Transient streaming potential in a finite length microchannel

Pressure-driven flow of an electrolyte solution in a microchannel with charged solid surfaces induces a streaming potential across the microchannel. Such a flow also causes rejection of ions by the microchannel, leading to different concentrations in the feed and permeate reservoirs connecting the capillary, which forms the basis of membrane based separation of electrolytes. Modeling approaches traditionally employed to assess the streaming potential development and ion rejection by capillaries often present a confusing picture of the governing electrochemical transport processes. In this paper, a transient numerical simulation of electrochemical transport process leading to the development of a streaming potential across a finite length circular cylindrical microchannel connecting two infinite reservoirs is presented. The solution based on finite element analysis shows the transient development of ionic concentrations, electric fields, and the streaming potential over the length of the microchannel. The transient analysis presented here resolves several contradictions between the two types of modeling approaches employed in assessing streaming potential development and ion rejection. The simulation results show that the streaming potential across the channel is predominantly set up at the timescale of the developing convective transport, while the equilibrium ion concentrations are developed over a considerably longer duration.     

Genes involved in <Emphasis Type="Italic">Drosophila</Emphasis>glutamate receptor expression and localization

Background  

A clear picture of the mechanisms controlling glutamate receptor expression, localization, and stability remains elusive, possibly due to an incomplete understanding of the proteins involved. We screened transposon mutants generated by the ongoing Drosophila Gene Disruption Project in an effort to identify the different types of genes required for glutamate receptor cluster development.     

A method for creating a non-equilibrium NT(P1-P2) ensemble in molecular dynamics simulation

A method is proposed for creating a non-equilibrium ensemble with a constant number of molecules, constant temperature and constant pressures with different target values in two reservoirs [referred to as NT(P(1)-P(2)) ensemble] that are connected by a finite length nanopore. This method includes two steps. The first step places a partition between the two reservoirs and then creates a static pressure field and a proper system volume by using two self-adjusting plates on which two external forces/pressures with different target values are exerted. The second step removes the partition and the two self-adjusting plates and the pressure difference between the two reservoirs is maintained by a "pump" designed to simultaneously create a periodic boundary condition between the two reservoirs and supply the necessary force (work) to a subset of molecules for a steady state flow. To examine this method, several cases using liquid argon with a truncated and shift Lennard-Jones potential under different target pressures and pump sizes were studied. Results show that the method proposed in this paper works well. In addition, the method proposed in this paper was compared with the other external force field methods. The results show that as long as the external force is applied to a restricted set of molecules away from the channel a constant pressure difference between two reservoirs is maintained. The advantage of the algorithm proposed here also sets the absolute pressures with different target levels in two reservoirs instead of it being arbitrary. Studies show that the fluid flow rate or permeability through a nanopore depends not only on the pressure difference between two reservoirs, but also on the absolute pressures in two reservoirs.     

Precision heterodyne measurements of ozone spectral lines near 9.5 μm

Precision measurements of 26 individual spectral lines of ozone near 9.5 μm were made using the GSFC infrared heterodyne spectrometer. The line profiles of 26 lines in near coincidence with the emission lines from a ¹²C¹⁶O₂ laser local oscillator were measured at resolving powers of 6 × 10⁶. The retrieved absolute line center frequencies, accurate to ±3 MHz, and the relative line intensities were compared to previous experimental results and theoretical calculations.     

Transient electrokinetic transport in a finite length microchannel: currents, capacitance, and an electrical analogy

Numerical simulations with the fluid mechanics based on the unsteady Navier-Stokes equations and the Poisson-Nernst-Planck formulation of electrostatics and ion transport were used to explore the transient transport of charge through a finite length cylindrical microchannel that is driven by a pressure difference. The evolution of the transcapillary potential from a no-flow equilibrium to the steady-state-steady-flow streaming potential was analyzed by following the convection, migration, and net currents. Observations of the unsteady characteristics of the streaming current, electrical resistance, and capacitance led to an electrical analogy. This electrical analogy was made from a current source (to represent convection current), which was placed in parallel with a capacitor (to allow the accumulation of charge) and a resistor (to permit a migration current). A parametric study involving a range of geometries, fluid mechanics, electrostatics, and mass transfer states allowed predictive submodels for the current source, capacitor, and resistor to be developed based on a dimensional analysis.     

Macrocylic bisbibenzyl natural products and their chemical synthesis

Covering: 1995 to June 2011. The macrocyclic bisbibenzyl family of natural products are commonly found in liverworts and other bryophytes, though the recent isolation of riccardin C from a primrose extract has demonstrated their existence in higher flowering plants. Each has a core comprising four aromatic rings and two ethano bridges, being derived in Nature from two molecules of lunularin. Sub-classes are distinguished by the connectivity between these lunularin units, while individual natural products are distinguished by the hydroxy- and/or alkoxy-substituents decorating the core structures. Further diversification results from halogenation and oxidation, which may lead to dimerization or the creation of additional rings. The review provides a timely update to a previous Natural Product Reports article by Keseru and Nógrádi (Nat. Prod. Rep. 1995, 12, 69-75) and largely focuses on new additions to the family and the strategies used to effect their chemical synthesis.     

Reflection of elastic waves in a crystal of Heusler alloy Ni<Subscript>2</Subscript>MnGa in the phase transition range

The reflection of longitudinal and transverse acoustic waves from the free surface of the ferromagnetic shape memory alloy Ni₂MnGa that is located in the ranges of the premartensite and martensite phase transformations is considered. The propagation directions and amplitudes of the waves reflected in the (001) plane of the crystal are determined. They acquire the character of substantially quasi-longitudinal and quasi-transverse vibrations rather than being pure modes. The angles of wave reflection and conversion are shown to be effectively controlled by temperature and a magnetic field due to the colossal acoustic anisotropy of the crystal over the wide range of its phase transitions. Beginning from a certain critical angle of incidence of a quasi-transverse wave, the quasi-longitudinal wave having appeared upon reflection becomes an accompanying surface vibration, and it can be emitted into the bulk of the crystal when the phase transition point is approached. Two angles of full conversion of an incident quasi-longitudinal wave into a quasi-transverse wave are established, and their temperature dependences are found. Trivisonno’s experimental data for the ultrasound velocity and absorption in an Ni₂MnGa crystal are used to numerically estimate these acoustic effects.     

High-performance liquid chromatographic determination of cholesteryl esters in the blood of obese children.

The serum of obese children and adolescents was analyzed for cholesteryl esters. The test substances were first separated from the sample matrix by solvent extraction and thin-layer chromatography and then resolved in a reversed-phase high-performance liquid chromatographic system involving a Separon SGX C18 column and a mobile phase of 2-propanol-acetonitrile (40:60, v/v), with ultraviolet detection at 206 nm. Cholesterol and 10-cholesteryl esters could be separated and determined within ca. 25 min at a flow-rate of 1 ml/min. The method was applied to a study of the effect of external conditions (physical stress, diet) on the content of cholesteryl esters in a test group of obese boys and girls aged from 13 to 16 years. The analyses have demonstrated that the above conditions do not affect the concentrations of the individual cholesteryl esters, although the total cholesterol concentration decreased significantly after spa treatment.