Capped semiconductor colloids: Synthesis and photochemistry of CdS capped SnO2 nanocrystallites

Cds-capped SnO₂ (CdS@SnO₂) and SiO₂ (CdS@SiO₂) colloids of 50–80 ? in diameter have been prepared in aqueous medium. Significant quenching of CdS emission is observed in the CdS@SnO₂ system as the electrons are injected from the excited CdS shell into the SnO₂ core. Photoinduced charge separation in this system also results in transient bleaching in the 450 nm region. Picosecond laser flash photolysis studies of composite semiconductor nanoclusters that highlight the effect of the metal oxide core on the photophysical properties of the outer CdS shell are described. Contribution No. 3961 from the Notre Dame Radiation Laboratory, USA     

The effect of hydrodynamic coupling on the axisymmetric drainage of thin films

The effect of hydrodynamic coupling of adjacent phases on the axisymmetric drainage of thin films is examined using a prototype model of coalescence. For long times, pressure forces in the film dominate flow in all three regions, and finally all move effectively as one, whereas for short times, profiles are sharp and initial flow differences in the three regions can dominate pressure effects. For intermediate times, temporal evolution of velocity profiles depends in a complicated way on the kinematic viscosity ratio and the parameter $\text{R = (?}{}_{\mathrm{A}}\text{μ}{}_{\mathrm{A}}\text{/?}{}_{\mathrm{B}}\text{/μ}{}_{\mathrm{B}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, as well as on initial conditions and pressure gradient. Generally speaking, the initial flows have less of an effect on overall drainage time than the presence of induced circulation in adjacent phases. Analytical solutions are plotted for a range of systems and representative initial conditions and pressure gradients. In a subsequent article, film-thinning equations are solved using this information.     

UV spectrophotometric determination of theobromine and caffeine in cocoa beans

A rapid, simple and accurate method for individually determining theobromine and caffeine in cocoa beans is presented. Caffeine alone is completely extracted into chloroform from an aqueous solution at a pH between 12.5 and 12.7, and can be determined by UV spectrophotometry at 275.9 nm. For the theobromine remaining in the aqueous solution, a wavelength of 272.7 nm is used. The results were reproducible with a relative standard deviation of about 0.65%.     

Correlated Rayleigh Scattering Spectroscopy and Scanning Electron Microscopy Studies of Au-Ag Bimetallic Nanoboxes and Nanocages

The optical properties of hollow nanoparticles (Au-Ag nanoboxes and nanocages) were investigated by recording Rayleigh scattering spectra of single particles, whose morphology and composition had been analyzed by scanning electron microscopy (SEM). This was achieved by depositing the particles on optically transparent substrates with registration marks, which are compatible with SEM imaging. Fitting the experimental spectra to a Lorentzian function yields the frequencies and homogeneous line widths of the plasmon resonance for the particles. The resonances are extremely broad, with dephasing times of 2-5 fs. Analysis of the line width data using the dimensions determined by SEM shows that the broadening is due to a combination of electron-surface scattering and radiation damping. The sensitivity of the plasmon resonance to the dielectric constant of the environment was also investigated by adding a drop of water to the substrate. The nanoboxes show similar dielectric sensitivities compared to other metal nanoparticle systems. A significant increase in the line width was also observed for the nanoboxes in water compared with air. This was attributed to increased radiation damping in the environment with a higher dielectric constant. Both the red shift and the increase in line width are reversible.     

The determination of interfacial tension by differential capillary rise

A method for directly determining interfacial tension without iteration from the difference in height between two capillaries is presented. An experimental technique is described for organic liquids both lighter and heavier than water, in which the menisci recede over surfaces wetted by the aqueous phase, thus providing the most favourable conditions for zero contact angle. The values determined experimentally using tables prepared from the known shapes of sessile interfaces agree well with accepted values of the interfacial tension.     

Effect of contact angle on capillary rise in annular meniscus

The shapes of axisymmetric annular menisci are calculated for a range of contact angles at the constraining cylindrical walls. From these shapes the capillary rise (or depression) Δh relative to an infinite external meniscus is obtained in terms of the radii of the inner and outer cylinders and physical properties of the liquids. This value of Δh may then be used to correct the measured rise in a concentric capillary tube relative to the annular meniscus. The capillary rise, Δh, has been experimentally measured for several liquids and their surface tensions directly calculated from these values. Excellent agreement with accepted values is obtained, even though the measured capillary rise is very small, thus confirming the accuracy of the theoretical calculations and experimental technique.     

Der Effekt hydrodynamischer Wechselwirkung auf die Annäherung einer Kugel an eine fluid-flüssige Grenzfläche für willkürliche Viskositätsverhältnisse

Zusammenfassung Die achsensymmetrische Str?mung in einem dünnen Film zwischen einer fluid-flüssigen Grenzfl?che und einer festen Kugel, die sich dieser n?hert, wird unter Berücksichtigung der Str?mung in der an den Film angrenzenden fluiden Phase untersucht. Die Geschwindigkeitsverteilungen im Film und in der angrenzenden Phase sind Funktionen der Zeit, des Druckgradienten im Film, der anf?nglichen Str?mungen in den beiden Phasen, der Krümmung des Systems und der physikalischen Eigenschaften, repr?sentiert durch die Parameterη (Verh?ltnis der dynamischen Viskosit?ten) undR ( =η (ν _B /ν _A )^1/2). Aus der Geschwindigkeitsverteilung im Film wird über eine Massen- und eine Kr?ftebilanz eine Gleichung für die ?nderung der Filmdicke hergeleitet, welche unter anderem eine tempor?re Zunahme der Dicke des Films vorauszusagen vermag. Ferner konnten damit aus der Literatur entnommene experimentelle Kurven der Abnahme der Filmdicke gut reproduziert werden.

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Summary The axisymmetric flow in a thin film between a solid sphere approaching a fluid-liquid interface has been investigated taking into account the hydrodynamic coupling with the adjacent phase. The velocity profiles in the film and the contiguous phase are functions of the time elapsed since drainage began, of the initial velocities in the two phases, of the pressure gradient in the film, and of the physical properties of the fluids as embodied in the ratioη of the dynamic viscosities ( =μ _A/μ _B) and in the hydrodynamic coupling parameter R ( =η(ν _B/ν _A)^1/2). A highly nonlinear integro-differential equation for the evolution of the film thickness with time as a function of the remaining parameters can be derived from the underlying film drainage profile, the macroscopic continuity equation, and the macroscopic force balance on the sphere. This equation has been solved numerically for a variety of circumstances and predicts, among other things, the experimentally often observed temporary thickening of draining films. Experimental film-thinning curves taken from the literature could be accurately predicted using this eauation.

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Mit 8 Abbildungen     

More Convenient and Suitable Methods for Sphere Tensiometry

Two new convenient and suitable gravimetric methods of measuring surface and interfacial tensions of liquids and contact angles of liquids on a spherical surface by pulling a sphere through the interface were examined. First, the accuracy of numerical integration was increased. Second, two methods which treat the experimental data were presented and compared. Both methods can be used for determination of the contact angle and interfacial tension at the same time.     

Gold nanostructures: engineering their plasmonic properties for biomedical applications

The surface plasmon resonance peaks of gold nanostructures can be tuned from the visible to the near infrared region by controlling the shape and structure (solid vs. hollow). In this tutorial review we highlight this concept by comparing four typical examples: nanospheres, nanorods, nanoshells, and nanocages. A combination of this optical tunability with the inertness of gold makes gold nanostructures well suited for various biomedical applications.