1,836 research outputs found
Where the linearized Poisson-Boltzmann cell model fails: (II) the planar case as a prototype study
The classical problem of two uniformly charged infinite planes in
electrochemical equilibrium with an infinite monovalent salt reservoir is
solved exactly at the mean-field nonlinear Poisson-Boltzmann (PB) level,
including an explicit expression of the associated nonlinear electrostatic
contribution to the semi-grand-canonical potential. A linearization of the
nonlinear functional is presented that leads to Debye-H\"uckel-like equations
agreeing asymptotically with the nonlinear PB results in the weak-coupling
(high-temperature) and counterionic ideal-gas limits. This linearization scheme
yields artifacts in the low-temperature, large-separation or high-surface
charge limits. In particular, the osmotic-pressure difference between the
interplane region and the salt reservoir becomes negative in the above limits,
in disagreement with the exact (at mean-field level) nonlinear PB solution. By
using explicitly gauge-invariant forms of the electrostatic potential we show
that these artifacts -- although thermodynamically consistent with quadratic
expansions of the nonlinear functional -- can be traced back to the
non-fulfillment of the underlying assumptions of the linearization. Explicit
comparison between the analytical expressions of the exact nonlinear solution
and the corresponding linearized equations allows us to show that the
linearized results are asymptotically exact in the weak-coupling and
counterionic ideal-gas limits, but always fail otherwise, predicting negative
osmotic-pressure differences.Comment: 24 pages, 3 PostScript figures, submitted to J. Chem. Phy
Liquid-state theory of charged colloids
A simple theory of the fluid state of a charged colloidal suspension is
proposed. The full free energy of a polyelectrolyte solution is calculated. It
is found that the counterions condense onto the polyions forming clusters
composed of one polyion and n counterions. The distribution of cluster sizes is
determined explicitly. In agreement with the current experimental and Monte
Carlo results, no liquid-gas phase separation was encountered.Comment: 4 pages, 2 Postscript figures, uses multicol.sty; changed conten
Donnan equilibrium and the osmotic pressure of charged colloidal lattices
We consider a system composed of a monodisperse charge-stabilized colloidal
suspension in the presence of monovalent salt, separated from the pure
electrolyte by a semipermeable membrane, which allows the crossing of solvent,
counterions, and salt particles, but prevents the passage of polyions. The
colloidal suspension, that is in a crystalline phase, is considered using a
spherical Wigner-Seitz cell. After the Donnan equilibrium is achieved, there
will be a difference in pressure between the two sides of the membrane. Using
the functional density theory, we obtained the expression for the osmotic
pressure as a function of the concentration of added salt, the colloidal volume
fraction, and the size and charge of the colloidal particles. The results are
compared with the experimental measurements for ordered polystyrene lattices of
two different particle sizes over a range of ionic strengths and colloidal
volume fractions.Comment: 8 pages, 4 Postscript figures, uses multicol.sty, to be published in
European Physical Journal
High-efficiency 20 GHz traveling wave tube development for space communications
A 75 watt CW high efficiency helix TWT operating at 20 GHz was developed for satellite communication systems. The purpose was to extend the performance capabilities of helix TWTs by using recent technology developments. The TWT described is a unique design because high overall efficiency is obtained with a low perveance beam. In the past, low perveance designs resulted in low beam efficiencies. However, due to recent breakthoughs in diamond rod technology and in collector electrode materials, high efficiencies can now be achieved with low perveance beams. The advantage of a low perveance beam is a reduction in space charge within the beam which translates to more efficient collector operation. In addition, this design incorporates textured graphite electrodes which further enhance collector operation by suppressing backstreaming secondaries. The diamond supported helix circuit features low RF losses, high interaction impedance, good thermal handling capability and has been designed to compensate for the low perveance beam. One more discussed tube feature is the use of a velocity taper in the output helix that achieves low signal distortion while maintaining high efficiency
Seedtime and Harvest: The Establishment of the Hawaiian Reformed Catholic Church, 1855-1870.
M.A. Thesis. University of Hawaiʻi at Mānoa 2018
The one-component plasma: a conceptual approach
The one-component plasma (OCP) represents the simplest statistical mechanical
model of a Coulomb system. For this reason, it has been extensively studied
over the last forty years. The advent of the integral equations has resulted in
a dramatic improvement in our ability to carry out numerical calculations, but
came at the expense of a physical insight gained in a simpler analytic theory.
In this paper we present an extension of the Debye-Hueckel (DH) theory to the
OCP. The theory allows for analytic calculations of all the thermodynamic
functions, as well as the structure factor. The theory explicitly satisfies the
Stillinger-Lovett and, for small couplings, the compressibility sum rules,
implying its internal self consistency.Comment: 28 pages, 5 Postscript figures, uses elsart.sty, accepted for
publication in Physica
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