309 research outputs found
Self-diffusion coefficients of charged particles: Prediction of Nonlinear volume fraction dependence
We report on calculations of the translational and rotational short-time
self-diffusion coefficients and for suspensions of
charge-stabilized colloidal spheres. These diffusion coefficients are affected
by electrostatic forces and many-body hydrodynamic interactions (HI). Our
computations account for both two-body and three-body HI. For strongly charged
particles, we predict interesting nonlinear scaling relations and depending on volume fraction
, with essentially charge-independent parameters and . These
scaling relations are strikingly different from the corresponding results for
hard spheres. Our numerical results can be explained using a model of effective
hard spheres. Moreover, we perceptibly improve the known result for of
hard sphere suspensions.Comment: 8 pages, LaTeX, 3 Postscript figures included using eps
Enhanced structural correlations accelerate diffusion in charge-stabilized colloidal suspensions
Theoretical calculations for colloidal charge-stabilized and hard sphere
suspensions show that hydrodynamic interactions yield a qualitatively different
particle concentration dependence of the short-time self-diffusion coefficient.
The effect, however, is numerically small and hardly accessible by conventional
light scattering experiments. Applying multiple-scattering decorrelation
equipment and a careful data analysis we show that the theoretical prediction
for charged particles is in agreement with our experimental results from
aqueous polystyrene latex suspensions.Comment: 1 ps-file (MS-Word), 14 page
A corresponding states approach to Small-Angle-Scattering for polydisperse ionic colloidal fluids
Approximate scattering functions for polydisperse ionic colloidal fluids are
obtained by a corresponding states approach. This assumes that all pair
correlation functions of a polydisperse fluid are
conformal to those of an appropriate monodisperse binary fluid (reference
system) and can be generated from them by scaling transformations. The
correspondence law extends to ionic fluids a {\it scaling approximation} (SA)
successfully proposed for nonionic colloids in a recent paper. For the
primitive model of charged hard spheres in a continuum solvent, the partial
structure factors of the monodisperse binary reference system are evaluated by
solving the Orstein-Zernike (OZ) integral equations coupled with an approximate
closure. The SA is first tested within the mean spherical approximation (MSA)
closure, which allows analytical solutions. The results are found in good
overall agreement with exact MSA predictions up to relevant polidispersity. The
SA is shown to be an improvement over the ``decoupling approximation'' extended
to the ionic case. The simplicity of the SA scheme allows its application also
when the OZ equations can be solved only numerically. An example is then given
by using the hypernetted chain (HNC) closure. Shortcomings of the SA approach,
its possible use in the analysis of experimental scattering data and other
related points are also briefly addressed.Comment: 29 pages, 7 postscript figures (included), Latex 3.0, uses aps.sty,
to appear in Phys. Rev. E (1999
First-principles GW-BSE excitations in organic molecules
We present a first-principles method for the calculation of optical
excitations in nanosystems. The method is based on solving the Bethe-Salpeter
equation (BSE) for neutral excitations. The electron self-energy is evaluated
within the GW approximation, with dynamical screening effects described within
time-dependent density-functional theory in the adiabatic, local approximation.
This method is applied to two systems: the benzene molecule, CH, and
azobenzene, CHN. We give a description of the
photoisomerization process of azobenzene after an excitation,
which is consistent with multi-configuration calculations
Ancient Yersinia pestis genomes from across Western Europe reveal early diversification during the First Pandemic (541â750)
The first historically documented pandemic caused by Yersinia pestis began as the Justinianic Plague in 541 within the Roman Empire and continued as the so-called First Pandemic until 750. Although paleogenomic studies have previously identified the causative agent as Y. pestis, little is known about the bacteriumâs spread, diversity, and genetic history over the course of the pandemic. To elucidate the microevolution of the bacterium during this time period, we screened human remains from 21 sites in Austria, Britain, Germany, France, and Spain for Y. pestis DNA and reconstructed eight genomes. We present a methodological approach assessing single-nucleotide polymorphisms (SNPs) in ancient bacterial genomes, facilitating qualitative analyses of low coverage genomes from a metagenomic background. Phylogenetic analysis on the eight reconstructed genomes reveals the existence of previously undocumented Y. pestis diversity during the sixth to eighth centuries, and provides evidence for the presence of multiple distinct Y. pestis strains in Europe. We offer genetic evidence for the presence of the Justinianic Plague in the British Isles, previously only hypothesized from ambiguous documentary accounts, as well as the parallel occurrence of multiple derived strains in central and southern France, Spain, and southern Germany. Four of the reported strains form a polytomy similar to others seen across the Y. pestis phylogeny, associated with the Second and Third Pandemics. We identified a deletion of a 45-kb genomic region in the most recent First Pandemic strains affecting two virulence factors, intriguingly overlapping with a deletion found in 17th- to 18th-century genomes of the Second Pandemic. © 2019 National Academy of Sciences. All rights reserved
Testing "microscopic" theories of glass-forming liquids
We assess the validity of "microscopic" approaches of glass-forming liquids
based on the sole k nowledge of the static pair density correlations. To do so
we apply them to a benchmark provided by two liquid models that share very
similar static pair density correlation functions while disp laying distinct
temperature evolutions of their relaxation times. We find that the approaches
are unsuccessful in describing the difference in the dynamical behavior of the
two models. Our study is not exhausti ve, and we have not tested the effect of
adding corrections by including for instance three-body density correlations.
Yet, our results appear strong enough to challenge the claim that the slowd own
of relaxation in glass-forming liquids, for which it is well established that
the changes of the static structure factor with temperature are small, can be
explained by "microscopic" appr oaches only requiring the static pair density
correlations as nontrivial input.Comment: 10 pages, 7 figs; Accepted to EPJE Special Issue on The Physics of
Glasses. Arxiv version contains an addendum to the appendix which does not
appear in published versio
Influence of Hydrodynamic Interactions on Mechanical Unfolding of Proteins
We incorporate hydrodynamic interactions in a structure-based model of
ubiquitin and demonstrate that the hydrodynamic coupling may reduce the peak
force when stretching the protein at constant speed, especially at larger
speeds. Hydrodynamic interactions are also shown to facilitate unfolding at
constant force and inhibit stretching by fluid flows.Comment: to be published in Journal of Physics: Condensed Matte
Effect of glycerol and dimethyl sulfoxide on the phase behavior of lysozyme: Theory and experiments
Salt, glycerol and dimethyl sulfoxide (DMSO) are used to modify the
properties of protein solutions. We experimentally determined the effect of
these additives on the phase behavior of lysozyme solutions. Upon the addition
of glycerol and DMSO, the fluid-solid transition and the gas-liquid coexistence
curve (binodal) shift to lower temperatures and the gap between them increases.
The experimentally observed trends are consistent with our theoretical
predictions based on the thermodynamic perturbation theory (TPT) and the
Derjaguin-Landau-Verwey-Overbeek (DLVO) model for the lysozyme-lysozyme pair
interactions. The values of the parameters describing the interactions, namely
the refractive indices, dielectric constants, Hamaker constant and cut-off
length, are extracted from literature or are experimentally determined by
independent experiments, including static light scattering to determine the
second virial coefficient. We observe that both, glycerol and DMSO, render the
potential more repulsive, while sodium chloride reduces the repulsion.Comment: Manuscript accepted for publication in The Journal of Chemical
Physic
Independent Ion Migration in Suspensions of Strongly Interacting Charged Colloidal Spheres
We report on sytematic measurements of the low frequency conductivity in
aequous supensions of highly charged colloidal spheres. System preparation in a
closed tubing system results in precisely controlled number densities between
1E16/m3 and 1E19/m^3 (packing fractions between 1E-7 and 1E-2) and electrolyte
concentrations between 1E-7 and 1E-3 mol/l. Due to long ranged Coulomb
repulsion some of the systems show a pronounced fluid or crystalline order.
Under deionized conditions we find s to depend linearily on the packing
fraction with no detectable influence of the phase transitions. Further at
constant packing fraction s increases sublinearily with increasing number of
dissociable surface groups N. As a function of c the conductivity shows
pronounced differences depending on the kind of electrolyte used. We propose a
simple yet powerful model based on independent migration of all species present
and additivity of the respective conductivity contributions. It takes account
of small ion macro-ion interactions in terms of an effectivly transported
charge. The model successfully describes our qualitatively complex experimental
observations. It further facilitates quantitative estimates of conductivity
over a wide range of particle and experimental parameters.Comment: 32 pages, 17 figures, 2 tables, Accepted by Physical Review
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