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Ultrafast science

Ultrafast science

Flash Physics: 3D stars shrink when hot, new laser sensor for gases, tracking solar waves

25 Oct 2016 Tushna Commissariat

Flash Physics is our daily pick of the latest need-to-know developments from the global physics community selected by Physics World's team of editors and reporters

Shrinking stars: the 3D-printed star structures

3D star structure shrinks when heated

While most solids expand when heated, some defy this thermodynamic trend and shrink instead. Now, an international team of researchers has combined two heat-expanding materials to fabricate a heat-contracting composite metamaterial. The team, led by Nicholas Fang at the Massachusetts Institute of Technology in the US, has created tiny, star-shaped structures out of interconnected beams. The sugar-cube-sized structures quickly contract in all three dimensions when heated to about 282 °C – previous such structures only shrunk in two dimensions. While each of the beams in the stars are made of materials that typically expand when heated, the team realized that when the beams were arranged in certain architectures, they pull inward when heated, causing the whole structure to effectively shrink and collapse much like a Hoberman sphere. The team also found that it could control the amount of contraction by changing the amount of copper nanoparticles added to one of the two materials used to build the structures. Such materials could have a wide variety of applications – they could be used in computer chips (which deform over time due to heat). They could also be used together with conventional materials to build objects that are subject to varying temperatures, including window frames, water pipes and space technologies. The research is published in Physical Review Letters.

New sensor is both laser and detector

A microscopic sensor that can be used to identify different gases simultaneously has been developed by researchers at the University of Vienna, Austria. The team uses quantum-cascade lasers, which emit light in the infrared range, to study gas samples. “Our quantum cascade lasers are circular, with a diameter of less than half a millimetre,” says team-member Gottfried Strasser, head of the University’s Center for Micro- and Nanostructures. “Their geometric properties help to ensure that the laser only emits light at a very specific wavelength.” This is particularly useful in carrying out chemical analyses of various gases, each of which only absorb very specific amounts of infrared light. Indeed, gases can be reliably detected using their own individual infrared “fingerprint”. Doing so requires a laser with the correct wavelength and a detector that measures the amount of infrared radiation swallowed up by the gas. The researchers’ device is both a laser and detector – they use two concentric quantum-cascade rings, which can both emit and detect light at varying wavelengths. One ring emits the laser light, which passes through the gas before being reflected by a mirror. The second ring receives and then measures the intensity of the reflected light. The two rings then immediately switch their roles, allowing the next measurement to be carried out. The sensor could have many applications in everything from environmental observations to medicine. The research is published in ACS Photonics.

Tracking solar waves from sunspots

An international team of astronomers has tracked a particular kind of solar wave for the first time, as it swept upward from the Sun’s surface through its atmosphere. Plasma and other material courses through the Sun and its atmosphere, and understanding the movement of such charged gas reveals more about our star, including how it heats up its atmosphere, how it creates a steady flow of solar wind streaming outward in all directions, and the dynamics of the star’s magnetic fields. Tracking solar waves in particular helps researchers to study the solar atmosphere, and by imaging the flow of the material the researchers hope to determine how and why the Sun’s upper atmosphere or corona is so hot. “We see certain kinds of solar seismic waves channelling upwards into the lower atmosphere, called the chromosphere, and from there, into the corona,” says lead-author Junwei Zhao at Stanford University in the US. “This research gives us a new viewpoint to look at waves that can contribute to the energy of the atmosphere.” The team made used of data and imagery from NASA’s Solar Dynamics Observatory, NASA’s Interface Region Imaging Spectrograph and the Big Bear Solar Observatory in Big Bear Lake, California. Together, these observatories watch the Sun in 16 wavelengths of light that show its surface and lower atmosphere. Although it has long been predicted that waves on the Sun’s surface (photosphere), are linked to those in its lower atmosphere (chromosphere), the new analysis is the first time that scientists have managed to actually watch the wave travel up through the various layers into the Sun’s atmosphere. The research is published in Astrophysical Journal Letters.

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