Molecular mass dependence of adsorbed amount and hydrodynamic thickness of polyelectrolyte layers

Highly charged polyelectrolytes adsorbed on oppositely charged colloidal particles are investigated by electrophoresis and dynamic light scattering. The dependence of the adsorbed amount and of the hydrodynamic layer thickness on the molecular mass and the salt level is analyzed. The adsorbed amount increases with increasing salt level and decreases with increasing molecular mass. The hydrodynamic layer thickness is independent of the molecular mass at low salt levels, but increases with the molecular mass as a power law with an exponent 0.10 ± 0.01 at high salt. The same behavior was observed for different polyelectrolytes and substrates and therefore is suspected to be generic. Due to semi-quantitative agreement with computer simulations carried out by Kong and Muthukumar in 1998, the observed behavior is interpreted with conformational changes of single adsorbed polyelectrolyte chains.     

μSR Investigation of magnetic phases in R1−xSrxCoO3 oxides (R=Pr, Nd)

We report results of our muon spin relaxation measurements in the series of polycrystalline compounds Pr_1−xSr_xCoO₃ (x=0.3, 0.4 and 0.5) and Nd_1−xSr_xCoO₃ (x=0.3 and 0.5). For the Pr-based samples our data clearly indicate the existence of two magnetic transitions, as also inferred from macroscopic measurements. While the high temperature transition is typical for cobaltites (∼200 K), the low temperature one is unusual. In our experiments it occurs below about 120 K and it manifests itself as a change in the slope of the temperature variation of the muon spin depolarization rate λ(T). For the Nd-based samples we found an increase of the muon spin depolarization rate below 45 K, temperature at which the sample is ferrimagnetic, when the Nd sublattice coupled antiparallel to the Co lattice. No phase separation could be evidenced in our samples by μSR experiments.     

Structural chemistry at Lomonosov Moscow State University: a special issue

Structural Chemistry -     

Sydney Brenner (1927–2019)—One of the greats of our science on new frontiers

Structural Chemistry - Sydney Brenner was among those who created the modern science of molecular genetics. He was primarily an experimentalist yet his philosophical views on science will also have...     

Nadrian C. Seeman (1945–2021): pioneer of DNA nanotechnology

Structural Chemistry - Nadrian (Ned) Seeman (1945-2021) was a crystallographer turned nanoscientist who pioneered a new direction of research at the borderline of the two fields. He constructed new...     

Forty years of quasicrystals: a bumpy road to triumph

Structural Chemistry - On April 8, 1982, Dan Shechtman conducted an electron diffraction experiment on an aluminum/manganese alloy. The diffraction pattern showed tenfold symmetry although the...     

Block-structured compressible Navier–Stokes solution using the OPS high-level abstraction

abstract

In this paper, we report the development and validation of a compressible solver with shock capturing, using a domain-specific high-level abstraction framework, OPS, that is being developed at the University of Oxford. OPS uses an active library approach for block-structured meshes, capable of generating codes for a variety of parallel implementations with different parallelisation strategies. Performance results on various architectures are reported for the 1D Shu–Osher test case.     

Single drop microextraction of homoserine lactones based quorum sensing signal molecules, and the separation of their enantiomers using gas chromatography mass spectrometry in the presence of biological matrices

N-acylated homoserine lactones (AHLs) are produced by Gram-negative bacteria as communication signals and are frequently studied as mediators of the “quorum sensing” response of bacterial communities. AHLs are optically active components and the L-form is known to have activity in the quorum sensing. However, the knowledge regarding the stereospecific production of the AHLs in bacterial cultures is limited; therefore, there is a need for a fast and easy method for their chiral analysis. A method was developed for the preconcentration of the AHLs using single drop microextraction or liquid-liquid microphase extraction in toluene for their analysis using GC-MS. The performance of the method were determined and discussed for the chiral separation of these autoinducers using a capillary column coated with heptakis-(2,3-di-O-acetyl-6-O-t-butyldimethyl-silyl)-β-cyclodextrin. The salient feature of this study is the demonstration, that Burkholderia cepacia LA3 produced D-decanoyl-homoserine lactone beside L-decanoyl- and L-octaonyl- enantiomers.     

On the law of the iterated logarithm for the discrepancy of 〈n k x〉

By a well known result of Philipp (1975), the discrepancy D _N (ω) of the sequence (n _k ω) _k≥1 mod 1 satisfies the law of the iterated logarithm under the Hadamard gap condition n _{k + 1}/n _k ≥ q > 1 (k = 1, 2, …). Recently Berkes, Philipp and Tichy (2006) showed that this result remains valid, under Diophantine conditions on (n _k ), for subexpenentially growing (n _k ), but in general the behavior of (n _k ω) becomes very complicated in the subexponential case. Using a different norming factor depending on the density properties of the sequence (n _k ), in this paper we prove a law of the iterated logarithm for the discrepancy D _N (ω) for subexponentially growing (n _k ) without number theoretic assumptions. C. Aistleitner, Research supported by FWF grant S9603-N13. I. Berkes, Research supported by FWF grant S9603-N13 and OTKA grants K 61052 and K 67961. Authors’ addresses: C. Aistleitner, Institute of Mathematics A, Graz University of Technology, Steyrergasse 30, 8010 Graz, Austria; I. Berkes, Institute of Statistics, Graz University of Technology, Steyrergasse 17/IV, 8010 Graz, Austria     

Characterization of Turbulent Structures in a Transonic Backward-Facing Step Flow

A transonic backward-facing step flow, at a free stream Mach number of 0.8 and a Reynolds number of 1.86 × 10⁵ with respect to the step height, was investigated experimentally by means of planar and stereo Particle Image Velocimetry (PIV) measurements for multiple fields of view. The primary aim of this analysis is to examine whether the large temporal variations of the reattachment location is associated with the presence of large scale coherent flow structures. The mean flow reattaches ≈6.1±0.2 times the step height downstream of the step. This value fluctuates temporally as much as ±3 step heights. Measurements of the wake flow in horizontal planes show that the strong variations of the reattachment length are associated with spanwise variations of the streamwise velocity. Two-point correlations revealed large–scale coherent regions with a length of up to 7 step heights and a dominant spanwise wave-length of 1.5…2.5 step heights. Furthermore, close to the step large structures are found, which span more than 5 step heights in spanwise direction. The Reynolds stress distribution of the separated region strongly suggests that the initial streamwise momentum is transferred to the vertical component as well as to the spanwise component in comparable portions by the deformation of the initial Kelvin-Helmholtz vortices and the generation of secondary ones. As a result, the separated shear layer is characterized by eddies of various sizes and orientations. The mean flow field only shows the primary separation bubble and a secondary recirculation region. No stationary streamwise vortices could be found for the tested Reynolds number.