Acoustic measurements of boundary layer flow and sediment flux

Results are reported on an assessment of the application of coherent Doppler and cross-correlation techniques to measure nearbed boundary layer flow. The approaches use acoustic backscattering from sediments entrained into the water column from the bed, to obtain high-resolution profiles of the nearbed hydrodynamics. Measurements are presented from a wave tunnel experiment in which sediment was entrained by unidirectional, oscillatory, and combined flows. The data collected have been used to evaluate the capability of the two flow techniques to measure boundary layer mean, turbulent, and intrawave velocity profiles. Further, the backscattered signal has been used to measure suspended sediment concentration profiles, which have been combined with the velocity profiles to obtain high-resolution measurements of boundary layer sediment flux.     

THE NATURE OF ULTRAVIOLET LIGHT-INDUCED STRAND BREAKAGE IN DNA CONTAINING BROMOURACIL

Abstract— Single-strand breaks are produced in the phosphodiester backbone of ultraviolet-light-irradiated 5–bromodeoxyuridine-containing DNA (BU-DNA) after treatment with alkali. No radiation dependent breakage is observed in thymine-containing DNA (thy-DNA). The relative yields of breaks terminated by 5'-hydroxyl and 5'-phosphate groups was determined by measuring the rate of phosphorylation achieved with polynucleotide kinase in BU-DNA single strands before and after treatment with alkaline phosphatase. The ratio of 5'-phosphate to 5' hydroxyl groups ranged from 2.3 to 2.9 in different experiments. When cysteamine was present during irradiation no new end groups were produced.
In order to identify the nucleoside(s) at the 5'-termini, phosphate groups were removed with alkaline phosphatase and the 5'-hydroxyl groups were phosphorylated with polynucleotide kinase. Electrophoresis of enzymatic digests showed a single ³²P-labeled component migrating more rapidly than any of the four usual 5'-mononucleotides. Upon column chromatography this component resolved into a major peak coincident with 5'-dUMP and a lesser unidentified constituent. No 5'-dBUM³²P was observed among these nucleotides.     

In-chain tunneling through charge-density-wave nanoconstrictions and break junctions

We have fabricated longitudinal nanoconstrictions in the charge-density wave conductor (CDW) NbSe3 using a focused ion beam and using a mechanically controlled break-junction technique. Conductance peaks are observed below the TP1=145 K and TP2=59 K CDW transitions, which correspond closely with previous values of the full CDW gaps 2Delta1 and 2Delta2 obtained from photoemission. These results can be explained by assuming CDW-CDW tunneling in the presence of an energy gap corrugation epsilon2 comparable to Delta2, which eliminates expected peaks at +/-|Delta1+Delta2|. The nanometer length scales our experiments imply indicate that an alternative explanation based on tunneling through back-to-back CDW-normal-conductor junctions is unlikely.     

Shear modulus and plasticity of a driven charge density wave

We have probed the effects of transverse variations in pinning strength on charge-density-wave (CDW) structure in NbSe3 by x-ray micro-beam diffraction. In ribbonlike crystals having a large longitudinal step in thickness, the CDW first depins on the thick side of the step, causing rotations of the CDW wave vector. By measuring these rotations as a function of position and electric field, the corresponding shear strains are determined, allowing the CDW's shear modulus to be estimated. These results demonstrate the usefulness of x-ray microdiffraction as a tool in studying collective dynamics in electronic crystals.     

Polymerizable derivatives of long-chain fatty acids. IV. Vinyl esters

Contrary to some published reports, the vinyl esters of saturated fatty acids polymerize readily and rapidly. Vinyl oleate, when present in excess of 5%, and oxygen exert marked retarding effects. Techniques are described for the free-radical-initiated polymerization of the vinyl esters of caprylic, capric, lauric, myristic, palmitic, and stearic acids in bulk, dispersion, solution, and emulsion. Some data are given for polymerization in the presence of chain-transfer agents, such as carbon tetrachloride, dodecylmercaptan, and ethylbenzene. Conditions are reported for obtaining degrees of polymerization from about 2 (when chain-transfer agents are employed) to 10,000 (weight average). The weight average degree of polymerization increases markedly as the conversion increases, particularly above 80%. Even up to extremely high conversions, soluble polymers are obtained in most cases. Solubility characteristics, transition point data, molecular weights (osmometric and light-scattering), and isolation and purification techniques are also reported.     

Controlling the electronic coupling between CdSe quantum dots and thiol capping ligands via pH and ligand selection

Comparison of the UV-vis absorption spectra of CdSe quantum dots (QDs) capped with various mercaptocarboxylic acid capping ligands reveals that only 4-mercaptobenzoic acid (MBzA) capping ligands lower the apparent optical band gap. We propose that the delocalization of the excitons in the CdSe QDs is extended onto the ligands via electronic coupling to the π system of the 4-mercaptobenzoic acid molecules through the Cd-S bond. Furthermore, we demonstrate that the electronic coupling between the QDs and the (MBzA) thiol ligands is influenced by the strength of the Cd-S bond that can be changed by protonating the S atom.     

Melting Point and Composition Distribution of Polyolefins by Fractionation

The fractionation technique described in this paper was used to characterize the melting-point, monomer, and blocking distributions for polymers and copolymers. It is different from the molecular-weight fractionation technique in that the fractions are obtained by using a single solvent to extract the solid polymer below its melting point at stepwise-increasing temperatures. The reproducibility of this technique is excellent, and the technique is sufficient to distinguish pellet-to-pellet variation in a commercially available polypropylene. It was used to show the influence of preparation variables on the melting-point distributions of polyethylene and polypropylene and on the monomer and blocking distribution of copolymers, and to distinguish copolymers from blends.