Comments on multiple oscillatory solutions in systems with time-delay feedback

A complex Ginzburg-Landau equation subjected to local and global time-delay feedback terms is considered. In particular, multiple oscillatory solutions and their properties are studied. We present novel results regarding the disappearance of limit cycle solutions, derive analytical criteria for frequency degeneration, amplitude degeneration, frequency extrema. Furthermore, we discuss the influence of the phase shift parameter and show analytically that the stabilization of the steady state and the decay of all oscillations (amplitude death) cannot happen for global feedback only. Finally, we explain the onset of traveling wave patterns close to the regime of amplitude death

Mechanically induced homochirality in nucleated enantioselective polymerization

Understanding how biological homochirality may have emerged during chemical evolution remains a challenge for origin of life research. In keeping with this goal, we introduce and solve numerically a kinetic rate equation model of nucleated cooperative enantioselective polymerization in closed systems. The microreversible scheme includes (i) solution phase racemization of the monomers, (ii) linear chain growth by stepwise monomer attachment, in both the nucleation and elongation phases, and (iii) annealing or fusion of homochiral chains. Mechanically induced breakage of the longest chains maintains the system out of equilibrium and drives a breakage-fusion recycling mechanism. Spontaneous mirror symmetry breaking (SMSB) can be achieved starting from small initial enantiomeric excesses due to the intrinsic statistical fluctuations about the idealized racemic composition. The subsequent chiral amplification confirms the model’s capacity for absolute asymmetric synthesis, and without chiral cross-inhibition and without explicit autocatalysis

Chiral and chemical oscillations in a simple dimerization model

We consider the APED model (activation-polymerization-epimerization- depolymerization) for describing the emergence of chiral solutions within a non-catalytic framework for chiral polymerization. The minimal APED model for dimerization can lead to the spontaneous appearance of chiral oscillations and we describe in detail the nature of these oscillations in the enantiomeric excess, which are the consequence of oscillations of the concentrations of the associated chemical species

Wasserverunreinigungen in Lithium-Ionen-Batterien

Lithium-Ionen Batterien (LIB) verwenden organische Elektrolyte, die auch bei hohen Spannungen von über 4 V stabil sind. Das Leitsalz, das für die meisten Elektrolyten genutzt wird, ist LiPF6. Dieses hat eine Reihe an positiven Eigenschaften, wie eine hohe Leitfähigkeit und chemische Stabilität, ist aber hydrolyseempfindlich. Die Hydrolyseprodukte können zu einer verringerten Batterieperformance und Zyklenstabilität sowie - im Falle einer Elektrolytleckage - zu Gesundheitsrisiken führen. Diese Arbeit untersucht deshalb, welche Materialien zu den Wasserverunreinigungen in einer LIB beitragen, wie die Wasserverunreinigungen mit LiPF6 reagieren und was die Effekte dieser Verunreinigungen auf das elektrochemische Verhalten der LIB sind. Das Trocknungsverhalten von Zellkomponenten wurde mittels Karl-Fischer Titration untersucht und zeigt, dass insbesondere das Anodenmaterial Graphit und der Glasfaserseparator eingehend getrocknet werden müssen, um einen Wassereintrag in die LIB zu minimieren. Unter den Kathodenmaterialien weist insbesondere LiFePO4 einen hohen Wassergehalt auf. Andere Zellkomponenten sind weniger hygroskopisch und können einfacher getrocknet werden, was bei der industriellen LIB Herstellung mit einer Kostenreduktion verbunden ist. Die Hydrolyse von LiPF6 wurde durch Ionenchromatographie im organischen Lösungsmittel EC/DEC (1:1 v/v) und in Wasser untersucht. Die Hydrolyseprodukte von LiPF6 in EC/DEC sind HF und HPO2F2. Um die komplexen Reaktionskinetiken zu untersuchen, wurde ein kinetisches Model erstellt, mit dem Vorhersagen über weniger hydrolyseempfindliche Elektrolyten möglich sind. Insbesondere zeigt sich, dass der Dissoziationsgrad eine große Rolle in der Hydrolyse von LiPF6 im spezifischen Lösungsmittel spielt, und ein höherer Dissoziationsgrad zu einer höheren Toleranz gegenüber Wasserverunreinigungen führt. Wasserverunreinigungen haben negative Effekte auf die Zyklenstabilität und führen zu einem erhöhten Ladungsübergangswiderstand, was auf eine veränderte SEI-Formierung und Zusammensetzung hindeutet. Die Erklärung wird auch durch die Erkenntnis gestützt, dass die coulomb’sche Effizienz im ersten Zyklus bei wasserkontaminierten Zellen niedriger ist. Diese Ergebnisse verdeutlichen, wie wichtig eine Limitierung und Kontrolle der Wasserkontamination in LIB für deren Langlebigkeit, Performance und Sicherheit ist.Lithium-ion batteries (LIBs) use organic electrolytes able to withstand high operating voltages of more than 4 V. The conducting salt in those electrolytes is predominantly LiPF6. This salt has a wide range of beneficial properties, such as high conductivity and electrochemical stability, but is susceptible to hydrolysis. The hydrolysis products can decrease the battery performance and lifetime as well as cause a health hazard in case of an electrolyte leakage. This work therefore investigates which materials contribute to the water impurities in a LIB, how the water impurities react with LiPF6 and what the effects of water impurities on the electrochemical behaviour of LIB are. Drying tests combined with Karl-Fischer titrations show that especially the anode material graphite and the glass fibre separator have to be dried thoroughly to limit their water carry-over in the LIB. Among the cathode materials especially LiFePO4 contains high water contents. Other cell components are less hygroscopic and can be dried easier, reducing drying costs in the industrial LIB production. The hydrolysis of LiPF6 was investigated by ion chromatography in an organic solvent (EC/DEC, 1:1 v/v) and water. Curiously, the hydrolysis of LiPF6 in water proceeds much slower than in EC/DEC with small water contaminations (70 mM). This is explained by the different dissociation powers of water and the organic solvent. The hydrolysis products of LiPF6 in EC/DEC are predominantly HF and HPO2F2. In order to investigate the complex reaction further, a kinetic model was established, able to accurately reproduce the experimental data and make predictions about electrolyte compositions less prone to hydrolysis. The degree of dissociation a was found to play a great role in the hydrolysis properties of LiPF6 in the specific solvent and a higher a would lead to an electrolyte more tolerant toward water impurities. Water impurities in LIB were shown to have a negative effect on the cycling stability and cause a higher charge transfer resistance, possibly related to a changed SEI formation and composition. This explanation also is in agreement with the finding that the coulombic efficiency in the first cycle is lower in water contaminated cells. These findings highlight the importance of limiting and controlling water contaminations inside a LIB for its longevity, performance and safety

Investitionsstrategien auf Basis des CO2-Ausstoßes

Das Niveau des CO2-Ausstoßes eines Unternehmens kann aus ökonomischer Sicht als Verpflichtung aufgefasst werden. Welche Auswirkungen hat der CO2-Ausstoß auf die Marktbewertung dieses Unternehmens? Und wie reagieren Investoren auf eine Veränderung des CO2-Ausstoßes

On the structural repertoire of pools of short, random RNA sequences

A detailed knowledge of the mapping between sequence and structure spaces in populations of RNA molecules is essential to better understand their present-day functional properties, to envisage a plausible early evolution of RNA in a prebiotic chemical environment and to improve the design of in vitro evolution experiments, among others. Analysis of natural RNAs, as well as in vitro and computational studies, show that certain RNA structural motifs are much more abundant than others, pointing out a complex relation between sequence and structure. Within this framework, we have investigated computationally the structural properties of a large pool (10 molecules) of single-stranded, 35 nt-long, random RNA sequences. The secondary structures obtained are ranked and classified into structure families. The number of structures in main families is analytically calculated and compared with the numerical results. This permits a quantification of the fraction of structure space covered by a large pool of sequences. We further show that the number of structural motifs and their frequency is highly unbalanced with respect to the nucleotide composition: simple structures such as stem-loops and hairpins arise from sequences depleted in G, while more complex structures require an enrichment of G. In general, we observe a strong correlation between subfamilies-characterized by a fixed number of paired nucleotides-and nucleotide composition. Our results are compared to the structural repertoire obtained in a second pool where isolated base pairs are prohibited

Phenotypic effect of mutations in evolving populations of RNA molecules

Abstract Background The secondary structure of folded RNA sequences is a good model to map phenotype onto genotype, as represented by the RNA sequence. Computational studies of the evolution of ensembles of RNA molecules towards target secondary structures yield valuable clues to the mechanisms behind adaptation of complex populations. The relationship between the space of sequences and structures, the organization of RNA ensembles at mutation-selection equilibrium, the time of adaptation as a function of the population parameters, the presence of collective effects in quasispecies, or the optimal mutation rates to promote adaptation all are issues that can be explored within this framework. Results We investigate the effect of microscopic mutations on the phenotype of RNA molecules during their in silico evolution and adaptation. We calculate the distribution of the effects of mutations on fitness, the relative fractions of beneficial and deleterious mutations and the corresponding selection coefficients for populations evolving under different mutation rates. Three different situations are explored: the mutation-selection equilibrium (optimized population) in three different fitness landscapes, the dynamics during adaptation towards a goal structure (adapting population), and the behavior under periodic population bottlenecks (perturbed population). Conclusions The ratio between the number of beneficial and deleterious mutations experienced by a population of RNA sequences increases with the value of the mutation rate μ at which evolution proceeds. In contrast, the selective value of mutations remains almost constant, independent of μ, indicating that adaptation occurs through an increase in the amount of beneficial mutations, with little variations in the average effect they have on fitness. Statistical analyses of the distribution of fitness effects reveal that small effects, either beneficial or deleterious, are well described by a Pareto distribution. These results are robust under changes in the fitness landscape, remarkably when, in addition to selecting a target secondary structure, specific subsequences or low-energy folds are required. A population perturbed by bottlenecks behaves similarly to an adapting population, struggling to return to the optimized state. Whether it can survive in the long run or whether it goes extinct depends critically on the length of the time interval between bottlenecks.Support from the Spanish MICINN through research project FIS2008-05273 is gratefully acknowledged.Peer Reviewe

Collective properties of evolving molecular quasispecies

<p>Abstract</p> <p>Background</p> <p>RNA molecules, through their dual appearance as sequence and structure, represent a suitable model to study evolutionary properties of quasispecies. The essential ingredient in this model is the differentiation between genotype (molecular sequences which are affected by mutation) and phenotype (molecular structure, affected by selection). This framework allows a quantitative analysis of organizational properties of quasispecies as they adapt to different environments, such as their robustness, the effect of the degeneration of the sequence space, or the adaptation under different mutation rates and the error threshold associated.</p> <p>Results</p> <p>We describe and analyze the structural properties of molecular quasispecies adapting to different environments both during the transient time before adaptation takes place and in the asymptotic state, once optimization has occurred. We observe a minimum in the adaptation time at values of the mutation rate relatively far from the phenotypic error threshold. Through the definition of a consensus structure, it is shown that the quasispecies retains relevant structural information in a distributed fashion even above the error threshold. This structural robustness depends on the precise shape of the secondary structure used as target of selection. Experimental results available for natural RNA populations are in qualitative agreement with our observations.</p> <p>Conclusion</p> <p>Adaptation time of molecular quasispecies to a given environment is optimized at values of the mutation rate well below the phenotypic error threshold. The optimal value results from a trade-off between diversity generation and fixation of advantageous mutants. The critical value of the mutation rate is a function not only of the sequence length, but also of the specific properties of the environment, in this case the selection pressure and the shape of the secondary structure used as target phenotype. Certain functional motifs of RNA secondary structure that withstand high mutation rates (as the ubiquitous hairpin motif) might appear early in evolution and be actually frozen evolutionary accidents.</p

Performance Enhancements for the Lattice-Boltzmann Solver in the LAVA Framework

Performance enhancements in NASA's recently developed Lattice Boltzmann solver within the Launch Ascent and Vehicle Aerodynamics (LAVA) framework are presented. Two key algorithmic developments are highlighted. A coarse-fine interface treatment that discretely conserves mass and momentum has been implemented and successfully verified and validated. Code optimizations targeting improved serial and parallel performance were presented. For a simple turbulent Taylor-Green Vortex problem, we were able to demonstrate a 2.3 times speedup over the baseline code for a single Skylake-SP node containing 40 physical cores, and a 2.14 times speedup for 64 nodes containing 2560 physical cores. In addition, we were able to show that the optimizations enabled us to scale the code almost perfectly to 20480 physical cores where, including ghost cells, the problem size was 10 billion cells