How do grains flow out of an emptying silo? And what about sugar poured out by a pastry chef? Like liquids (2), grains can flow, but there is no attraction between the grains to trigger cohesion. However, by studying the waves that form and propagate on the surface of flowing sand, the physicists have observed telltale signs of cohesion.
Like the very small ripples that form on the surface of water, these waves point to the existence of a “taut elastic skin” on the surface of volumes of grain.This “skin” on flowing grain is its surface tension.
By measuring wave propagation speed, the researchers have shown that this cohesion effect is a result of a decrease in air pressure between flowing grains. Therefore, when a mass of grains flows, there is a depressed area at the middle of the flow, which pulls straying grains back towards the mass. These results should improve our understanding of the details of what happens in grain flows –materials which are common, but not yet well understood.
(1) CPMOH(2) The surface of a liquid is similar to an elastic membrane under
The taming of the light screw
22.03.2019 | Max-Planck-Institut für Struktur und Dynamik der Materie
21.03.2019 | Max-Planck-Institut für Polymerforschung
DESY and MPSD scientists create high-order harmonics from solids with controlled polarization states, taking advantage of both crystal symmetry and attosecond electronic dynamics. The newly demonstrated technique might find intriguing applications in petahertz electronics and for spectroscopic studies of novel quantum materials.
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The Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT) in Arizona released its first image of the surface magnetic field of another star. In a paper in the European journal Astronomy & Astrophysics, the PEPSI team presents a Zeeman- Doppler-Image of the surface of the magnetically active star II Pegasi.
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Researchers at Chalmers University of Technology and the University of Gothenburg, Sweden, have proposed a way to create a completely new source of radiation. Ultra-intense light pulses consist of the motion of a single wave and can be described as a tsunami of light. The strong wave can be used to study interactions between matter and light in a unique way. Their research is now published in the scientific journal Physical Review Letters.
"This source of radiation lets us look at reality through a new angle - it is like twisting a mirror and discovering something completely different," says...
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