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

Field theoretic description of charge regulation interaction

In order to find the exact form of the electrostatic interaction between two proteins with dissociable charge groups in aqueous solution, we have studied a model system composed of two macroscopic surfaces with charge dissociation sites immersed in a counterion-only ionic solution. Field-theoretic representation of the grand canonical partition function is derived and evaluated within the mean-field approximation, giving the Poisson-Boltzmann theory with the Ninham-Parsegian boundary condition. Gaussian fluctuations around the mean-field are then analyzed in the lowest order correction that we calculate analytically and exactly, using the path integral representation for the partition function of a harmonic oscillator with time-dependent frequency. The first order (one loop) free energy correction gives the interaction free energy that reduces to the zero-frequency van der Waals form in the appropriate limit but in general gives rise to a mono-polar fluctuation term due to charge fluctuation at the dissociation sites. Our formulation opens up the possibility to investigate the Kirkwood-Shumaker interaction in more general contexts where their original derivation fails.Comment: 12 pages, 9 figures, submitted to EPJ

Space power distribution system technology. Volume 2: Autonomous power management

Electrical power subsystem requirements, power management system functional requirements, algorithms, power management subsystem, hardware development, and trade studies and analyses are discussed

One-dimensional metallic behavior of the stripe phase in La2−x_{2-x}Srx_xCuO4_4

Using an exact diagonalization method within the dynamical mean-field theory we study stripe phases in the two-dimensional Hubbard model. We find a crossover at doping $\delta\simeq 0.05$ from diagonal stripes to vertical site-centered stripes with populated domain walls, stable in a broad range of doping, $0.05<\delta<0.17$. The calculated chemical potential shift $\propto -\delta^2$ and the doping dependence of the magnetic incommensurability are in quantitative agreement with the experimental results for doped La$_{2-x}$Sr$_x$CuO$_4$. The electronic structure shows one-dimensional metallic behavior along the domain walls, and explains the suppression of spectral weight along the Brillouin zone diagonal.Comment: 4 pages, 4 figure