211 research outputs found
"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
Electrically-driven phase transition in magnetite nanostructures
Magnetite (FeO), an archetypal transition metal oxide, has been
used for thousands of years, from lodestones in primitive compasses[1] to a
candidate material for magnetoelectronic devices.[2] In 1939 Verwey[3] found
that bulk magnetite undergoes a transition at T 120 K from a
high temperature "bad metal" conducting phase to a low-temperature insulating
phase. He suggested[4] that high temperature conduction is via the fluctuating
and correlated valences of the octahedral iron atoms, and that the transition
is the onset of charge ordering upon cooling. The Verwey transition mechanism
and the question of charge ordering remain highly controversial.[5-11] Here we
show that magnetite nanocrystals and single-crystal thin films exhibit an
electrically driven phase transition below the Verwey temperature. The
signature of this transition is the onset of sharp conductance switching in
high electric fields, hysteretic in voltage. We demonstrate that this
transition is not due to local heating, but instead is due to the breakdown of
the correlated insulating state when driven out of equilibrium by electrical
bias. We anticipate that further studies of this newly observed transition and
its low-temperature conducting phase will shed light on how charge ordering and
vibrational degrees of freedom determine the ground state of this important
compound.Comment: 17 pages, 4 figure
Poisson-Boltzmann for oppositely charged bodies: an explicit derivation
The interaction between charged bodies in an ionic solution is a general
problem in colloid physics and becomes a central topic in the study of
biological systems where the electrostatic interaction between proteins,
nucleic acids, membranes is involved. This problem is often described starting
from the simple one-dimensional model of two parallel charged plates. Several
different approaches to this problem exist, focusing on different features. In
many cases, an intuitive expression of the pressure exerted on the plates is
proposed, which includes an electrostatic plus an osmotic contribution. We
present an explicit and self-consistent derivation of this formula for the
general case of any charge densities on the plates and any salt solution,
obtained in the framework of the Poisson-Boltzmann theory. We also show that,
depending on external constraints, the correct thermodynamic potential can
differ from the usual PB free energy. The resulting expression predicts, for
asymmetric, oppositely charged plates, the existence of a non trivial
equilibrium position with the plates separated by a finite distance. It is
therefore crucial, in order to study the kinetic stability of the corresponding
energy minimum, to obtain its explicit dependence on the plates charge
densities and on the ion concentration. An analytic expression for the position
and value of the corresponding energy minimum has been derived in 1975 by
Ohshima [Ohshima H., Colloid and Polymer Sci. 253, 150-157 (1975)] but,
surprisingly, this important result seems to be overlooked today. We retrieve
the expressions obtained by Ohshima in a simpler formalism, more familiar to
the physics community, and give a physical interpretation of the observed
behavior.Comment: 11 pages, 7 figures, submitted to Molecular Physic
Clay micromechanics : an analysis of elementary mechanisms of clay particle interactions to gain insight into compression behaviour of clay
The macroscopic response of geomaterials is controlled by the processes occurring at the microscale. Understanding these processes is key to interpret experimental data, understand fundamental modes of stress-strain behaviour, inform âcontinuumâ macroscopic constitutive models, and develop quantitative predictive tools based on Discrete Element Method (DEM) approaches. Unlike granular materials, mechanisms at the particle scale controlling macro-mechanical behaviour of clays are still largely ignored. This paper presents an analysis of elementary mechanisms of clay particle interactions with the aim of gaining an insight into behaviour of clay and advance the process of defining suitable contact laws to be implemented into DEM formulations
Yukawa potentials in systems with partial periodic boundary conditions I : Ewald sums for quasi-two dimensional systems
Yukawa potentials are often used as effective potentials for systems as
colloids, plasmas, etc. When the Debye screening length is large, the Yukawa
potential tends to the non-screened Coulomb potential ; in this small screening
limit, or Coulomb limit, the potential is long ranged. As it is well known in
computer simulation, a simple truncation of the long ranged potential and the
minimum image convention are insufficient to obtain accurate numerical data on
systems. The Ewald method for bulk systems, i.e. with periodic boundary
conditions in all three directions of the space, has already been derived for
Yukawa potential [cf. Y., Rosenfeld, {\it Mol. Phys.}, \bm{88}, 1357, (1996)
and G., Salin and J.-M., Caillol, {\it J. Chem. Phys.}, \bm{113}, 10459,
(2000)], but for systems with partial periodic boundary conditions, the Ewald
sums have only recently been obtained [M., Mazars, {\it J. Chem. Phys.}, {\bf
126}, 056101 (2007)]. In this paper, we provide a closed derivation of the
Ewald sums for Yukawa potentials in systems with periodic boundary conditions
in only two directions and for any value of the Debye length. A special
attention is paid to the Coulomb limit and its relation with the
electroneutrality of systems.Comment: 40 pages, 5 figures and 4 table
Long range electronic phase separation in CaFe3O5
Electronic phase separation is an important feature of many correlated perovskite compounds but hasnât been seen in other complex oxides with similar physical behaviour such as magnetite. Hong et al. find phase separation between a magnetite-like charge ordered phase and a charge averaged phase in CaFe3O5
Calibrative approaches to protein solubility modeling of a mutant series using physicochemical descriptors
A set of physicochemical properties describing a protein of known structure is employed for a calibrative approach to protein solubility. Common hydrodynamic and electrophoretic properties routinely measured in the bio-analytical laboratory such as zeta potential, dipole moment, the second osmotic virial coefficient are first estimated in silico as a function a pH and solution ionic strength starting with the protein crystal structure. The utility of these descriptors in understanding the solubility of a series of ribonuclease Sa mutants is investigated. A simple two parameter model was trained using solubility data of the wild type protein measured at a restricted number of solution pHs. Solubility estimates of the mutants demonstrate that zeta potential and dipole moment may be used to rationalize solubility trends over a wide pH range. Additionally a calibrative model based on the proteinâs second osmotic virial coefficient, B22 was developed. A modified DVLO type potential along with a simplified representation of the protein allowed for efficient computation of the second viral coefficient. The standard error of prediction for both models was on the order of 0.3 log S units. These results are very encouraging and demonstrate that these models may be trained with a small number of samples and employed extrapolatively for estimating mutant solubilities
Convection and Retro-Convection Enhanced Delivery: Some Theoretical Considerations Related to Drug Targeting
Delivery of drugs and macromolecules into the brain is a challenging problem, due in part to the bloodâbrain barrier. In this article, we focus on the possibilities and limitations of two infusion techniques devised to bypass the bloodâbrain barrier: convection enhanced delivery (CED) and retro-convection enhanced delivery (R-CED). CED infuses fluid directly into the interstitial space of brain or tumor, whereas R-CED removes fluid from the interstitial space, which results in the transfer of drugs from the vascular compartment into the brain or tumor. Both techniques have shown promising results for the delivery of drugs into large volumes of tissue. Theoretical approaches of varying complexity have been developed to better understand and predict brain interstitial pressures and drug distribution for these techniques. These theoretical models of flow and diffusion can only be solved explicitly in simple geometries, and spherical symmetry is usually assumed for CED, while axial symmetry has been assumed for R-CED. This perspective summarizes features of these models and provides physical arguments and numerical simulations to support the notion that spherical symmetry is a reasonable approximation for modeling CED and R-CED. We also explore the potential of multi-catheter arrays for delivering and compartmentalizing drugs using CED and R-CED
Ligand-hole localization in oxides with unusual valence Fe
Unusual high-valence states of iron are stabilized in a few oxides. A-site-ordered perovskite-structure oxides contain such iron cations and exhibit distinct electronic behaviors at low temperatures, e.g. charge disproportionation (4Fe4+ â 2Fe3+ + 2Fe5+) in CaCu3Fe4O12 and intersite charge transfer (3Cu2+ + 4Fe3.75+ â 3Cu3+ + 4Fe3+) in LaCu3Fe4O12. Here we report the synthesis of solid solutions of CaCu3Fe4O12 and LaCu3Fe4O12 and explain how the instabilities of their unusual valence states of iron are relieved. Although these behaviors look completely different from each other in simple ionic models, they can both be explained by the localization of ligand holes, which are produced by the strong hybridization of iron d and oxygen p orbitals in oxides. The localization behavior in the charge disproportionation of CaCu3Fe4O12 is regarded as charge ordering of the ligand holes, and that in the intersite charge transfer of LaCu3Fe4O12 is regarded as a Mott transition of the ligand holes
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