Correlations of synthetic,spectroscopic, structural,and speciation studies in the biologically relevant cobalt(II)-citrate system: the tale of the first aqueous dinuclear cobalt(II)-citrate complex

Synthetic efforts targeting soluble species of Co(II) with the low molecular mass physiological ligand citric acid led to the isolation of the first dinuclear complex [Co(2)(C(6)H(5)O(7))(2)(H(2)O)(4)](2-), at pH approximately 5, in the form of its K+ (1) and Na+ (2) salts. Both 1 and 2 were characterized analytically, spectroscopically (FT-IR, UV/visible, EPR), and magnetically. Complex 1 crystallizes in the monoclinic space group P2(1)/n, with a = 10.348(5) A, b = 11.578(6) A, c = 12.138(6) A, beta = 112.62(2) degrees, V = 1342(1) A(3), and Z = 2. Complex 2 crystallizes in the monoclinic space group P2(1)/c, with a = 9.234(4) A, b = 11.913(4) A, c = 11.728(6) A, beta = 99.93(2) degrees, V = 1271(1) A(3), and Z = 2. X-ray crystallography on 1 and 2 reveals the presence of two Co(II) ions, in a dinuclear assembly, octahedrally coordinated by two citrate ligands in a tridentate fashion. The octahedral environment around each Co(II) is complemented by another singly bonded citrate belonging to the adjacent Co(II) unit and two water molecules. Magnetic susceptibility and EPR studies on 1, in the solid state, corroborate the X-ray results, indicating a weak interaction between the two Co(II) ions. Moreover, EPR and UV/visible studies in solution suggest that 1 does not retain its dimeric structure, yielding a mononuclear octahedral Co(II)-citrate species. Detailed speciation studies suggest the presence of a number of species including the mononuclear complex [Co(C(6)H(5)O(7))](-), optimally present around pH approximately 5. In consonance with EPR and UV/visible spectroscopy, [Co(C(6)H(5)O(7))](-) is likely the scaffolding unit on the basis of which the dimer [Co(2)(C(6)H(5)O(7))(2)(H(2)O)(4)](2-) is isolated from aqueous solutions. Collectively, this comprehensive study offers significant structural insight into the Co(II)-citrate speciation and the elucidation of the role of Co(II) in biological fluids.     

High resolution multigrid cross-correlation digital PIV measurements of a turbulent starting jet using half frame image shift film recording

This paper describes a novel approach of acquiring two single-exposed, non-overlapping images using half frame image shift (HFIS) recording on photographic film. This technique permits the recording of two single-exposed, non-overlapping images of seed particles in a flow plane on high spatial resolution film with any arbitrary time delay between exposures. A new multigrid CCDPIV (MCCDPIV) analysis method is used to analyse the single-exposed, non-overlapping sequential images resulting in PIV measurements with a larger velocity dynamic range, lower random error and better spatial resolution than standard CCDPIV analysis. HFIS recording followed by MCCDPIV analysis was employed to measure the spatio-temporal evolution of the in-plane velocity vector and the out-of-plane vorticity fields of a turbulent starting jet at Reynolds numbers based on the orifice diameter and piston velocity of 10,780 and 13,860.     

Magnetohydrodynamics and Plasma Cosmology

We study the linear magnetohydrodynamic (MHD) equations, both in the Newtonian and the general-relativistic limit, as regards a viscous magnetized fluid of finite conductivity and discuss instability criteria. In addition, we explore the excitation of cosmological perturbations in anisotropic spacetimes, in the presence of an ambient magnetic field. Acoustic, electromagnetic (e/m) and fast-magnetosonic modes, propagating normal to the magnetic field, can be excited, resulting in several implications of cosmological significance.     

Dynamic K-L analysis of coherent structures based on DNS of turbulent Newtonian and viscoelastic flows

Earlier work [1, 2], using Karhunen-Loeve (K-L) analysis, demonstrated a dramatic enhancement of the importance of large scale motions with increased viscoelasticity and an equally dramatic decrease in the K-L dimension of the flow (an order of magnitude) as viscoelasticity increases versus similar Newtonian results. In this work we look into dynamics of viscoelastic turbulent flows by projecting the DNS data in time into a selected set of K-L modes. The dynamics of the coherent structures embedded in turbulent flows were investigated through calculations of mode auto and cross correlations. This allows for a more systematic examination of the role of large-scale structures in turbulence and drag reduction. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Extensional behavior influence on viscoelastic turbulent channel flow

In this work we use in the simulation of a viscoelastic turbulent channel flow a modification of the finitely extensible of non-linear elastic dumbbells with the Peterlin approximation (FENE-P) constitutive model for dilute polymer solutions, applicable to high extensional deformations. The new feature introduced by this modification is that the free energy of the polymer (since it is assumed to be entirely entropically driven) remains always bounded (FENE-PB). The characteristics of the model under steady shear flow, pure elongational flow and transient extensional behavior are presented. It is found that the FENE-PB model is more shear thinning than FENE-P. Most importantly, it also shows a higher extensional viscosity than the FENE-P model. Although the steady-state Trouton ratio asymptotically reaches at high extensional rates the same limit as the FENE-P model, the transition from the Newtonian value is sharper and faster. We use the FENE-PB model in direct numerical simulations (DNS) of viscoelastic turbulent channel flow using spectral approximations. The results for various statistics of the flow and the polymer conformation, when compared against those obtained with the original FENE-P model and the same rheological parameters, show an enhanced polymer-induced drag reduction effect and enhanced deformation of the polymer molecules. This indicates that it is not only the asymptotic but also details from the extensional rheological behavior that matter in quantitatively specifying turbulent viscoelastic flow behavior.     

Rheological characterization of polyethylene terephthalate resins using a multimode Phan-Tien-Tanner constitutive relation

Linear viscoelastic, shear, and extensional rheological characterization of linear and branched Poly(Ethylene Terephthalate) resins (PET) was carried out by means of both a parallel-plate and capillary rheometers. Before loading into the rheometers, the polymer pellets were thoroughly dried at well-characterized conditions long enough to obtain consistent and reproducible results. Continuing polymer degradation and poly-condensation reactions in the relatively open environment of the parallel-plate rheometer were accounted for by correcting the data using material-time super-position. The rheological data obtained were used to fit by nonlinear optimization, the linear relaxation spectrum and nonlinear parameters of a multi-mode Phan-Thien and Tanner (PTT) constitutive relation. It was found that this model can represent rheological data for PET resins very well and as a result may be used in relevant processing flow simulations, i.e. film casting.