Superhydrophobic surfaces have attracted much attention lately as they present the possibility of achieving a substantial skin-friction drag reduction in turbulent flows. In this paper, the effects of a superhydrophobic surface, consisting of microgrates aligned in the flow direction, on skin-friction drag in turbulent flows were investigated through direct numerical simulation of turbulent channel flows. The superhydrophobic surface was modeled through a shear-free boundary condition on the air-water interface. Dependence of the effective slip length and resulting skin-friction drag on Reynolds number and surface geometry was examined. In laminar flows, the effective slip length depended on surface geometry only, independent of Reynolds number, consistent with an existing analysis. In turbulent flows, the effective slip length was a function of Reynolds number, indicating its dependence on flow conditions near the surface. The resulting drag reduction was much larger in turbulent flows than in laminar flows, and near-wall turbulence structures were significantly modified, suggesting that indirect effects resulting from modified turbulence structures played a more significant role in reducing drag in turbulent flows than the direct effect of the slip, which led to a modest drag reduction in laminar flows. It was found that the drag reduction in turbulent flows was well correlated with the effective slip length normalized by viscous wall units. C 2013 AIP Publishing LLC. [http://dx.

Gadolinium-labeled magnetite nanoparticles (GMNPs) were synthesized via a bioinspired manner to use as dual contrast agents for T1- and T2-weighted magnetic resonance imaging. A mussel-derived adhesive moiety, 3,4-dihydroxy-l-phenylalanine (DOPA), was utilized as a robust anchor to form a mixed layer of poly(ethylene glycol) (PEG) chains and dopamine molecules on the surface of iron oxide nanoparticles. Gadolinium ions were subsequently complexed at the distal end of the dopamine molecules that were prefunctionalized with a chelating ligand for gadolinium. The resultant GMNPs exhibited high dispersion stability in aqueous solution. Crystal structure and superparamagnetic properties of magnetite nanocrystals were also maintained after the complexation of gadolinium. The potential of GMNPs as dual contrast agents for T1 and T2-weighted magnetic resonance imaging was demonstrated by conducting in vitro and in vivo imaging and relaxivity measurements.

No abstract

We show that the dipole-dipole interaction between two Rydberg atoms can give rise to long range molecules. The binding potential arises from two states that converge to different separated atom asymptotes. These states interact weakly at large distances, but start to repel each other strongly as the van der Waals interaction turns into a resonant dipole-dipole interaction with decreasing separation between the atoms. This mechanism leads to the formation of an attractive well for one of the potentials. If the two separated atom asymptotes come from the small Stark splitting of an atomic Rydberg level, which lifts the Zeeman degeneracy, the depth of the well and the location of its minimum are controlled by the external electric field. We discuss two different geometries that result in a localized and a donut shaped potential, respectively.

Water was directly injected into the cylinder with an injection pressure of 5 MPa to investigate its effect on engine performance and emissions in a gasoline engine. The test engine was a 1.6-L naturally aspirated prototype engine consisted of water direct injection and port fuel injection systems. The engine featured a compression ratio of 13.5. Commercial gasoline direct injection injectors were used to inject the water. The water was injected at a fixed timing of 2120 crank angle degrees after top dead center. The addition of water showed potential to mitigate the knock occurrence at partload condition where the knock initially started to occur due to the high compression ratio. It allowed a further advance of spark timing; thus, the brake-specific fuel consumption was improved. The effects of water injection were further investigated under full-load condition within the engine speed range of 1500-3000 r/min. The water effectively reduced the in-cylinder temperature and the exhaust gas temperature; therefore, charge cooling through over-fueling (fuel enrichment) was eliminated with reduced brake-specific fuel consumption. Increase in the injected water mass resulted in further spark advance without the knock occurrence and provided room for further brake-specific fuel consumption reduction. An optimum water mass existed because too much water deteriorated the combustion efficiency, burn duration, and cycle efficiency. The positive effects of water injection were dulled with increased engine speed because the knocking resistance was already high intrinsically with the higher engine speed.

High-energy phase-stable sub-cycle mid-infrared pulses can provide unique opportunities to explore phase-sensitive strong-field light–matter interactions in atoms, molecules and solids. At the mid-infrared wavelength, the Keldysh parameter could be much smaller than unity even at relatively modest laser intensities, enabling the study of the strong-field sub-cycle electron dynamics in solids without damage. Here we report a high-energy sub-cycle pulse synthesiser based on a mid-infrared optical parametric amplifier and its application to high-harmonic generation in solids. The signal and idler combined spectrum spans from 2.5 to 9.0 µm. We coherently synthesise the passively carrier-envelope phase-stable signal and idler pulses to generate 33 μJ, 0.88-cycle, multi-gigawatt pulses centred at ~4.2 μm, which is further energy scalable. The mid-infrared sub-cycle pulse is used for driving high-harmonic generation in thin silicon samples, producing harmonics up to ~19th order with a continuous spectral coverage due to the isolated emission by the sub-cycle driver.

Currently available methods to stably disperse iron oxide nanoparticles (IONPs) in aqueous solution need to be improved due to potential aggregation, reduction of superparamagnetism, and the use of toxic reagents. Herein, we present a facile strategy for aqueous transfer and dispersion of organic-synthesized IONPs using only polyethylene glycol (PEG), a biocompatible polymer. A library of PEG derivatives was screened, and it was determined that amine-functionalized six-armed PEG, 6(PEG-NH2), was the most effective dispersion agent. The 6(PEG-NH2)-modified IONPs (IONP-6PEG) were stable after extensive washing, exhibited high superparamagnetism, and could be used as a platform material for secondary surface functionalization with bioactive polymers. IONP-6PEG biofunctionalized with hyaluronic acid (IONP-6PEG-HA) was shown to specifically label mesenchymal stem cells and demonstrate MR contrast potential with high r2 relaxivity (442.7 s−1mM−1) compared to the commercially available Feridex (182.1 s−1mM−1).

The dipole-dipole broadening of ns − np microwave transitions of cold Rb Rydberg atoms in a magneto-optical trap has been recorded for 28 n 51. Since the electric dipole transition matrix elements scale as n 2 , a broadening rate scaling as n 4 is expected and a broadening rate of 8.2 × 10 −15 n 4 MHz cm 3 is observed. The observed broadening is smaller than expected from a classical picture due to the spin-orbit interaction in the np atoms. The broadened resonances are asymmetric and cusp shaped, and their line shapes can be reproduced by a diatomic model which takes into account the dipole-dipole interaction, including the spin-orbit interaction, the strengths of the allowed microwave transitions, and the distribution of the atomic spacings in the trap.