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.

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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.