Diffusion within ultrathin, dense nanoporous silica films

In this work the origin of permselectivity in dense silica films which possess a pore structure with pore sizes commensurate with the molecular size of the diffusing gas species is investigated. Much of the recently reported work in this field has involved the development of composite membrane films, and while it is generally assumed that the transport process of the gas species within the selective layer of these films is activated in nature, there are anomalies with this simplified picture. In this paper a new model is developed which, for the first time, explains the permselective behavior of the thin selective coatings ubiquitous to membrane separation processes. The model involves the existence of two primary transport domains within the solid film, one of which rapidly conducts the permeating gas (under non-Fickian conditions), while the second domain involves a slow diffusion mode characterized by normal Fickian transport. To validate the model, molecular dynamics simulations are conducted for diffusion of a number of simple gases (He, N(2), and CO(2)) within silica glasses over a range of solid densities. The silica media employed in these studies are based on a novel approach developed in this work for the construction of three-dimensionally periodic atomistic structures of silica of arbitrary density in which network bond connectivity is ensured. The results obtained from this work are in qualitative agreement with experimental observations and confirm the existence of dual mode transport which is central to the interpretation of the permselectivity in composite membranes systems.     

Cell-like equivalences and boundaries of CAT(0) groups

In 2000, Croke and Kleiner showed that a CAT(0) group G can admit more than one boundary. This contrasted with the situation for ??-hyperbolic groups, where it was well-known that each such group admitted a unique boundary??in a very stong sense. Prior to Croke and Kleiner??s discovery, it had been observed by Geoghegan and Bestvina that a weaker sort of uniquness does hold for boundaries of torsion free CAT(0) groups; in particular, any two such boundaries always have the same shape. Hence, the boundary really does carry significant information about the group itself. In an attempt to strengthen the correspondence between group and boundary, Bestvina asked whether boundaries of CAT(0) groups are unique up to cell-like equivalence. For the types of space that arise as boundaries of CAT(0) groups, this is a notion that is weaker than topological equivalence and stronger than shape equivalence. In this paper we explore the Bestvina Cell-like Equivalence Question. We describe a straightforward strategy with the potential for providing a fully general positive answer. We apply that strategy to a number of test cases and show that it succeeds??often in unexpectedly interesting ways.     

The geometry of spheres in free abelian groups

We study word metrics on ${\mathbb{Z}^d}$ by developing tools that are fine enough to measure dependence on the generating set. We obtain counting and distribution results for the words of length n. With this, we show that counting measure on spheres always converges to cone measure on a polyhedron (strongly, in an appropriate sense). Using the limit measure, we can reduce probabilistic questions about word metrics to problems in convex geometry of Euclidean space. We give several applications to the statistics of ??size-like?? functions.     

Alginate hydrogels as biomaterials

[Image: see text] Alginate hydrogels are proving to have a wide applicability as biomaterials. They have been used as scaffolds for tissue engineering, as delivery vehicles for drugs, and as model extracellular matrices for basic biological studies. These applications require tight control of a number of material properties including mechanical stiffness, swelling, degradation, cell attachment, and binding or release of bioactive molecules. Control over these properties can be achieved by chemical or physical modifications of the polysaccharide itself or the gels formed from alginate. The utility of these modified alginate gels as biomaterials has been demonstrated in a number of in vitro and in vivo studies.Micro-CT images of bone-like constructs that result from transplantation of osteoblasts on gels that degrade over a time frame of several months leading to improved bone formation.     

Evaluation of a polymerase chain reaction-based heteroduplex assay for detecting the adulteration of processed orange juice with mandarin juice

A polymerase chain reaction (PCR)-based heteroduplex assay was evaluated for the detection of mandarin juice in processed orange juice. PCR amplification of a fragment of the chloroplast trnT-trnL intergenic spacer derived from mixtures of DNA extracted from orange and mandarin juice resulted in heteroduplex formation. The heteroduplex resulted from the co-amplification of a fragment containing an 8 base-pair indel that distinguished mixtures of orange and mandarin juice from orange juice and mandarin juice alone. The heteroduplex assay was evaluated against authentic juices obtained from different citrus species and confirmed that the marker was homogeneous within Citrus. The data obtained demonstrated maternal inheritance of chloroplast type in Citrus sp. and allowed the identification and confirmation of the maternal parentage of unknown and known citrus hybrids. Analysis of the quantitative potential of the PCR and polyacrylamide gel electrophoresis (PAGE) analysis demonstrated good repeatability with a coefficient of variation of 7.5%. Greatest sources of variance in experimental results were attributable to species and varietal differences in the levels of the PCR target. Mandarin juice contained approximately 18% (w/v) less PCR target sequence than did orange juice. The assay was tested in a blind trial using processed juices and correctly identified 20/22 samples with no false-positive results.