zero. This latter model is more
realistic. For the ideal conductor model we find that the disjoining pressure
is positive behaves as for large separations with a prefactor that is
universal, i.e. independent of the microscopic constitution of the system. For
the good conductor boundaries the disjoining pressure is negative and it has an
exponential decay for large . We also compute the density and electric
potential profiles inside the electrolyte. These are the same in both models.
If the electrolyte is charge asymmetric we find that the system is not locally
neutral and that a non-zero potential difference builds up between any
electrode and the interior of the system although both electrodes are grounded.Comment: 16 pages, 5 figures, added a new appendix B and a discussion on ideal
conductors vs. good conductor
Virial series for inhomogeneous fluids applied to the Lennard-Jones wall-fluid surface tension at planar and curved walls
We formulate a straightforward scheme of statistical mechanics for
inhomogeneous systems that includes the virial series in powers of the activity
for the grand free energy and density distributions. There, cluster integrals
formulated for inhomogeneous systems play a main role. We center on second
order terms that were analyzed in the case of hard-wall confinement, focusing
in planar, spherical and cylindrical walls. Further analysis was devoted to the
Lennard-Jones system and its generalization the 2k-k potential. For this
interaction potentials the second cluster integral was evaluated analytically.
We obtained the fluid-substrate surface tension at second order for the planar,
spherical and cylindrical confinement. Spherical and cylindrical cases were
analyzed using a series expansion in the radius including higher order terms.
We detected a dependence of the surface tension for the
standard Lennard-Jones system confined by spherical and cylindrical walls, no
matter if particles are inside or outside of the hard-walls. The analysis was
extended to bending and Gaussian curvatures, where exact expressions were also
obtained.Comment: 15 pages, 6 figure
General Non-equilibrium Theory of Colloid Dynamics
A non-equilibrium extension of Onsager's canonical theory of thermal
fluctuations is employed to derive a self-consistent theory for the description
of the statistical properties of the instantaneous local concentration profile
n(r,t) of a colloidal liquid in terms of the coupled time evolution equations
of its mean value n(r,t) and of the covariance {\sigma}(r,r';t) \equiv
of its fluctuations {\delta}n(r, t) = n(r, t) -
n(r, t). These two coarse-grained equations involve a local mobility function
b(r, t) which, in its turn, is written in terms of the memory function of the
two-time correlation function C(r, r' ; t, t') \equiv <{\delta}n(r,
t){\delta}n(r',t')>. For given effective interactions between colloidal
particles and applied external fields, the resulting self-consistent theory is
aimed at describing the evolution of a strongly correlated colloidal liquid
from an initial state with arbitrary mean and covariance n^0(r) and
{\sigma}^0(r,r') towards its equilibrium state characterized by the equilibrium
local concentration profile n^(eq)(r) and equilibrium covariance
{\sigma}^(eq)(r,r').
This theory also provides a general theoretical framework to describe
irreversible processes associated with dynamic arrest transitions, such as
aging, and the effects of spatial heterogeneities
The properties of fully flexible Lennard-Jones chains in the solid phase: Wertheim theory and simulation
NpT ensemble Monte Carlo simulations were performed for fully flexible
Lennard-Jones chains in the solid phase. The bond length between monomers
within the chains is fixed to and the molecule is free to adopt any
configuration. The solid structure of fully flexible chains is obtained by
randomly locating the bonds of the chain within a face centered cubic close
packed arrangement of atoms. For fully flexible chains it is believed that the
stable solid phase is disordered. Such a solid is considered in this work.
Computer simulations were performed for chains with and 5 monomer
units, and results were obtained for the equation of state and internal energy
of the chains. An extension of Wertheim's TPT1 to the solid phase of
Lennard-Jones chains (C.Vega, F.J.Blas and A.Galindo, J.Chem.Phys.,{\bf
116},7645,2002) has been proposed recently. The simulation results of this work
provide a check on the performance of this theory. It is found that Wertheim's
TPT1 successfully predicts the equation of state and internal energies of fully
flexible LJ chains in the solid phase. Finally, a rigid LJ chain in a linear
configuration has been considered. Computer simulations were also performed for
the rigid chain in an ordered solid structure. It is found that fully flexible
and linear rigid chains present quite different equations of state and
different thermodynamic properties in the solid phase.Comment: PrEprint of the paper published in Molecular Physics volume 101, pp.
2241-2255 (2003
Dimensional crossover of a boson gas in multilayers
We obtain the thermodynamic properties for a non-interacting Bose gas
constrained on multilayers modeled by a periodic Kronig-Penney delta potential
in one direction and allowed to be free in the other two directions. We report
Bose-Einstein condensation (BEC) critical temperatures, chemical potential,
internal energy, specific heat, and entropy for different values of a
dimensionless impenetrability between layers. The BEC critical
temperature coincides with the ideal gas BEC critical temperature
when and rapidly goes to zero as increases to infinity for
any finite interlayer separation. The specific heat \textit{vs} for
finite and plane separation exhibits one minimum and one or two maxima
in addition to the BEC, for temperatures larger than which highlights
the effects due to particle confinement. Then we discuss a distinctive
dimensional crossover of the system through the specific heat behavior driven
by the magnitude of . For the crossover is revealed by the change
in the slope of and when , it is evidenced by a broad
minimum in .Comment: Ten pages, nine figure
Networked urban governance: a socio-structural analysis of transport strategies in London and New York
This paper investigates urban governance empirically by applying social network analysis methods to data gathered through structured interviews in London and New York. We explore how decisions are made in complex institutional environments inhabited by various types of actors. Owing to the time-consuming data collection and treatment processes, the research zooms in on transport. The comparative approach enabled the detection of different structural features in the governance networks shaping transport strategies in both cities. The perceived relative power, influence, dependence and/or affinity between the actors involved is discussed based on network attributes. The evidence suggests that transport governance in London is more centralised (and, arguably, more technocratic and integrated), in the sense that a few prestigious entities are clearly more prominent. In New York the institutional environment is typified by many checks and balances (and, arguably, more democratic and fragmented), where central actors are less obvious
- …