43,883 research outputs found
Thermodynamics and phase behavior of the lamellar Zwanzig model
Binary mixtures of lamellar colloids represented by hard platelets are
studied within a generalization of the Zwanzig model for rods, whereby the
square cuboids can take only three orientations along the , or axes.
The free energy is calculated within Rosenfeld's ''Fundamental Measure Theory''
(FMT) adapted to the present model. In the one-component limit, the model
exhibits the expected isotropic to nematic phase transition, which narrows as
the aspect ratio ( is the width and the thickness of the
platelets) increases. In the binary case the competition between nematic
ordering and depletion-induced segregation leads to rich phase behaviour.Comment: 9 pages, 6 figure
Random Phase Approximation and Extensions Applied to a Bosonic Field Theory
An application of a self-consistent version of RPA to quantum field theory
with broken symmetry is presented. Although our approach can be applied to any
bosonic field theory, we specifically study the theory in 1+1
dimensions. We show that standard RPA approach leads to an instability which
can be removed when going to a superior version,i.e. the renormalized RPA. We
present a method based on the so-called charging formula of the many electron
problem to calculate the correlation energy and the RPA effective potential.Comment: 30 pages, LaTeX file, 10 figures included, final version accepted in
EPJ
Screened electrostatic interactions between clay platelets
An effective pair potential for systems of uniformly charged lamellar
colloids in the presence of an electrolytic solution of microscopic co- and
counterions is derived. The charge distribution on the discs is expressed as a
collection of multipole moments, and the tensors which determine the
interactions between these multipoles are derived from a screened Coulomb
potential. Unlike previous studies of such systems, the interaction energy may
now be expressed for discs at arbitrary mutual orientation. The potential is
shown to be exactly equivalent to the use of linearized Poisson-Boltzmann
theory.Comment: 23 pages, 10 figures, created with Revtex. To appear in Molecular
Physic
Conditional quantum state engineering in repeated 2-photon down conversion
The U(1,1) and U(2) transformations realized by three-mode interaction in the
respective parametric approximations are studied in conditional measurement,
and the corresponding non-unitary transformation operators are derived. As an
application, the preparation of single-mode quantum states using an optical
feedback loop is discussed, with special emphasis of Fock state preparation.
For that example, the influence of non-perfect detection and feedback is also
considered.Comment: 17 pages, 4 figures, using a4.st
Tolerance and Sensitivity in the Fuse Network
We show that depending on the disorder, a small noise added to the threshold
distribution of the fuse network may or may not completely change the
subsequent breakdown process. When the threshold distribution has a lower
cutoff at a finite value and a power law dependence towards large thresholds
with an exponent which is less than , the network is not sensitive
to the added noise, otherwise it is. The transition between sensitivity or not
appears to be second order, and is related to a localization-delocalization
transition earlier observed in such systems.Comment: 12 pages, 3 figures available upon request, plain Te
A universal velocity distribution of relaxed collisionless structures
Several general trends have been identified for equilibrated,
self-gravitating collisionless systems, such as density or anisotropy profiles.
These are integrated quantities which naturally depend on the underlying
velocity distribution function (VDF) of the system. We study this VDF through a
set of numerical simulations, which allow us to extract both the radial and the
tangential VDF. We find that the shape of the VDF is universal, in the sense
that it depends only on two things namely the dispersion (radial or tangential)
and the local slope of the density. Both the radial and the tangential VDF's
are universal for a collection of simulations, including controlled collisions
with very different initial conditions, radial infall simulation, and
structures formed in cosmological simulations.Comment: 13 pages, 6 figures; oversimplified analysis corrected; changed
abstract and conclusions; significantly extended discussio
Burst avalanches in solvable models of fibrous materials
We review limiting models for fracture in bundles of fibers, with
statistically distributed thresholds for breakdown of individual fibers. During
the breakdown process, avalanches consisting of simultaneous rupture of several
fibers occur, and the distribution of the magnitude of
such avalanches is the central characteristics in our analysis. For a bundle of
parallel fibers two limiting models of load sharing are studied and contrasted:
the global model in which the load carried by a bursting fiber is equally
distributed among the surviving members, and the local model in which the
nearest surviving neighbors take up the load. For the global model we
investigate in particular the conditions on the threshold distribution which
would lead to anomalous behavior, i.e. deviations from the asymptotics
, known to be the generic behavior. For the local
model no universal power-law asymptotics exists, but we show for a particular
threshold distribution how the avalanche distribution can nevertheless be
explicitly calculated in the large-bundle limit.Comment: 28 pages, RevTeX, 3 Postscript figure
Investigation of Polymer–Plasticizer Blends as SH-SAW Sensor Coatings for Detection of Benzene in Water with High Sensitivity and Long-Term Stability
We report the first-ever direct detection of benzene in water at concentrations below 100 ppb (parts per billion) using acoustic wave (specifically, shear-horizontal surface acoustic wave, SH-SAW) sensors with plasticized polymer coatings. Two polymers and two plasticizers were studied as materials for sensor coatings. For each polymer–plasticizer combination, the influence of the mixing ratio of the blend on the sensitivity to benzene was measured and compared to commercially available polymers that were used for BTEX (benzene, toluene, ethylbenzene, and xylene) detection in previous work. After optimizing the coating parameters, the highest sensitivity and lowest detection limit for benzene were found for a 1.25 μm thick sensor coating of 17.5%-by-weight diisooctyl azelate-polystyrene on the tested acoustic wave device. The calculated detection limit was 45 ppb, with actual sensor responses to concentrations down to 65 ppb measured directly. Among the sensor coatings that showed good sensitivity to benzene, the best long-term stability was found for a 1.0 μm thick coating of 23% diisononyl cyclohexane-1,2-dicarboxylate-polystyrene, which was studied here because it is known to show no detectable leaching in water. The present work demonstrates that, by varying type of plasticizer, mixing ratio, and coating thickness, the mechanical and chemical properties of the coatings can be conveniently tailored to maximize analyte sorption and partial chemical selectivity for a given class of analytes as well as to minimize acoustic-wave attenuation in contact with an aqueous phase at the operating frequency of the sensor device
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