240 research outputs found
Ultrafiltration modeling of non-ionic microgels
Membrane ultrafiltration (UF) is a pressure driven process allowing for the
separation and enrichment of protein solutions and dispersions of nanosized
microgel particles. The permeate flux and the near-membrane
concentration-polarization (CP) layer in this process is determined by
advective-diffusive dispersion transport and the interplay of applied and
osmotic transmembrane pressure contributions. The UF performance is thus
strongly dependent on the membrane properties, the hydrodynamic structure of
the Brownian particles, their direct and hydrodynamic interactions, and the
boundary conditions. We present a macroscopic description of cross-flow UF of
non-ionic microgels modeled as solvent-permeable spheres. Our filtration model
involves recently derived semi-analytic expressions for the
concentration-dependent collective diffusion coefficient and viscosity of
permeable particle dispersions [Riest et al., Soft Matter, 2015, 11, 2821].
These expressions have been well tested against computer simulation and
experimental results. We analyze the CP layer properties and the permeate flux
at different operating conditions and discuss various filtration process
efficiency and cost indicators. Our results show that the proper specification
of the concentration-dependent transport coefficients is important for reliable
filtration process predictions. We also show that the solvent permeability of
microgels is an essential ingredient to the UF modeling. The particle
permeability lowers the particle concentration at the membrane surface, thus
increasing the permeate flux.Comment: 19 pages, 11 figures (Electronic Supplementary Information included:
2 pages, 1 figure
Aggregation of self-propelled colloidal rods near confining walls
Non-equilibrium collective behavior of self-propelled colloidal rods in a
confining channel is studied using Brownian dynamics simulations and dynamical
density functional theory. We observe an aggregation process in which rods
self-organize into transiently jammed clusters at the channel walls. In the
early stage of the process, fast-growing hedgehog-like clusters are formed
which are largely immobile. At later stages, most of these clusters dissolve
and mobilize into nematized aggregates sliding past the walls.Comment: 5 pages, 4 figure
Colloidal glass transition: Beyond mode-coupling theory
A new theory for dynamics of concentrated colloidal suspensions and the
colloidal glass transition is proposed. The starting point is the memory
function representation of the density correlation function. The memory
function can be expressed in terms of a time-dependent pair-density correlation
function. An exact, formal equation of motion for this function is derived and
a factorization approximation is applied to its evolution operator. In this way
a closed set of equations for the density correlation function and the memory
function is obtained. The theory predicts an ergodicity breaking transition
similar to that predicted by the mode-coupling theory, but at a higher density.Comment: to be published in PR
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
Gaussian density fluctuations, mode coupling theory, and all that
We consider a toy model for glassy dynamics of colloidal suspensions: a
single Brownian particle diffusing among immobile obstacles. If Gaussian
factorization of static density fluctuations is assumed, this model can be
solved without factorization approximation for any dynamic correlation
function. The solution differs from that obtained from the ideal mode coupling
theory (MCT). The latter is equivalent to including only some, positive
definite terms in an expression for the memory function. An approximate
re-summation of the complete expression suggests that, under the assumption of
Gaussian factorization of static fluctuations, mobile particle's motion is
always diffusive. In contrast, MCT predicts that the mobile particle becomes
localized at a high enough obstacle density. We discuss the implications of
these results for models for glassy dynamics.Comment: to be published in Europhys. Let
Assessment of electrophoresis and electroosmosis in construction materials: effect of enhancing electrolytes and heavy metals contamination
Electrokinetic effects are those that take place by application of an electric field to porous materials, with the zeta potential as the key parameter. Specifically, in the case of contaminated construction materials, the generation of an electroosmotic flux, with the corresponding dragging due to water transport, is a crucial mechanism to succeed in the treatment of decontamination. Therefore, it is of great interest trying to optimize the treatment by the addition of specific electrolytes enhancing the electrokinetic phenomena. Most of the data of zeta potential found in literature for construction materials are based in micro-electrophoresis measurements, which are quite far of the real conditions of application of the remediation treatments. In this paper, electrophoretic and electroosmotic experiments, with monolithic and powdered material respectively, have been carried out for mortar, brick and granite clean and contaminated with Cs, Sr, Co, Cd, Cu and Pb. The electrolytes tested have been distilled water (DW), Na2–EDTA, oxalic acid, acetic acid and citric acid. The zeta potential values have been determined through the two different techniques and the results compared and critically analysed
Hot drought reduces the effects of elevated COâ‚‚ on tree water use efficiency and carbon metabolism
- Trees are increasingly exposed to hot droughts due to CO2-induced climate change. However, the direct role of [CO2] in altering tree physiological responses to drought and heat stress remains ambiguous.
- Pinus halepensis (Aleppo pine) trees were grown from seed under ambient (421 ppm) or elevated (867 ppm) [CO2]. The 1.5-yr-old trees, either well watered or drought treated for 1 month, were transferred to separate gas-exchange chambers and the temperature gradually increased from 25°C to 40°C over a 10 d period. Continuous whole-tree shoot and root gasexchange measurements were supplemented by primary metabolite analysis.
- Elevated [CO2] reduced tree water loss, reflected in lower stomatal conductance, resulting in a higher water-use efficiency throughout amplifying heat stress. Net carbon uptake declined strongly, driven by increases in respiration peaking earlier in the well-watered (31– 32°C) than drought (33–34°C) treatments unaffected by growth [CO2]. Further, drought altered the primary metabolome, whereas the metabolic response to [CO2] was subtle and mainly reflected in enhanced root protein stability.
- The impact of elevated [CO2] on tree stress responses was modest and largely vanished with progressing heat and drought. We therefore conclude that increases in atmospheric [CO2] cannot counterbalance the impacts of hot drought extremes in Aleppo pine
Short-time Rheology and Diffusion in Suspensions of Yukawa-type Colloidal Particles
A comprehensive study is presented on the short-time dynamics in suspensions
of charged colloidal spheres. The explored parameter space covers the major
part of the fluid-state regime, with colloid concentrations extending up to the
freezing transition. The particles are assumed to interact directly by a
hard-core plus screened Coulomb potential, and indirectly by solvent-mediated
hydrodynamic interactions (HIs). By comparison with accurate accelerated
Stokesian Dynamics (ASD) simulations of the hydrodynamic function H(q), and the
high-frequency viscosity, we investigate the accuracy of two fast and
easy-to-implement analytical schemes. The first scheme, referred to as the
pairwise additive (PA) scheme, uses exact two-body hydrodynamic mobility
tensors. It is in good agreement with the ASD simulations of H(q) and the
high-frequency viscosity, for smaller volume fractions up to about 10% and 20%,
respectively. The second scheme is a hybrid method combining the virtues of the
\delta\gamma-scheme by Beenakker and Mazur with those of the PA scheme. It
leads to predictions in good agreement with the simulation data, for all
considered concentrations, combining thus precision with computational
efficiency. The hybrid method is used to test the accuracy of a generalized
Stokes-Einstein (GSE) relation proposed by Kholodenko and Douglas, showing its
severe violation in low salinity systems. For hard spheres, however, this GSE
relation applies decently well
Ancient Yersinia pestis genomes from across Western Europe reveal early diversification during the First Pandemic (541–750)
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 bacterium’s 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
Polydisperse star polymer solutions
We analyze the effect of polydispersity in the arm number on the effective
interactions, structural correlations and the phase behavior of star polymers
in a good solvent. The effective interaction potential between two star
polymers with different arm numbers is derived using scaling theory. The
resulting expression is tested against monomer-resolved molecular dynamics
simulations. We find that the theoretical pair potential is in agreement with
the simulation data in a much wider polydispersity range than other proposed
potentials. We then use this pair potential as an input in a many-body theory
to investigate polydispersity effects on the structural correlations and the
phase diagram of dense star polymer solutions. In particular we find that a
polydispersity of 10%, which is typical in experimental samples, does not
significantly alter previous findings for the phase diagram of monodisperse
solutions.Comment: 14 pages, 7 figure
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