Micro-structure evolution of wall based crystals after casting of model suspensions as obtained from Bragg microscopy

Growth of heterogeneously nucleated, wall based crystals plays a major role in determining the micro-structure during melt casting. This issue is here addressed using a model system of charged colloidal spheres in deionized aqueous suspension observed by Bragg microscopy which is a combination of light scattering and microscopy. We examine the evolution of the three-dimensional size, shape, and orientation of twin domains in monolithic crystals growing from two opposing planar walls into a meta-stable (shear-) melt. At each wall crystal orientation and twinning emerges during nucleation with small domains. During growth these widen and merge. From image analysis we observe the lateral coarsening velocities to follow a power law behaviour L(XY) ∝ t(1∕2) as long as the vertical growth continues at constant speed. Lateral coarsening terminates upon intersection of the two solids and hardly any further ripening is seen. Initial lateral coarsening velocities show a Wilson Frenkel type dependence on the melt meta-stability.     

Measurement of coherent pion-photoproduction on the 3He/3H isodoublet

Neutral and charged pion-photo production on ${}^3\text{H}$ and ${}^3\text{He}$ nuclei have been observed in the Δ(1232) resonance region. Resonance averaged cross-sections are presented as functions of momentum transfer.     

Coherentπ 0 photoproduction on12C,13C and16O in theΔ (1232) resonance region

π ⁰ photoproduction has been measured on¹²C,¹³C and¹⁶O in theΔ-resonance region. By limiting the acceptance of theπ ⁰ spectrometer to small angles (Θ≦10°), only the coherent channel could contribute. The data show a broadening of the resonance width whereas the peak energy remains essentially unchanged. Theoretical calculations give considerably lower values compared with our data.     

Atomic modifications by synchrotron radiation at the calcite–ethanol interface

This article reports on studies of the chemical alterations induced by synchrotron radiation at the calcite–ethanol interface, a simple model system for interfaces between minerals and more complex organic molecules containing OH groups. A combination of X‐ray reflectivity and X‐ray photoelectron spectroscopy of natural calcite, cleaved in distilled ethanol to obtain new clean interfaces, indicated that, during a 5 h period, the two top atomic layers of calcite, CaCO₃, transform into calcium oxide, CaO, by releasing CO₂. Also, the occupation of the first ordered layer of ethanol attached to calcite by hydrogen bonds almost doubles. Comparison between radiated and non‐radiated areas of the same samples demonstrate that these effects are induced only by radiation and not caused by aging. These observations contribute to establishing a time limit for synchrotron experiments involving fluid–mineral interfaces where the polar OH group, as present in ethanol, plays a key role in their molecular structure and bonding. Also, the chemical evolution observed in the interface provides new insight into the behavior of some complex organic molecules involved in biomineralization processes.     

Predicting hydration energies for multivalent ions

We have predicted the free energy of hydration for 40 monovalent and multivalent cations and anions using density functional theory and the implicit solvent model COnductor like Screening MOdel for Real Solvents (COSMO‐RS) at the Becke‐Perdew (BP)/Triple zeta valence with polarization functions (TZVP) level. Agreement with experimental data for monovalent and divalent ions is good and shows no significant systematic errors. Predictions are noticeably better than with standard COSMO. The agreement with experimental data for trivalent and tetravalent ions is slightly worse and shows systematic errors. Our results indicate that quantum chemical calculations combined with COSMO‐RS solvent treatment is a reliable method for treating multivalent ions in solution, provided one hydration shell of explicit water molecules is included for metal cations. The accuracy is not high enough to allow absolute predictions of hydration energies but could be used to investigate trends for several ions, thanks to the low computational cost, in particular for ligand exchange reactions. © 2014 Wiley Periodicals, Inc.     

Imaging cytoplasmic cAMP in mouse brainstem neurons

Background  

cAMP is an ubiquitous second messenger mediating various neuronal functions, often as a consequence of increased intracellular Ca²⁺ levels. While imaging of calcium is commonly used in neuroscience applications, probing for cAMP levels has not yet been performed in living vertebrate neuronal tissue before.     

Stable isotopic studies of earthworm feeding ecology in tropical ecosystems of Puerto Rico

Feeding strategies of earthworms and their influence on soil processes are often inferred from morphological, behavioral and physiological traits. We used (13)C and (15)N natural abundance in earthworms, soils and plants to explore patterns of resource utilization by different species of earthworms in three tropical ecosystems in Puerto Rico. In a high altitude dwarf forest, native earthworms Trigaster longissimus and Estherella sp. showed less (15)N enrichment ((15)N = 3-6 per thousand) than exotic Pontoscolex corethrurus ((15)N =7-9 per thousand) indicating different food sources or stronger isotopic discrimination by the latter. Conversely, in a lower altitude tabonuco forest, Estherella sp. and P. corethrurus overlapped completely in (15)N enrichment ((15)N = 6-9 per thousand), suggesting the potential for interspecific competition for N resources. A tabonuco forest converted to pasture contained only P. corethrurus which were less enriched in (15)N than those in the forest sites, but more highly enriched in (13)C suggesting assimilation of C from the predominant C(4) grass. These results support the utility of stable isotopes to delineate resource partitioning and potential competitive interactions among earthworm species. Copyright 1999 John Wiley & Sons, Ltd.     

Molecular ordering of ethanol at the calcite surface

To produce biominerals, such as shells, bones, and teeth, living beings create organic compounds that control the growth of the solid phase. Investigating the atomic scale behavior of individual functional groups at the mineral-fluid interface provides fundamental information that is useful for constructing accurate predictive models for natural systems. Previous investigations of the activity of coccolith-associated polysaccharides (CAP) on calcite, using atomic force microscopy (AFM) [Henriksen, K., Young, J. R., Bown, P. R., and Stipp, S. L. S. Palentology 2004, 43 (Part 3), 725-743] and molecular dynamics (MD) modeling [Yang, M., Stipp, S. L. S., and Harding, J. H. Cryst. Growth Des. 2008, 8 (11), 4066-4074], have suggested that OH functional groups control polysaccharide attachment. The purpose of this work was to characterize, using X-ray reflectivity (XR) combined with molecular dynamics (MD) simulations, the structuring on calcite of a layer of the simplest carbon chain molecule that contains an OH group, ethanol (CH(3)-CH(2)-OH). We found evidence that EtOH forms a highly ordered structure at the calcite surface, where the first layer molecules bond with calcite. The ethanol molecules stand up perpendicularly at the interface or nearly so. As a consequence, the fatty, CH(3) ends form a new surface, about 6 ? from the termination of the bulk calcite, and beyond that, there is a thin gap where ethanol density is low. Following is a more disordered layer that is two to three ethanol molecules thick, about 14 ?, where density more resembles that of bulk liquid ethanol. The good agreement between theory and experiment gives confidence that a theoretical approach can offer information about behavior in more complex systems.     

Scaling in colloidal crystallization

Summary The process of colloidal crystallization has already been separately investigated by light scattering mearurements of the crystalline order (Bragg peaks) and of long-ranged density fluctuations (small-angle scattering). We present the firstsimultaneous small- and large-angle light scattering experiment. Our measurements have been performed on hard-sphere colloids of different particle densities. We show results for the temporal evolution of crystallization and find two distinct regimes: homogeneous nucleation and diffusion- or reaction-limited growth at early times, followed by ripening of the Lifshitz-Slyozov or Lifshitz-Allen-Cahn type. To our preliminary experience, we can already state that the results are always consistent in both partial experiments. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. With deep regret we have to announce that Prof. Klaus Sch?tzel, who inspired and guided the work presented here, has died in October 1994.     

Measurement of coherent π0 photoproduction on3He and3H in the resonance region

Neutral pion photoproduction has been measured on³He and³H nuclei in theΔ(1232) resonance region. Resonance averaged cross-sections are presented as a function of momentum transfer and compared to theoretical calculations.