Novel materials: Sound waves traveling backwards
by Staff Writers
Karlsruhe, Germany (SPX) Jun 11, 2021
Acoustic waves in gases, liquids, and solids usually travel at an almost constant speed of sound. So-called rotons are an exception: their speed of sound changes significantly with the wavelength, and it is also possible that the waves travel backwards. Researchers at Karlsruhe Institute of Technology (KIT) are studying the possibilities of using rotons in artificial materials.
These computer-designed metamaterials, produced by ultra-precise 3D laser printing, might be used in the future to manipulate or direct sound in ways that have never been possible before. A report on the researchers' work has been published in Nature Communications. (DOI: 10.1038/s41467-021-23574-2)
Rotons are quasiparticles, which means that they behave similarly to free particles. Unlike ordinary acoustic waves in gases, liquids, and solids, the speed of sound changes significantly with the wavelength. In addition, certain frequencies generate three different partial waves.
"The slowest among them is a backward wave: the energy flow and the wavefronts run in exactly opposite directions," explains Professor Martin Wegener from the Institute of Applied Physics (APH) and KIT's Institute of Nanotechnology (INT). Understanding and benefiting from quasiparticles such as rotons is one of the great challenges of quantum physics.
Physicist Lev Landau, who won a Nobel Prize in 1962 for his groundbreaking work, predicted their existence in the context of superfluidity, a condition in which a fluid loses its internal friction and becomes thermally conductive in a nearly ideal way. Until now, rotons could only be observed under special quantum-physical conditions at very low temperatures - and were therefore not suitable to technical applications.
Rotons without Any Quantum Effects
Thus, it might be possible in the future to better manipulate sound waves in air or in materials, for example, to bounce them back, redirect them, or create echoes. These materials have not been demonstrated experimentally yet; however, it should be possible to produce them by using technologies such as ultra-precise 3D laser printing. "We have even made some of these metamaterials in the meantime," Professor Martin Wegener says. "Currently, we are working intensively on the direct experimental proof for the existence of rotons."
3D printing - the Gateway from the Digital to the Physical World
"In general, our dream is to design materials on the computer and then turn them directly into reality - without years of trial and error. So 3D printing is just an automated converter, as it were, from the digital to the physical world," Professor Martin Wegener explains.(or)
Research Report: "Roton-like acoustical dispersion relations in 3D metamaterials"
Quark-gluon plasma flows like water, according to new study
London, UK (SPX) May 28, 2021
What does quark-gluon plasma - the hot soup of elementary particles formed a few microseconds after the Big Bang - have in common with tap water? Scientists say it's the way it flows. A new study, published in the journal SciPost Physics, has highlighted the surprising similarities between quark-gluon plasma, the first matter thought to have filled the early Universe, and water that comes from our tap. The ratio between the viscosity of a fluid, the measure of how runny it is, and its densit ... read more
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