Phase Behavior of Polyelectrolyte-Surfactant Complexes at Planar Surfaces

We investigate theoretically the phase diagram of an insoluble charged surfactant monolayer in contact with a semi-dilute polyelectrolyte solution (of opposite charge). The polyelectrolytes are assumed to have long-range and attractive (electrostatic) interaction with the surfactant molecules. In addition, we introduce a short-range (chemical) interaction which is either attractive or repulsive. The surfactant monolayer can have a lateral phase separation between dilute and condensed phases. Three different regimes of the coupled system are investigated depending on system parameters. A regime where the polyelectrolyte is depleted due to short range repulsion from the surface, and two adsorption regimes, one being dominated by electrostatics, whereas the other by short range chemical attraction (similar to neutral polymers). When the polyelectrolyte is more attracted (or at least less repelled) by the surfactant molecules as compared with the bare water/air interface, it will shift upwards the surfactant critical temperature. For repulsive short-range interactions the effect is opposite. Finally, the addition of salt to the solution is found to increase the critical temperature for attractive surfaces, but does not show any significant effect for repulsive surfaces.Comment: 23 pages, 10 figure

Pattern formation during diffusion limited transformations in solids

We develop a description of diffusion limited growth in solid-solid transformations, which are strongly influenced by elastic effects. Density differences and structural transformations provoke stresses at interfaces, which affect the phase equilibrium conditions. We formulate equations for the interface kinetics similar to dendritic growth and study the growth of a stable phase from a metastable solid in both a channel geometry and in free space. We perform sharp interface calculations based on Green's function methods and phase field simulations, supplemented by analytical investigations. For pure dilatational transformations we find a single growing finger with symmetry breaking at higher driving forces, whereas for shear transformations the emergence of twin structures can be favorable. We predict the steady state shapes and propagation velocities, which can be higher than in conventional dendritic growth.Comment: submitted to Philosophical Magazin

Nachhaltige Verbesserung der Verfügbarkeit ökologisch vermehrten Gemüsesaatguts für den Erwerbsanbau durch on-farm Erhaltung, Züchtung und Vermehrung

Die Verfügbarkeit von ökologisch vermehrtem Saatgut bzw. die Sortenauswahl wird insbesondere bei Gemüse als unzureichend eingestuft. Die verwendeten Sorten sind bei vielen Gemüsekulturen zum überwiegenden Teil Hybriden, die für eine /On-farm/-Erhaltung ungeeignet sind oder im Falle von aus Zellfusion (Protoplasten- oder Cytoplastenfusion) stammenden CMS-Hybriden nicht mehr fertil und auf Betrieben der Ökolandbauverbände per Richtlinie verboten sind. Diese beiden Aspekte der Saatgutfrage waren maßgebend für den Start dieses Projektes. Mit dem Ziel, die Eignung der Sorten für den ökologischen Gemüsebau zu prüfen, wurden 164 Saatgutmuster noch verfügbarer Populationssorten der Kulturen Kohlrabi (15), Möhren (86) und Porree (63) am europäischen Saatgutmarkt erworben und als Ausgangsmaterial für die Sichtungen verwendet. Dafür wurden an elf Standorten in Deutschland, den Niederlanden und der Schweiz über zwei Jahre 143 Sorten gesichtet und von mehr als der Hälfte der Sorten erhaltungszüchterischer Samenbau durchgeführt. Sorten, die auf Basis der Sichtungen als für den ökologischen Erwerbsgemüsebau unmittelbar geeignet eingestuft worden sind, werden in weiterführenden (teilweise externen) Versuchsanbau gestellt. Außerdem werden dafür ökologische Erhaltungszuchten bzw. Saatgutvermehrungen aufgebaut, die teilweise bereits in der Projektlaufzeit initiiert wurden. Sorten, die einer züchterischen Weiterentwicklung bedürfen, stehen als Ausgangsmaterial für ökologische Zuchtprogramme zur Verfügung. Das Saatgut wird über eine vereinseigene Genbank zentral verwaltet. Die Ergebnisse der Sichtungen, sowie Sortenbeschreibungen sind in einer Online-Datenbank veröffentlicht: www.kultursaat.org/index.ph

Counterion density profiles at charged flexible membranes

Counterion distributions at charged soft membranes are studied using perturbative analytical and simulation methods in both weak coupling (mean-field or Poisson-Boltzmann) and strong coupling limits. The softer the membrane, the more smeared out the counterion density profile becomes and counterions pentrate through the mean-membrane surface location, in agreement with anomalous scattering results. Membrane-charge repulsion leads to a short-scale roughening of the membrane.Comment: 4 pages, 4 figure

Community Beat: Revealing the Hidden Rhythms of a Neighbourhood

We live our lives in world of hidden rhythms, but many of these rhythms were socially constructed many years ago and may no longer be best serving us. In this paper we present our concept for an interactive public display that aims to make people aware of some of these hidden rhythms and help us question how we make use of our time. We raise a number of design decisions to bring this concept to reality, and a number of research questions arising out of it

Magnetism in the complex cobaltates Y1−xSrxCoO3−δ (0.7 ≤ x ≤ 0.95) and Ca3Co2O6

The magnetic phases in the complex cobaltates Y1−xSrxCoO3−δ (0.7 ≤ x ≤ 0.95) and Ca3Co2O6 have been investigated by susceptibility, heat capacity, X-ray and neutron scattering techniques. These measurements have shown that the super- structure ordering in the perovskite cobaltate Y1−xSrxCoO3−δ which evolves as a function of temperature heavily influences the ferrimagnetic behaviour of this mate- rial. Neutron scattering has also been used to probe the unusual time and magnetic field dependent behaviour of the spin-chain compound Ca3Co2O6, and to further our understanding of the magnetic phase diagram of this system. Both polycrystalline and single crystal samples have been used in this study. High quality single crystals of the A-site (Sr/Y) and oxygen vacancy ordered form of the perovskite Y1−xSrxCoO3−δ have been produced using the floating zone technique and characterised using EDAX and TGA. The single crystals produced were large enough to perform polarised and inelastic neutron scattering experiments on this compound for the first time, revealing anisotropic quasi-elastic scattering above the magnetic transition temperature. In addition, diffraction experiments on these samples found evidence of coincident structural and magnetic transitions in Y1−xSrxCoO3−δ at both 370 and 280 K. Neutron diffraction measurements were also performed on the geometrically frustrated compound Ca3Co2O6. The low temperature magnetisation process was found to be accompanied by clearly visible steps in the intensity of the ferromagnetic and antiferromagnetic Bragg peaks. Detailed measurements have shown that the presence of short-range correlations cannot account for the reduction in intensity of the antiferromagnetic Bragg peaks at low temperatures. Instead, the origin of this drop in intensity was found to be a slow time-dependent magnetic transition from one long-range ordered antiferromagnetic state to another. This transition occurs over a timescale of hours and is never complete. The experimental work detailed in this thesis provides new information about the phase diagrams of Y1−xSrxCoO3−δ and Ca3Co2O6 and contributes to our overall understanding of the physics of these complex cobaltate compounds

The Dynamics of Energy Systems and the Logistic Substitution Model. Executive Summary

This is a report on the work done at the International Institute for Applied Systems Analysis in 1976 to 1977 under a grant from Volkswagenwerk Foundation, whose objective was to explore the potential and the mechanisms of logistic analysis to describe the structure and the evolution of energy systems. Volume One contains the phenomenological part. About 300 cases were examined, some of which are reported in detail. The quality of the logistic description is generally excellent, even for cases extending 150 years into the past and with all the perturbations such a long time span entails, and consequently we thought it appropriate to extend the description into the future and use it for prediction. This was not really the objective of the grant but 'it naturally arises from the work and provides food for thought. Projections in the current literature appear to be in fact strongly inconsistent with the past, which casts doubts on their realizability, and are even internally inconsistent, which reinforces these doubts. The fact that numerous "free" choices at the social level lead to very regular overall patterns should perhaps temper the feeling of being caught in a deterministic clockwork. In Volume Two, devoted to the theoretical work, F. Fleck deals specifically with this problem showing the final regularity derived from a set of stochastic, i.e. "free", decisions. V. Peterka, on the other hand, operates at a more aggregated level, where one can start to speak of economic determinism. He describes a form of fate we are more ready to accept, if only grudgingly. Our exploration has generated more problems than we have solved; thus the field appears very fertile for future research

Radiative transfer in very optically thick circumstellar disks

In this paper we present two efficient implementations of the diffusion approximation to be employed in Monte Carlo computations of radiative transfer in dusty media of massive circumstellar disks. The aim is to improve the accuracy of the computed temperature structure and to decrease the computation time. The accuracy, efficiency and applicability of the methods in various corners of parameter space are investigated. The effects of using these methods on the vertical structure of the circumstellar disk as obtained from hydrostatic equilibrium computations are also addressed. Two methods are presented. First, an energy diffusion approximation is used to improve the accuracy of the temperature structure in highly obscured regions of the disk, where photon counts are low. Second, a modified random walk approximation is employed to decrease the computation time. This modified random walk ensures that the photons that end up in the high-density regions can quickly escape to the lower density regions, while the energy deposited by these photons in the disk is still computed accurately. A new radiative transfer code, MCMax, is presented in which both these diffusion approximations are implemented. These can be used simultaneously to increase both computational speed and decrease statistical noise. We conclude that the diffusion approximations allow for fast and accurate computations of the temperature structure, vertical disk structure and observables of very optically thick circumstellar disks.Comment: Accepted for publication in A&