Creeping flow analysis of an integrated microfluidic device for rheometry

This paper analyzes flow of a power-law fluid in a microfluidic device for the purpose of discovering an algorithm for rheometry. Previous investigations have shown that measurement of the velocity field or the pressure field and the inlet flow rate in a microfluidic T-junction allow determination of rheological parameters uniquely. However, the range of shear induced within the flow domain was limited by the constant pressure drop applied across the micro-device. To avoid this control restriction and further develop our inverse technique, a constant flow rate system was investigated. With this configuration, the flow rate can be set appropriately to achieve a desired shear range and the rheological parameters can be inferred from the measurement of mean pressure at the inlet and at the junction. By assuming creeping flow conditions and the existence of a Hagen-Poiseuille-like law for the relationship between the pressure drop and the volumetric flow rate, the analysis produces an algorithm that is self-consistent (demonstrates the Hagen-Poiseuille law) and permits the inference of the power-law parameters from the ratio of any two field variables measured over a region (averaged), the pressure drop, and the volumetric flow rate.     

Polar effects in nitride coupling reactions

The nucleophilic molybdenum nitride (Et(2)NCS(2))(3)MoN (1) reacts with the electrophilic osmium nitride complex TpOsNCl(2) (2, Tp = hydrotris(1-pyrazolyl)borate) to produce molecular nitrogen. Reaction of 1 at the nitride is accompanied by a substantial amount of reaction at a sulfur atom of the dithiocarbamate ligand, forming the osmium thionitrosyl complex TpOs(NS)Cl(2) (4). Labeling experiments establish that the N(2) produced comes specifically (>96%) from mixed-metal (molybdenum-osmium) coupling. The major transition-metal-containing product of the reaction is the mu-nitrido complex TpOsCl(2)(mu-N)Mo(S(2)CNEt(2))(3) (3), where the bridging nitride derives primarily (82%) from the osmium nitride 2. The mu-nitrido complex 3 has been characterized crystallographically, and shows a nitride bridge that is very asymmetric (Mo-N = 1.721(3) A, Os-N = 1.906(3) A), with less multiple bonding toward osmium and more toward molybdenum. Heterometallic coupling is much faster than either homometallic coupling reaction, in particular the osmium-osmium coupling, despite the greater oxidizing power of osmium over molybdenum. The origin and implications of this kinetic effect on nitride coupling and dinitrogen cleavage are discussed.     

Synthetic receptors for CG base pairs

Hydrogen-bond-mediated complexation of a CG base pair by a hexylureido phthalimide and a hexylureido isoindolin-1-one was studied by (1)H NMR spectroscopy in an organic solvent. Chemical shift data indicate that both receptors effectively bind the CG base pair from the major groove side.     

The tri-pi-methane rearrangement: mechanistic and exploratory organic photochemistry(1)

The di-pi-methane rearrangement is firmly established as a mode of synthesizing three-membered-ring compounds. We now report the tri-pi-methane counterpart.     

Analysis of repetitive mechanism signatures

A new signal decomposition for repetitive signatures is introduced. This is based on the concept of derived time series, produced by rearranging the original signal by using points one period apart. Time domain averaging is shown to be a special case of such rearrangement, and a second technique of variance analysis is presented. Applications to rotating machinery show that the decomposition can be very sensitive to various generating mechanisms, making them suitable for signature analysis of mechanical systems.     

Pressure ionization in laser-fusion target simulation

Accurate simulation of high density target implosion requires material properties (ionization, pressure, energy, opacity and transport coefficients) at densities where bound electrons are significantly perturbed by neighboring atoms. In modern laser-fusion simulation codes, this data is supplied by tables and/or calculated from a Stromgren model for ionization equilibrium. Improvements have been made in the Stromgren average-atom model which aim at assuring thermodynamic consistency and obtaining better agreement with more elaborate calculations. Arbitrary degeneracy is allowed for the free electrons. Consistent Coulomb contributions to pressure and continuum lowering are obtained. A new pressure ionization scheme merges bound electrons into the continuum as a smooth function of density and the corresponding contribution to pressure is calculated. Results are shown for aluminum.     

Nematic phase transitions in mixtures of thin and thick colloidal rods

We report experimental measurements of the phase behavior of mixtures of thin (charged semiflexible fd virus) and thick (fd-PEG, fd virus covalently coated with polyethylene glycol) rods with diameter ratio varying from 3.7 to 1.1. The phase diagrams of the rod mixtures reveal isotropic-nematic, isotropic-nematic-nematic, and nematic-nematic coexisting phases with increasing concentration. In stark contrast to predictions from earlier theoretical work, we observe a nematic-nematic coexistence region bound by a lower critical point. Moreover, we show that a rescaled Onsager-type theory for binary hard-rod mixtures qualitatively describes the observed phase behavior.     

Shape memory and pseudoelasticity in metal nanowires

Structural reorientations in metallic fcc nanowires are controlled by a combination of size, thermal energy, and the type of defects formed during inelastic deformation. By utilizing atomistic simulations, we show that certain fcc nanowires can exhibit both shape memory and pseudoelastic behavior. We also show that the formation of defect-free twins, a process related to the material stacking fault energy, nanometer size scale, and surface stresses is the mechanism that controls the ability of fcc nanowires of different materials to show a reversible transition between two crystal orientations during loading and thus shape memory and pseudoelasticity.