Self‐Assembly of Lead Chalcogenide Nanocrystals

This review focuses on recent developments in the self‐assembly of lead chalcogenide nanocrystals into two‐ and three‐dimensional superstructures. Self‐assembly is categorized by the shapes of building blocks, including nanospheres, nanocubes, nano‐octahedra, and nanostars. In the section on nanospheres, rapid assemblies of lead chalcogenide‐based multicomponent nanocrystals with additional components, such as semiconductors, noble metals, and magnetic nanocrystals, are further highlighted. In situ self‐assembly of lead chalcogenide nanocrystals into one‐dimensional nanostructures at elevated temperatures is also covered. Each section of this paper highlights examples extracted from recent publications. Finally, relatively novel properties and applications arising from lead chalcogenide superlattices as typical examples are also discussed.     

Manipulation of Single‐Stranded DNA by Using an Artificial Site‐Selective DNA Cutter Composed of Cerium(IV)/EDTA and Phosphonate–Oligonucleotide Conjugates

An artificial site‐selective DNA cutter to hydrolyze single‐stranded DNA at a desired site was prepared from Ce^IV/ethylenediamintetraacetic acid (EDTA) and two ethylenediamine‐N,N,N′,N′‐tetrakis(methylenephosphonic acid)–oligonucleotide conjugates. By using this cutter, the sense strand of a blue fluorescent protein (BFP) gene was selectively cut at a predetermined site in the chromophore‐coding region. The upstream fragment obtained by the site‐selective scission was ligated with the downstream fragment of the closely related green fluorescent protein (GFP) gene so that the 5′‐ and 3′‐end portions of the chromophore came from the BFP fragment and the GFP fragment, respectively. The recombinant gene was successfully expressed in E. coli and the chimeric chromophore emitted green fluorescence as expected.     

Desorption Dynamics,Internal Energies,and Imaging of Organic Molecules from Surfaces with Laser Desorption and Vacuum Ultraviolet (VUV) Photoionization

There is enormous interest in visualizing the chemical composition of organic material that comprises our world. A convenient method to obtain molecular information with high spatial resolution is imaging mass spectrometry. However, the internal energy deposited within molecules upon transfer to the gas phase from a surface can lead to increased fragmentation and to complications in analysis of mass spectra. Here it is shown that in laser desorption with postionization by tunable vacuum ultraviolet (VUV) radiation, the internal energy gained during laser desorption leads to minimal fragmentation of DNA bases. The internal temperature of laser‐desorbed triacontane molecules approaches 670 K, whereas the internal temperature of thymine is 800 K. A synchrotron‐based VUV postionization technique for determining translational temperatures reveals that biomolecules have translational temperatures in the range of 216–346 K. The observed low translational temperatures as well as their decrease with increased desorption laser power is explained by collisional cooling. An example of imaging mass spectrometry on an organic polymer by using laser‐desorption VUV postionization shows 5 μm feature details while using a 30 μm laser spot size and 7 ns pulse duration. Applications of laser‐desorption postionization to the analysis of cellulose, lignin, and humic acids are briefly discussed.     

Ultrasonic devulcanization of waste rubbers: Experimentation and modeling

Continuous ultrasonic devulcanization of ground tire rubber (GRT) and styrene-butadiene rubber (SBR) is considered. Experiments are performed under various processing conditions. Two recipes of SBR with different amounts of polysulfidic linkages are utilized. Gel fraction and crosslink density of devulcanized rubbers are measured and a unique relationship between them is established. Die characteristics with and without imposition of ultrasonic waves are determined. Devulcanized samples are revulcanized and mechanical properties are measured. In some cases, properties of revulcanized SBR samples exceeded those of virgin vulcanizates. This is explained based on the presence of a double network in the revulcanized rubber. A modification of acoustic cavitation and flow modeling of ultrasonic devulcanization of SBR and GRT is proposed using a concept of effective viscosity characterizing the flow of vulcanized particles before devulcanization combined with a shear rate, temperature and gel fraction-dependent viscosity of devulcanized rubber. Velocity, shear rate, pressure, and temperature field along with gel fraction, crosslink density and number of bonds broken are simulated. Predicted data on gel fraction, crosslink density, and pressure using the present modification of the model are found to be closer to experimental data then previously reported.Dedicated to the memory of Professor Tasos C. Papanastasiou     

Induced drag of a straight wing in a wind tunnel of circular cross section

Summary The Induced drag of a wing with optimum lift distributions is studied, which lies on a diameter of a closed wind tunnel of circular cross section. Exact expressions of the spanwise lift distribution and the induced drag are obtained from the Trefftz plane flow field. When the ratio b/d of wing span to tunnel diameter increases, the lift distribution changes gradually from elliptic to uniform ones, and the induced drag also gradually decreases until it becomes zero. An alternative solution based on the method of matched asymptotic expansions confirms the exact theory to be valid. Throughout this paper the flow is assumed inviscid and incompressible, whereas the viscous effect are briefly descirbed.

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Übersicht Es wird der induzierte Widerstand eines Tragflügels mit optimaler Auftriebsverteilung untersucht. Der Tragflügel Hegt diametral in einem geschlossenen Windkanal mit Kreisquerschnitt. Aus einer Betrachtung des Strömungsfeldes in der Trefftz-Ebene werden exakte Ausdrücke für die Auftriebs- und Widerstandsverteilung abgeleitet. Wenn das Verhältnis b/d von Flügelspannweite zum Kanaldurchmesser vergrößert wird, dann ändert sich die Auftriebsverteilung allmählich von einer elliptischen zu einer gleichförmigen, und der induzierte Widerstand nimmt bis auf Null ab. Eine alternativ gewonnene Lösung mit Hilfe von gemischten asymptotischen Reihenentwicklungen bestätigt die Richtigkeit der Theorie. Bei den Untersuchungen wird die Strömung als reibungsfrei und inkompressibel vorausgesetzt, jedoch werden Zähigkeitseffekte kurz beschrieben.

     

A steam-soak production model for thick gravity drainage petroleum reservoirs

Summary This paper deals with the production performance for the steam-soak process as applied to a reservoir where oil is produced by gravity drainage. An improved analytical temperature profile which is simple, less time consuming and comparable to the exact solution is used to compute the viscosity variation inside the reservoir. Based on this temperature profile the continuity equation is solved by finite difference technique to find the oil height as a function of radius and time which in turn gives the flow rate and cumulative oil production. Comparison of the developed model with available field data shows that the model gives better prediction of steam-soak oil recovery than other comparable methods.

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Ein Modell zur Ausbeute von Rohölfeldern durch Schwerkraftsickerung bei zyklischer Dampfinjektion

Übersicht Behandelt wird das Förderverhalten eines Rohölspeichers bei Dampfinjektion und Schwerkraftsickerströmung. Zur Berechnung der Viskositätsverteilung im Reservoir wird ein vereinfachtes Temperaturfeld, welches das wahre annähert, benutzt. Mit dieser Temperaturverteilung wird die Kontinuitätsgleichung nach der Finite-Differenzen-Methode gelöst und die Ölstandshöhe als Funktion von Radius und Zeit bestimmt. Daraus ergibt sich der Volumenstrom und die kumulierte Fördermenge. Ein Vergleich des hier entwickelten Modells mit Daten eines Ölfeldes zeigt, daß es bessere Vorhersagen liefert als vergleichbare Methoden.

     

Berechnungsverfahren für stochastische Fahrzeugschwingungen

Übersicht Fahrzeuge auf unebenen Fahrwegen führen Zufallsschwingungen aus, durch die Fahrkomfort und Fahrsicherheit beeinträchtigt werden können. Die Analyse solcher Fahrweg-Fahrzeug-Mensch-Systeme bedarf einer umfassenden Untersuchung der zufälligen Fahrwegunebenheiten, der nichtlinearen Fahrzeugdynamik und der menschlichen Schwingungswahrnehmung. Hierfür werden sowohl im Frequenz- als auch im Zeitbereich mathematische Modelle für diese drei Teilsysteme angegeben. Bei mehrachsigen Fahrzeugen wird dabei die Erregung in mehreren Kontaktpunkten berücksichtigt. Eventuell auftretende Nichtlinearitäten werden mit Hilfe der Statistischen Linearisierung behandelt. Die Berechnung der interessierenden stochastischen Kennwerte, d. h. der Varianzen der dynamischen Radlasten, der Fahrzeugaufbaubeschleunigungen und der menschlichen Schwingungswahrnehmung, geschieht mit der Spektraldichtemethode im Frequenzbereich oder der Kovarianzmethode im Zeitbereich. Die Gegenüberstellung der beiden Methoden ergibt Vorteile der Kovarianzmethode sowohl bei linearen als auch insbesondere bei nichtlinearen Problemen. Die dargestellten Rechenverfahren für stochastische Fahrzeugschwingungen werden anhand eines nichtlinearen Automobilmodells beispielhaft veranschaulicht.

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Summary The dynamic analysis of random vehicle vibrations and the consequences especially to human comfort requires an integrated investigation of guideway roughness, nonlinear vehicle dynamics and human response to vibration exposure. Therefore, the complete system is characterized in the frequency as well as in the time domain. Multi-axle excitation inputs are included in the vehicle dynamics. Nonlinearities are considered by statistical linearization techniques. Then, the variances of wheel load variation, car body acceleration and human perception are determined by spectral analysis in frequency domain as well as by covariance analysis in time domain. Comparing both methods the covariance analysis is favourable for linear and, particularly, for nonlinear problems. Its efficiency is illustrated by the random vibration analysis of a nonlinear automobile model.

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Herrn Professor Dr. E. Mettler zum 70. Gebrutstag gewidmet