Influence of the Dufour effect on convection in binary gas mixtures

Linear and nonlinear properties of convection in binary fluid layers heated from below are investigated, in particular for gas parameters. A Galerkin approximation for realistic boundary conditions that describes stationary and oscillatory convection in the form of straight parallel rolls is used to determine the influence of the Dufour effect on the bifurcation behaviour of convective flow intensity, vertical heat current, and concentration mixing. The Dufour--induced changes in the bifurcation topology and the existence regimes of stationary and traveling wave convection are elucidated. To check the validity of the Galerkin results we compare with finite--difference numerical simulations of the full hydrodynamical field equations. Furthermore, we report on the scaling behaviour of linear properties of the stationary instability.Comment: 14 pages and 10 figures as uuencoded Postscript file (using uufiles

Influence of the Soret effect on convection of binary fluids

Convection in horizontal layers of binary fluids heated from below and in particular the influence of the Soret effect on the bifurcation properties of extended stationary and traveling patterns that occur for negative Soret coupling is investigated theoretically. The fixed points corresponding to these two convection structures are determined for realistic boundary conditions with a many mode Galerkin scheme for temperature and concentration and an accurate one mode truncation of the velocity field. This solution procedure yields the stable and unstable solutions for all stationary and traveling patterns so that complete phase diagrams for the different convection types in typical binary liquid mixtures can easily be computed. Also the transition from weakly to strongly nonlinear states can be analyzed in detail. An investigation of the concentration current and of the relevance of its constituents shows the way for a simplification of the mode representation of temperature and concentration field as well as for an analytically manageable few mode description.Comment: 30 pages, 12 figure

Electro-optic sensor for static fields

A sensor has been developed for low frequency and DC electric fields E. The device is capable of measuring fields with Delta-E = 4 (1) V/cm resolution. It is based on a Y-cut Z-propagation lithium niobate electro-optic crystal. For a particular commercially available bare crystal, we achieved an in air time constant tau_c(air) = 6.4(1.8) h for the decay of the electro-optic signal. This enables field monitoring for several hours. As an application, we demonstrated that a constant electric field Eext = 640 V/cm applied via external electrodes to a particular spherical glass container holding an Xe/He gas mixture decays inside this cell with a time constant tau-E-glass = 2.5(5) h. This is sufficient for the needs of experiments searching for a permanent electric dipole moment in 129Xe. An integrated electric field sensor has been constructed which is coupled to a light source and light detectors via optical fibers. The sensor head does not contain any electrically conducting material

Convection in Binary Fluid Mixtures. II. Localized Traveling Waves. (Physical Review E, in press)

Nonlinear, spatially localized structures of traveling convection rolls are investigated in quantitative detail as a function of Rayleigh number for two different Soret coupling strengths (separation ratios) with Lewis and Prandtl numbers characterizing ethanol-water mixtures. A finite-difference method was used to solve the full hydrodynamic field equations numerically. Structure and dynamics of these localized traveling waves (LTW) are dominated by the concentration field. Like in the spatially extended convective states ( cf. accompanying paper), the Soret-induced concentration variations strongly influence, via density changes, the buoyancy forces that drive convection. The spatio-temporal properties of this feed-back mechanism, involving boundary layers and concentration plumes, show that LTW's are strongly nonlinear states. Light intensity distributions are determined that can be observed in side-view shadowgraphs. Detailed analyses of all fields are made using colour-coded isoplots, among others. In the frame comoving with their drift velocity, LTW's display a nontrivial spatio-temporal symmetry consisting of time-translation by half an oscillation period combined with vertical reflection through the horizontal midplane of the layer. A time-averaged concentration current is driven by a phase difference between the waves of concentration and vertical velocity in the bulk of the LTW state. The associated large-scale concentration redistribution stabilizes the LTW and controls its drift velocity into the quiescent fluid by generating a buoyancy-reducing concentration "barrier" ahead of the leading LTW front. The selection of the width of the LTW's is investigated and comparisons with experiments are presented.Comment: 18 pages and 6 figures as uuencoded Postscript file (using uufiles) 1 color figure as uuencoded Postscript file, a high resolution version of the color figure (about 10MB) can be requested from [email protected] or [email protected].: (Barten)present address: PSI, CH-5232 Villigen PSI, Switzerlan

Identification of an Aptamer Binding to Human Osteogenic-Induced Progenitor Cells

The aim of this study was to generate a specific aptamer against human jaw periosteal cells (JPCs) for tissue engineering applications in oral and maxillofacial surgery. This aptamer should serve as a capture molecule to enrich or even purify osteogenic progenitor cells from JPCs or from adult stem cells of other sources. Using systematic evolution of ligands by exponential enrichment (SELEX), we generated the first aptamer to specifically bind to human osteogenically induced JPCs. We did not detect any binding of the aptamer to undifferentiated JPCs, adipogenically and chondrogenically induced JPCs, or to any other cell line tested. However, similar binding patterns of the identified aptamer 74 were detected with mesenchymal stromal cells (MSCs) derived from placental tissue and bone marrow. After cell sorting, we analyzed the expression of osteogenic marker genes in the aptamer 74-positive and aptamer 74-negative fractions and detected no significant differences. Additionally, the analysis of the mineralization capacity revealed a slight tendency for the aptamer positive fraction to have a higher osteogenic potential. In terms of proliferation, JPCs growing in aptamer-coated wells showed increased proliferation rates compared with the controls. Herein, we report the development of an innovative approach for tissue engineering applications. Further studies should be conducted to modify and improve the specificity of the generated aptamer