Physics and Physicists: Self-Assembling Nanoparticles

Physics and Physicists: Self-Assembling Nanoparticles

Physics and Physicists: "I Want To Ride My Bicycle, I Want To Ride My Bike...."

Physics and Physicists: "I Want To Ride My Bicycle, I Want To Ride My Bike...."

Physics and Physicists: "How many dominoes does one need to topple a domino as tall as the Domtoren?"

Physics and Physicists: "How many dominoes does one need to topple a domino as tall as the Domtoren?"

Physics and Physicists: Measuring The Speed Of Light - Using Chocolates!

Physics and Physicists: Measuring The Speed Of Light - Using Chocolates!

Physics and Physicists: The Physics Of Ponytails

Physics and Physicists: The Physics Of Ponytails

The physics of floating pyramids

An unexpected result is reported here.

Results just in from an experiment that levitated open-bottomed paper pyramids on gusts of air reveal a curious phenomenon: When it comes to drifting through the air, top-heavy designs are more stable than bottom-heavy ones. The finding may lead to robots that fly not like insects or birds but like jellyfish.
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The researchers placed hollow paper pyramids inside the cylinder. The objects were about 1 to 5 centimeters high and were made of tissue paper or letter paper on carbon fiber supports, like tiny homemade kites. Physicist Bin Liu led the experiments, attaching a beadlike weight to a post running down the center of the pyramid and changing the height of the bead to give the object a different center of mass. Common sense says that the pyramid should be most stable when the bead is at the bottom of the post, like ballast in the hold of a ship. But when the team released the pyramids over the subwoofer, the opposite was true: the bottom-heavy pyramids were likely to flip over and fall, whereas the top-heavy ones remained upright and continued to hover (see first video), the group reports in an upcoming issue of Physical Review Letters.
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The team suspected that the effect was due to swirls of air that develop along the pyramid's sides. To see the swirls in action, Zhang's group examined a two-dimensional version of the pyramid experiment in water. They placed upside-down V shapes into a pan of water and rocked it to create currents. As the water ran past the V, it created tiny whirlpools at the ends of the V's two legs (see second video). These swirls pushed away from the upside-down V, moving downward, which exerted an upward force on the V-the same mechanism that creates lift in the pyramids.

If the V was tilted, however, the swirls went in different directions: Those on the higher leg shoved it sideways, while the lower leg got a weaker upward push. This would straighten the upside-down V. Team member Leif Ristroph showed that the same sorts of swirls roll off the sides of the pyramids: They push the pyramid upright as long as the center of mass is above the tilted-up side, much in the same way that you can balance a vertical stick on the end of your finger by moving the bottom of the stick in the direction of the tilt, Zhang says. For bottom-heavy pyramids, this same mechanism causes them to flip over-it's like moving the top of the stick in the direction of the tilt, encouraging it to fall.

Paring with spin fluctuations

A review of an interesting work observing the paring mechanism of an exotic superconductor.

Hattori et al. are able to correlate this field-angle-dependence of the magnetic fluctuations with another striking property of UCoGe, which is that its superconductivity is exceptionally sensitive to the direction of an applied magnetic field. When the magnetic field is perpendicular to the c axis the superconductivity is very robust, surviving to around 10 tesla; however, as the field direction is rotated towards the c axis, the critical field for destruction of superconductivity falls precipitously. An obvious interpretation of this behavior would be that the component of the applied field that is parallel to the c axis induces a large magnetic polarization, and the large internal field thus generated disrupts the paired electrons either through coupling to their spins or their orbital motion. This sort of physics is very well understood (indeed this is why ordinary superconductors don’t like magnetic fields) so it can be modeled quite accurately and, surprisingly, it doesn’t fit the measurements in UCoGe. Rather, Hattori et al. argue that their results are better explained if the magnetic field is disrupting not the pairs directly, but rather the underlying pairing mechanism. This, in particular, explains the striking parallel in the suppression of the magnetic fluctuations and the suppression of the superconductivity as the magnetic field is rotated towards the c axis. It is strong evidence that magnetic fluctuations are the ones doing the pairing.