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Skydiving birds, holograms that offer a 360° view, and more in our monthly wrap-up of papers from physics literature.
Some species of seabirds are known for their unique hunting skills — diving headfirst at speeds over 20 m/s, they then pack a punch when hitting the waters below. Sporting long, slender necks, it was a mystery how these birds could avoid injury during this endeavor. As reported in the Proceedings of the National Academy of Science, researchers have now identified the dynamics behind a seabird’s plunge-dive. Using simulations with a cone attached to a rod, along with a salvaged seabird carcass, Change et al. studied the mechanisms of the fast dive and identified the axial forces affecting the skull and neck during the dive. These turned out to be primarily hydrodynamic drag on the skull during water impact, and hydrostatic pressure on the neck, caused by an air pocket created after the cone-shaped skull became fully submerged. This phase, which researchers called the "air cavity phase," showed that the bird’s neck would be less affected by hydrostatic pressure as the size of the skull and the length of the neck decreased, and the impact speed increased. In theory, the added stability from the bird’s neck muscles should reduce the likelihood that the neck would buckle under increased drag during the dive. According to their analysis, the team determined that these plunge-divers could keep up the daredevil acts without sustaining injury, provided their speeds were less than 80 m/s.
Some seabirds hunt for food by plunging into the water at high speed, yet they avoid neck injury.
A newly derived universal law shows that the rotation of disk galaxies is determined entirely by the visible matter they contain, even if they are mostly filled with dark matter. As reported in Physical Review Letters, McGaugh et al. have investigated 153 disk galaxies spanning four orders of magnitude in mass and three orders in density. Using available observations, the team determined the rotational acceleration as a function of radius, as well as the radial distribution of visible matter, for each galaxy. They found that the two distributions display a remarkably simple relation that holds for all observed galaxies. Since dark matter is the main component of these galaxies — and thus the main determinant of galactic rotation — the result implies that the distribution of conventional matter in the disk specifies the density profile of the surrounding dark matter halo. While it was expected that more massive dark matter halos host more massive galaxy disks, the tight correlation between the radial distribution of dark and visible matter is surprising. Simulation work on galaxy formation will need to assess if this result is compatible with the standard cosmological model or requires its substantial revision.
(For more, see the Viewpoint in Physics "Connecting the Bright and Dark Sides of Galaxies" by Arthur Kosowsky)
Researchers have grown high-quality graphene sheets on the surface of a diamond crystal, offering a robust new method for engineering graphene devices. Graphene is a one-atom-thick layer of carbon with amazing properties, such as high electron mobility, that make it potentially useful for touch-screens, solar cells, and many other electronic applications. However, one challenge has been finding a suitable support material. Simply placing graphene on top of an insulator, like silicon dioxide, can degrade its properties, whereas growing it directly on a substrate, such as silicon carbide, can introduce defects. As reported in Applied Physics Letters, Gu et al. doped a single crystal of diamond with boron atoms. They were guided by their own first-principles calculations, which showed that carefully controlled boron doping can deform the crystal lattice such that the diamond surface forms a hexagonal structure — ideal for graphene growth. Using chemical vapor deposition, the team produced single-layer, as well as multilayer, graphene on their doped diamond. Spectral analysis revealed no sign of defects in the graphene, and electronic measurements confirmed that the sheets featured high electron mobility.
Photo: Appl. Phys. Lett.
High-quality graphene has been grown on diamond substrates.
Star Wars fans rejoice! Tabletop holograms like those of the Jedi knights in faraway galaxies are starting to approach reality now that researchers have created a 360-degree holographic viewing experience. Lim et al. published their results in Optics Express describing an optical system consisting of four high-speed digital micromirror displays that formed a holographic image. This image is sent through a rotating mirror system that sweeps it horizontally so that viewers can see it from any position while walking around the device. The resulting hologram was just over an inch in size, so the team used additional optics to magnify it, almost tripling its size. Since this magnification system was composed of two confocal parabolic mirrors, the optical rays exiting such mirrors would be distorted at the angle of the observer. To get around this, the team inserted an aspheric lens between the mirrors, which compensated for the distortion. The next challenges to undertake are reducing the system to a practical size and adapting it to full-color operation.
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