836 research outputs found
Where the linearized Poisson-Boltzmann cell model fails: (I) spurious phase separation in charged colloidal suspensions
We perform a linearization of the Poisson-Boltzmann (PB) density functional
for spherical Wigner-Seitz cells that yields Debye-H\"uckel-like equations
agreeing asymptotically with the PB results in the weak-coupling
(high-temperature) limit. Both the canonical (fixed number of microions) as
well as the semi-grand-canonical (in contact with an infinite salt reservoir)
cases are considered and discussed in a unified linearized framework. In the
canonical case, for sufficiently large colloidal charges the linearized theory
predicts the occurrence of a thermodynamical instability with an associated
phase separation of the homogeneous suspension into dilute (gas) and dense
(liquid) phases. In the semi-grand-canonical case it is predicted that the
isothermal compressibility and the osmotic-pressure difference between the
colloidal suspension and the salt reservoir become negative in the
low-temperature, high-surface charge or infinite-dilution (of polyions) limits.
As already pointed out in the literature for the latter case, these features
are in disagreement with the exact nonlinear PB solution inside a Wigner-Seitz
cell and are thus artifacts of the linearization. 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 and osmotic pressure, may be traced back to the
non-fulfillment of the underlying assumptions of the linearization.Comment: 32 pages, 3 PostScript figures, submitted to J. Chem. Phy
Electrophoretic Properties of Highly Charged Colloids: A Hybrid MD/LB Simulation Study
Using computer simulations, the electrophoretic motion of a positively
charged colloid (macroion) in an electrolyte solution is studied in the
framework of the primitive model. Hydrodynamic interactions are fully taken
into account by applying a hybrid simulation scheme, where the charged ions
(i.e. macroion and electrolyte), propagated via molecular dynamics (MD), are
coupled to a Lattice Boltzmann (LB) fluid. In a recent experiment it was shown
that, for multivalent salt ions, the mobility initially increases with
charge density , reaches a maximum and then decreases with further
increase of . The aim of the present work is to elucidate the behaviour
of at high values of . Even for the case of monovalent microions,
we find a decrease of with . A dynamic Stern layer is defined
that includes all the counterions that move with the macroion while subject to
an external electrical field. The number of counterions in the Stern layer,
, is a crucial parameter for the behavior of at high values of
. In this case, the mobility depends primarily on the ratio
(with the valency of the macroion). The previous contention that
the increase in the distortion of the electric double layer (EDL) with
increasing leads to the lowering of does not hold for high
. In fact, we show that the deformation of the EDL decreases with
increase of . The role of hydrodynamic interactions is inferred from
direct comparisons to Langevin simulations where the coupling to the LB fluid
is switched off. Moreover, systems with divalent counterions are considered. In
this case, at high values of the phenomenon of charge inversion is
found.Comment: accepted in J. Chem Phys., 10 pages, 9 figure
Numerical electrokinetics
A new lattice method is presented in order to efficiently solve the
electrokinetic equations, which describe the structure and dynamics of the
charge cloud and the flow field surrounding a single charged colloidal sphere,
or a fixed array of such objects. We focus on calculating the electrophoretic
mobility in the limit of small driving field, and systematically linearise the
equations with respect to the latter. This gives rise to several subproblems,
each of which is solved by a specialised numerical algorithm. For the total
problem we combine these solvers in an iterative procedure. Applying this
method, we study the effect of the screening mechanism (salt screening vs.
counterion screening) on the electrophoretic mobility, and find a weak
non-trivial dependence, as expected from scaling theory. Furthermore, we find
that the orientation of the charge cloud (i. e. its dipole moment) depends on
the value of the colloid charge, as a result of a competition between
electrostatic and hydrodynamic effects.Comment: accepted for publication in Journal of Physics Condensed Matter
(proceedings of the 2012 CODEF conference
Impact of EMA regulatory label changes on systemic diclofenac initiation, discontinuation, and switching to other pain medicines in Scotland, England, Denmark, and The Netherlands
Purpose: In June 2013 a European Medicines Agency referral procedure concluded that diclofenac was associated with an elevated risk of acute cardiovascular events and contraindications, warnings, and changes to the product information were implemented across the European Union. This study measured the impact of the regulatory action on the prescribing of systemic diclofenac in Denmark, The Netherlands, England, and Scotland. Methods: Quarterly time series analyses measuring diclofenac prescription initiation, discontinuation and switching to other systemic nonsteroidal anti-inflammatory (NSAIDs), topical NSAIDs, paracetamol, opioids, and other chronic pain medication in those who discontinued diclofenac. Absolute effects were estimated using interrupted time series regression. Results: Overall, diclofenac prescription initiations fell during the observation periods of all countries. Compared with Denmark where there appeared to be amore limited effect, the regulatory action was associated with significant immediate reductions in diclofenac initiation in The Netherlands (−0.42%, 95% CI, −0.66% to −0.18%), England (−0.09%, 95% CI, −0.11% to −0.08%), and Scotland (−0.67%, 95% CI, −0.79% to −0.55%); and falling trends in diclofenac initiation in the Netherlands (−0.03%, 95% CI, −0.06% to −0.01% per quarter) and Scotland (−0.04%, 95% CI, −0.05% to −0.02% per quarter). There was no significant impact on diclofenac discontinuation in any country. The regulatory action was associated with modest differences in switching to other pain medicines following diclofenac discontinuation. Conclusions: The regulatory action was associated with significant reductions in overall diclofenac initiation which varied by country and type of exposure. There was no impact on discontinuation and variable impact on switching
High-level characteristics of or-and independent and-parallelism in prolog
Although studies of a number of parallel implementations of logic programming languages are now available, their results are difficult to interpret due to the multiplicity of factors involved, the effect of each of which is difficult to sepárate. In this paper we present the results of a high-level simulation study of or- and independent
and-parallelism with a wide selection of Prolog programs that aims to determine the intrinsic amount of parallelism,
independently of implementation factors, thus facilitating this separation. We expect this study will be instrumental in better understanding and comparing results from actual implementations, as shown by some examples provided in the paper. In addition, the paper examines some of the issues and tradeoffs associated with the combination of and- and or-parallelism and proposes reasonable solutions based on the simulation data obtained
Diffuse-Charge Dynamics in Electrochemical Systems
The response of a model micro-electrochemical system to a time-dependent
applied voltage is analyzed. The article begins with a fresh historical review
including electrochemistry, colloidal science, and microfluidics. The model
problem consists of a symmetric binary electrolyte between parallel-plate,
blocking electrodes which suddenly apply a voltage. Compact Stern layers on the
electrodes are also taken into account. The Nernst-Planck-Poisson equations are
first linearized and solved by Laplace transforms for small voltages, and
numerical solutions are obtained for large voltages. The ``weakly nonlinear''
limit of thin double layers is then analyzed by matched asymptotic expansions
in the small parameter , where is the
screening length and the electrode separation. At leading order, the system
initially behaves like an RC circuit with a response time of
(not ), where is the ionic diffusivity, but nonlinearity
violates this common picture and introduce multiple time scales. The charging
process slows down, and neutral-salt adsorption by the diffuse part of the
double layer couples to bulk diffusion at the time scale, . In the
``strongly nonlinear'' regime (controlled by a dimensionless parameter
resembling the Dukhin number), this effect produces bulk concentration
gradients, and, at very large voltages, transient space charge. The article
concludes with an overview of more general situations involving surface
conduction, multi-component electrolytes, and Faradaic processes.Comment: 10 figs, 26 pages (double-column), 141 reference
Complex networks theory for analyzing metabolic networks
One of the main tasks of post-genomic informatics is to systematically
investigate all molecules and their interactions within a living cell so as to
understand how these molecules and the interactions between them relate to the
function of the organism, while networks are appropriate abstract description
of all kinds of interactions. In the past few years, great achievement has been
made in developing theory of complex networks for revealing the organizing
principles that govern the formation and evolution of various complex
biological, technological and social networks. This paper reviews the
accomplishments in constructing genome-based metabolic networks and describes
how the theory of complex networks is applied to analyze metabolic networks.Comment: 13 pages, 2 figure
The osmotic pressure of charged colloidal suspensions: A unified approach to linearized Poisson-Boltzmann theory
We study theoretically the osmotic pressure of a suspension of charged
objects (e.g., colloids, polyelectrolytes, clay platelets, etc.) dialyzed
against an electrolyte solution using the cell model and linear
Poisson-Boltzmann (PB) theory. From the volume derivative of the grand
potential functional of linear theory we obtain two novel expressions for the
osmotic pressure in terms of the potential- or ion-profiles, neither of which
coincides with the expression known from nonlinear PB theory, namely, the
density of microions at the cell boundary. We show that the range of validity
of linearization depends strongly on the linearization point and proof that
expansion about the selfconsistently determined average potential is optimal in
several respects. For instance, screening inside the suspension is
automatically described by the actual ionic strength, resulting in the correct
asymptotics at high colloid concentration. Together with the analytical
solution of the linear PB equation for cell models of arbitrary dimension and
electrolyte composition explicit and very general formulas for the osmotic
pressure ensue. A comparison with nonlinear PB theory is provided. Our analysis
also shows that whether or not linear theory predicts a phase separation
depends crucially on the precise definition of the pressure, showing that an
improper choice could predict an artificial phase separation in systems as
important as DNA in physiological salt solution.Comment: 16 pages, 5 figures, REVTeX4 styl
Septicemia Caused by Tick-borne Bacterial Pathogen Candidatus Neoehrlichia mikurensis
We have repeatedly detected Candidatus Neoehrlichia mikurensis, a bacterium first described in Rattus norvegicus rats and Ixodes ovatus ticks in Japan in 2004 in the blood of a 61-year-old man with signs of septicemia by 16S rRNA and groEL gene PCR. After 6 weeks of therapy with doxycycline and rifampin, the patient recovered
Anaerobic Carbon Monoxide Dehydrogenase Diversity in the Homoacetogenic Hindgut Microbial Communities of Lower Termites and the Wood Roach
Anaerobic carbon monoxide dehydrogenase (CODH) is a key enzyme in the Wood-Ljungdahl (acetyl-CoA) pathway for acetogenesis performed by homoacetogenic bacteria. Acetate generated by gut bacteria via the acetyl-CoA pathway provides considerable nutrition to wood-feeding dictyopteran insects making CODH important to the obligate mutualism occurring between termites and their hindgut microbiota. To investigate CODH diversity in insect gut communities, we developed the first degenerate primers designed to amplify cooS genes, which encode the catalytic (β) subunit of anaerobic CODH enzyme complexes. These primers target over 68 million combinations of potential forward and reverse cooS primer-binding sequences. We used the primers to identify cooS genes in bacterial isolates from the hindgut of a phylogenetically lower termite and to sample cooS diversity present in a variety of insect hindgut microbial communities including those of three phylogenetically-lower termites, Zootermopsis nevadensis, Reticulitermes hesperus, and Incisitermes minor, a wood-feeding cockroach, Cryptocercus punctulatus, and an omnivorous cockroach, Periplaneta americana. In total, we sequenced and analyzed 151 different cooS genes. These genes encode proteins that group within one of three highly divergent CODH phylogenetic clades. Each insect gut community contained CODH variants from all three of these clades. The patterns of CODH diversity in these communities likely reflect differences in enzyme or physiological function, and suggest that a diversity of microbial species participate in homoacetogenesis in these communities
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