928 research outputs found
Electrostatic Disorder-Induced Interactions in Inhomogeneous Dielectrics
We investigate the effect of quenched surface charge disorder on
electrostatic interactions between two charged surfaces in the presence of
dielectric inhomogeneities and added salt. We show that in the linear
weak-coupling regime (i.e., by including mean-field and Gaussian-fluctuations
contributions), the image-charge effects lead to a non-zero disorder-induced
interaction free energy between two surfaces of equal mean charge that can be
repulsive or attractive depending on the dielectric mismatch across the
bounding surfaces and the exact location of the disordered charge distribution.Comment: 7 pages, 2 figure
The Poisson-Boltzmann Theory for Two Parallel Uniformly Charged Plates
We solve the nonlinear Poisson-Boltzmann equation for two parallel and likely
charged plates both inside a symmetric elecrolyte, and inside a 2 : 1
asymmetric electrolyte, in terms of Weierstrass elliptic functions. From these
solutions we derive the functional relation between the surface charge density,
the plate separation, and the pressure between plates. For the one plate
problem, we obtain exact expressions for the electrostatic potential and for
the renormalized surface charge density, both in symmetric and in asymmetric
electrolytes. For the two plate problems, we obtain new exact asymptotic
results in various regimes.Comment: 17 pages, 9 eps figure
Long range polarization attraction between two different likely charged macroions
It is known that in a water solution with multivalent counterions (Z-ions),
two likely charged macroions can attract each other due to correlations of
Z-ions adsorbed on their surfaces. This "correlation" attraction is
short-ranged and decays exponentially with increasing distance between
macroions at characteristic distance A/2\pi, where A is the average distance
between Z-ions on the surfaces of macroions. In this work, we show that an
additional long range "polarization" attraction exists when the bare surface
charge densities of the two macroions have the same sign, but are different in
absolute values. The key idea is that with adsorbed Z-ions, two insulating
macroions can be considered as conductors with fixed but different electric
potentials. Each potential is determined by the difference between the entropic
bulk chemical potential of a Z-ion and its correlation chemical potential at
the surface of the macroion determined by its bare surface charge density. When
the two macroions are close enough, they get polarized in such a way that their
adjacent spots form a charged capacitor, which leads to attraction. In a salt
free solution this polarization attractive force is long ranged: it decays as a
power of the distance between the surfaces of two macroions, d. The
polarization force decays slower than the van der Waals attraction and
therefore is much larger than it in a large range of distances. In the presence
of large amount of monovalent salt, when A/2\pi<< d<< r_s (r_s is the
Debye-H\"{u}ckel screening radius), this force is still much stronger than the
van der Waals attraction and the correlation attraction mentioned above.Comment: 12 pages, 7 figures. Small change in the text, no change in result
Casimir-Polder interatomic potential between two atoms at finite temperature and in the presence of boundary conditions
We evaluate the Casimir-Polder potential between two atoms in the presence of
an infinite perfectly conducting plate and at nonzero temperature. In order to
calculate the potential, we use a method based on equal-time spatial
correlations of the electric field, already used to evaluate the effect of
boundary conditions on interatomic potentials. This method gives also a
transparent physical picture of the role of a finite temperature and boundary
conditions on the Casimir-Polder potential. We obtain an analytical expression
of the potential both in the near and far zones, and consider several limiting
cases of interest, according to the values of the parameters involved, such as
atom-atom distance, atoms-wall distance and temperature.Comment: 11 page
Electroneutrality and Phase Behavior of Colloidal Suspensions
Several statistical mechanical theories predict that colloidal suspensions of
highly charged macroions and monovalent microions can exhibit unusual
thermodynamic phase behavior when strongly deionized. Density-functional,
extended Debye-H\"uckel, and response theories, within mean-field and
linearization approximations, predict a spinodal phase instability of charged
colloids below a critical salt concentration. Poisson-Boltzmann cell model
studies of suspensions in Donnan equilibrium with a salt reservoir demonstrate
that effective interactions and osmotic pressures predicted by such theories
can be sensitive to the choice of reference system, e.g., whether the microion
density profiles are expanded about the average potential of the suspension or
about the reservoir potential. By unifying Poisson-Boltzmann and response
theories within a common perturbative framework, it is shown here that the
choice of reference system is dictated by the constraint of global
electroneutrality. On this basis, bulk suspensions are best modeled by
density-dependent effective interactions derived from a closed reference system
in which the counterions are confined to the same volume as the macroions.
Linearized theories then predict bulk phase separation of deionized suspensions
only when expanded about a physically consistent (closed) reference system.
Lower-dimensional systems (e.g., monolayers, small clusters), depending on the
strength of macroion-counterion correlations, may be governed instead by
density-independent effective interactions tied to an open reference system
with counterions dispersed throughout the reservoir, possibly explaining
observed structural crossover in colloidal monolayers and anomalous
metastability of colloidal crystallites.Comment: 12 pages, 5 figures. Discussion clarified, references adde
"Blue energy" from ion adsorption and electrode charging in sea- and river water
A huge amount of entropy is produced at places where fresh water and seawater
mix, for example at river mouths. This mixing process is a potentially enormous
source of sustainable energy, provided it is harnessed properly, for instance
by a cyclic charging and discharging process of porous electrodes immersed in
salt and fresh water, respectively [D. Brogioli, Phys. Rev. Lett. 103, 058501
(2009)]. Here we employ a modified Poisson-Boltzmann free-energy density
functional to calculate the ionic adsorption and desorption onto and from the
charged electrodes, from which the electric work of a cycle is deduced. We
propose optimal (most efficient) cycles for two given salt baths involving two
canonical and two grand-canonical (dis)charging paths, in analogy to the
well-known Carnot cycle for heat-to-work conversion from two heat baths
involving two isothermal and two adiabatic paths. We also suggest a slightly
modified cycle which can be applied in cases that the stream of fresh water is
limited.Comment: 7 Figure
Paediatric spirometry guideline of the South African Thoracic Society: Part 1
Spirometry forms an important component in the diagnosis and management of pulmonary diseases in children. In the paediatric setting, there are different challenges in terms of performance and interpretation of good quality and reliable tests. An awareness of the physiological and developmental aspects that exist in children is necessary to improve the quality and reliability of spirometry. We reviewed the recommendations on the technical aspects of performing spirometry in children, from the available guidelines and clinical trials. The focus was on the indications, methods and the interpretation of lung function tests in children <12 years of age. Reliable lung function testing can be performed in children, but an awareness of the limitations, the use of incentives and a dedicated lung function technologist are necessary
Resonant X-ray diffraction studies on the charge ordering in magnetite
Here we show that the low temperature phase of magnetite is associated with
an effective, although fractional, ordering of the charge. Evidence and a
quantitative evaluation of the atomic charges are achieved by using resonant
x-ray diffraction (RXD) experiments whose results are further analyzed with the
help of ab initio calculations of the scattering factors involved. By
confirming the results obtained from X-ray crystallography we have shown that
RXD is able to probe quantitatively the electronic structure in very complex
oxides, whose importance covers a wide domain of applications.Comment: 4 pages 4 figures, accepted for publication in PR
Direct test of defect mediated laser induced melting theory for two dimensional solids
We investigate by direct numerical solution of appropriate renormalization
flow equations, the validity of a recent dislocation unbinding theory for laser
induced freezing/melting in two dimensions. The bare elastic moduli and
dislocation fugacities which are inputs to the flow equations are obtained for
three different 2-d systems (hard disk, inverse power and the
Derjaguin-Landau-Verwey-Overbeek potentials) from a restricted Monte Carlo
simulation sampling only configurations {\em without} dislocations. We conclude
that (a) the flow equations need to be correct at least up to third order in
defect fugacity to reproduce meaningful results, (b) there is excellent
quantitative agreement between our results and earlier conventional Monte Carlo
simulations for the hard disk system and (c) while the qualitative form of the
phase diagram is reproduced for systems with soft potentials there is some
quantitative discrepancy which we explain.Comment: 11 pages, 14 figures, submitted to Phys. Rev.
Electromagnetic vacuum of complex media II: the Lamb shift and the total vacuum energy
We study the physical content of the electromagnetic vacuum energy of a
random medium made of atomic electric dipoles. First, we evaluate the
contribution of statistical fluctuations to the average total vacuum energy,
which is made out of the integration of the variations of the Lamb shift with
respect to the coupling constant. While the Lamb shift is a function of the
electrical susceptibility only, the vacuum energy is generally not. Second, we
make clear why the effective medium bulk energy does not account for the total
vacuum energy of a molecular dielectric. Consequently, the Lamb shift does not
derive from the effective medium bulk energy except at leading order in the
molecular density. The local field factors provide natural cutoffs for the
spectrum of the total vacuum energy at a wavelength of the order of the
correlation length. Third, we investigate to what extent shifts in the spectrum
of the dielectric constant may be attributed to the binding energy of a
dielectric. In particular, in the continuum approximation we have found a
relation between the electrostatic binding energy and the Lorentz-Lorenz shift.
Nonetheless, we conclude that the knowledge of the spectrum of the refractive
index is insufficient either to quantify the energy of radiative modes or to
estimate the electrostatic binding energy of molecular clusters.Comment: Comments added, some sections rewritte
- …