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Topics - saikat07

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31
EEE / A new fabric becomes more breathable as you work up a sweat
« on: February 28, 2020, 09:16:22 PM »
Magnetism transforms a weird new material from soft to rigid in a split second.

This metamaterial — a synthetic structure designed to behave in ways that natural materials don’t — comprises a gridlike network of plastic tubes filled with fluid that becomes more viscous in a magnetic field, causing the tubes to firm up. The material could help make more adaptable robots or body armor, researchers report online December 7 in Science Advances.

Christopher Spadaccini, a materials engineer at Lawrence Livermore National Laboratory in California, and colleagues 3-D printed lattices composed of plastic struts 5 millimeters long and injected them with a mixture of tiny iron particles and oil. In the absence of a magnetic field, the iron microparticles remain scattered randomly throughout the oil, so the liquid is runny. But close to a magnet, these iron microparticles align into chains along the magnetic field lines, making the fluid viscous and the lattices stiffer.
A solid hunk of iron microparticle–filled material would be heavy and expensive to make. Building tubular structures that are mostly open space makes this tunable material more lightweight, says coauthor Julie Jackson, an engineer at Lawrence Livermore.
The researchers tested individual “unit cells” of the new material — hollow, die-shaped structures that can collectively form the larger lattices. Moving one unit cell from about eight centimeters to one centimeter away from a magnet increased its stiffness by about 62 percent.

In future technologies, this material could be paired with devices that use electricity to generate magnetic fields, called electromagnets. Material that becomes softer or stiffer on demand could be used to make next-generation sports pads or helmets with tunable impact absorption, Jackson says. Robots with changeable stiffness could squeeze into small spaces, but then be sturdy enough to carry or move other objects.

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EEE / How droplets of oil or water can glow vibrant colors
« on: February 28, 2020, 09:15:10 PM »
Oil and water may not mix, but the two have now revealed a new example of structural color, in which an object’s hue arises from its shape.

Studying droplets made of two layers of clear oil, researchers discovered that, depending on a viewer’s perspective, the tiny blobs glowed a variety of vibrant colors under white light. In a petri dish, same-sized droplets changed color as the dish was rotated (see video below). The same phenomenon, described in the Feb. 28 Nature, occurred with tiny water droplets that collected on the underside of a petri dish’s lid.

Materials chemist Lauren Zarzar of Penn State and colleagues found that the iridescent hues appear when light strikes a bowl-shaped boundary between two substances — in this case, the water-air barrier on the underside of the water droplets hanging off a flat surface, or a basin-shaped divide between the two layers of oil. Light that enters near a droplet’s edge bounces along this this concave surface multiple times before being reflected and exiting near the opposite edge.

Under a microscope, that reflected light creates an iridescent ring whose apparent color depends on the viewer’s perspective. That’s because light waves can take many different ricocheting routes through the droplet on their way from the light source to an observer. When waves of a specific wavelength — for instance, yellow light — line up, they reinforce each other and produce a bright color. But light rays of other wavelengths taking these same routes may get misaligned and wash each other out. Changing the viewing angle changes which pathways a viewer sees, and thus the color.
The researchers also found that changing a droplet’s diameter or curvature altered its apparent color even when viewed from the same angle (see image above). A petri dish filled with two differently curved kinds of oil droplets produced a two-color picture of a penguin (see image below).

The effect also worked with tiny, transparent polymer bumps on flat surfaces. Such research could help make color-changing materials that could be used in cosmetics, camouflage or other products. “You could also design surfaces where you have effectively two different images from different directions, or you see an image only from a very specific direction,” Zarzar says.

33
EEE / Bacteria can be coaxed into making the toughest kind of spider silk
« on: February 28, 2020, 09:14:38 PM »
Bacteria are helping to make engineered silk that rivals the strength and stretchiness of a spider’s stiff dragline silk, the type from which the arachnids dangle.

Pound for pound, dragline silk is stronger and tougher than steel. Engineers have tried for decades to create a synthetic mimic from genetically modified bacteria, yeast and even goat milk, but have always fallen short.

Part of the challenge is that the genetic information for dragline silk is a long string of repeating DNA. And those previously tested organisms’ cell machinery haphazardly alters or chops up such series.   
To overcome this issue, researchers precisely separated the repeating DNA into bits and inserted each repeating piece into an E. coli microbe. These smaller pieces were less likely to be further altered within the bacteria, and each microbe then followed the genetic instructions to produce a short strand of silk. The researchers added to the end of each strand a chemical tag that glued the individual fibers together.
The resulting material behaved like dragline silk. Its tensile strength, or resistance to being pulled apart, was measured at 1.03 gigapascals, about the same as for naturally produced dragline silk. The engineered silk’s toughness measured 114 megajoules per cubic meter, compared with around 100 megajoules for silk made by spiders. And the engineered silk strands could stretch 18 percent before breaking, the same as natural dragline silk.

“We can now use bacteria to produce something as good as nature,” says synthetic biologist Fuzhong Zhang of Washington University in St. Louis who presented the research April 2 at the American Chemical Society’s annual meeting in Orlando, Fla.

The new silk was developed in part with NASA funding for applications such as giving astronauts a means of creating tough materials while on Mars. But the substance could be used in designing stronger materials for medical or textile applications, Zhang says, such as impact-resistant fabrics or surgical sutures.

The team is working to scale up the process by engineering bacteria able to produce full silk strands rather than just segments.

34
EEE / A new graphene foam stays squishy at the coldest temperatures
« on: February 28, 2020, 09:14:04 PM »
A new graphene-based foam is the first material to remain soft and squishy even at deep cryogenic temperatures.

Most materials become stiff and brittle in extreme cold. But the new foam stays superelastic even when it’s subjected to the temperature of liquid helium: –269.15° Celsius. A material that remains pliable at such low temperatures could be used to build devices for use in space, researchers report online April 12 in Science Advances.

Inside this foam, oxygen atoms connect micrometer-sized patches of the superthin 2-D material graphene to create a meshlike structure (SN: 8/13/11, p. 26). The resulting material is flexible in deep cryogenic conditions because, even at such low temperatures, sheets of graphene are easily bendable and resistant to tearing, and the carbon-oxygen bonds that link these sheets together remain strong.
ongsheng Chen, a materials scientist at Nankai University in Tianjin, China, and colleagues compressed samples of the material repeatedly at different temperatures. At –269.15° C, the foam behaved just as it did at room temperature, bouncing back to almost full size even after being compressed to one-tenth its original thickness. The material kept this resilience even when heated to about 1000° C and flattened hundreds of times.

Chen’s team suspects that different superthin materials, like 2-D semiconductors (SN Online: 2/13/18) or 2-D inorganic compounds (SN Online: 9/21/18) may create foams that might boast other unique properties.

35
EEE / How seafood shells could help solve the plastic waste problem
« on: February 28, 2020, 09:13:32 PM »
Lobster bisque and shrimp cocktail make for scrumptious meals, but at a price. The food industry generates 6 million to 8 million metric tons of crab, shrimp and lobster shell waste every year. Depending on the country, those claws and legs largely get dumped back into the ocean or into landfills.

In many of those same landfills, plastic trash relentlessly accumulates. Humans have produced over 8 billion tons of plastic since mass production began in the 1950s. Only 10 percent of plastic packaging gets recycled successfully. Most of the rest sits in landfills for a very long time (a plastic bottle takes about 450 years to break down), or escapes into the environment, perhaps sickening seabirds that swallow tiny pieces or gathering in the Pacific Ocean’s floating garbage patch (SN Online: 3/22/18)
Some scientists think it’s possible to tackle the two problems at once. Crustaceans’ hardy shells contain chitin, a material that, along with its derivative chitosan, offers many of plastic’s desirable properties and takes only weeks or months to biodegrade, rather than centuries.

The challenge is getting enough pure chitin and chitosan from the shells to make bio-based “plastic” in cost-effective ways. “There’s no blueprint or operating manual for what we’re doing,” says John Keyes, CEO of Mari Signum, a start-up company based just outside of Richmond, Va., that is devising ways to make environmentally friendly chitin. But a flurry of advances in green chemistry is providing some guideposts.

36
EEE / 50 years ago, bulletproof armor was getting light enough to wear
« on: February 28, 2020, 09:12:49 PM »
Many boron carbide armor components have been replaced by Kevlar, which was developed around the same time (SN Online: 4/8/15). Made of woven synthetic polymer chains, Kevlar fast became essential wear for soldiers and law enforcement officers. More than eight times the tensile strength of steel, the textile distributes the energy of a bullet impact over a large area. Some modern body armor systems today weigh a tenth of their boron carbide counterparts. Scientists are testing engineered spider silk, another strong and flexible textile, for body armor (SN: 5/11/19, p. 24).

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hysicists are crushing it — hydrogen, that is. Squeezing the chemical element to extremely high pressure transforms it into a metal, a trio of researchers claims.

The purported metallic hydrogen appeared at a pressure more than 4 million times that of Earth’s atmosphere, the scientists report June 13 at arXiv.org. If confirmed, the achievement would fulfill a long-standing quest to produce the elusive metal, first predicted in the 1930s to exist.

The work has yet to undergo peer review. But it is “definitely a very substantial step ahead” of previous research, says physicist Alexander Goncharov of the Carnegie Institution for Science in Washington, D.C., who was not involved with the study. “I think it’s quite conclusive it is metallic.”

Other researchers are more skeptical. Physicist Eugene Gregoryanz of the University of Edinburgh says that he does not find the new experiment convincing, and notes that many previous claims of metallic hydrogen have been proven wrong in the end.



Some aspects of the result disagree with previously published measurements of hydrogen at high pressure, says physicist Isaac Silvera of Harvard University. In 2017, he and his colleagues reported spotting hydrogen turning into a metal, a claim that was criticized by other scientists, including Gregoryanz (SN: 2/18/17, p. 14).

Physicist Paul Loubeyre of the French Alternative Energies and Atomic Energy Commission in Arpajon, a coauthor of the new study, did not respond to requests for comment for this article.

Metallic hydrogen is particularly enticing to physicists because of predictions that it may be a superconductor, a material that allows electricity to flow without resistance. But unlike other known superconductors, which must be chilled to very low temperatures, metallic hydrogen might be a superconductor at room temperature. Scientists’ eventual goal is to find a superconductor that requires neither cooling nor high pressure. If such materials are found, they could be integrated into electronics and save vast amounts of energy (SN: 8/20/16, p. 18).

To create the purported metal, Loubeyre and colleagues squeezed hydrogen gas between two diamonds. An improved setup, with diamonds etched with a doughnut shape, allowed the researchers to reach higher pressures than previous experiments.

Using a powerful source of light called a synchrotron, the team sent infrared light through the diamonds and the squeezed hydrogen. As the physicists ratcheted up the pressure, the hydrogen suddenly became opaque to the light, a sign of a transition to a metal.

“It’s the best experimental data to date on hydrogen in this pressure range,” says physicist Russell Hemley of the University of Illinois at Chicago. The behavior of hydrogen at high pressures is complex, taking on a variety of different phases depending on the conditions, as molecules and atoms rearrange under the squeeze. For example, there may be semimetallic phases as well as a true metal.

It’s unclear what the properties of the claimed metal are, such as whether it is superconducting. But the new result adds to the complexities of hydrogen that scientists have already uncovered, Hemley says. “It’s part of this developing story about the nature of hydrogen at high pressure.”

38
EEE / Permanent liquid magnets have now been created in the lab
« on: February 28, 2020, 09:11:44 PM »
The rules about what makes a good magnet may not be as rigid as scientists thought. Using a mixture containing magnetic nanoparticles, researchers have now created liquid droplets that behave like tiny bar magnets. 

Magnets that generate persistent magnetic fields typically are composed of solids like iron, where the magnetic poles of densely packed atoms are all locked in the same direction (SN: 2/17/18, p. 18). While some liquids containing magnetic particles can become magnetized when placed in a magnetic field, the magnetic orientations of those free-floating particles tend to get jumbled when the field goes away — causing the liquid to lose its magnetism.

Now, adding certain polymers to their recipe has allowed researchers to concoct permanently magnetized liquid droplets. These tiny, moldable magnets, described in the July 19 Science, could be used to build soft robots or capsules that can be magnetically steered through the body to deliver drugs to specific cells.

To make liquid magnets, the team submerged millimeter-sized droplets of a watery solution containing iron oxide nanoparticles in oil peppered with polymers. Those polymers drew many of the magnetic nanoparticles to the droplets’ surfaces and pinned them there, forming a densely packed shell of nanoparticles around each particle-rich droplet. Exposing one of these droplets to a magnetic field forces the magnetic poles of its nanoparticles to point in the same direction. Nanoparticles on the droplet’s surface are crowded so closely that, when the magnetic field shuts off, their magnetic orientations can’t jostle out of alignment, the team found.

What’s more, the surface particles’ collective magnetism is strong enough to keep nanoparticles free-floating throughout the rest of the droplet in line. “So the whole droplet behaves like a solid magnet,” says study coauthor Xubo Liu, a materials scientist at Beijing University of Chemical Technology.


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EEE / Scientists seek materials that defy friction at the atomic level
« on: February 28, 2020, 09:11:13 PM »
t’s a moonless night. The wind howls outside. A door opens slowly, as if pushed by an invisible hand.

“Cre-e-e-a-k.”

That sound — a horror movie cliché — is the result of friction. A stealthier entrance calls for oiling the door’s hinges.

Friction is everywhere — from a violinist bowing a string to children skidding down a slide. In the right situation, the ubiquitous force can have big effects: Interleave the pages of two phone books, and the friction between the pages will hold the books together so tightly that they become strong enough to suspend a car above the ground.
But scientists can’t fully explain, at the scale of atoms and molecules, why one pair of materials sticks while another moves with ease. The extreme slipperiness of ice, for example, has been a puzzle for more than 160 years. The multitude of water molecules on an icy surface creates a sheen that can send a car spinning or a penguin tobogganing. But getting a handle on the details of how this slippery surface arises from the water molecules is surprisingly tricky.

Despite its everyday nature, “we still don’t really understand a lot of things about friction,” says mechanical engineer Ali Erdemir of Argonne National Laboratory in Lemont, Ill. On its most basic level, friction results from the interactions between atoms in two materials that are butted up against one another. But, Erdemir says, “there is a disconnect” between the large-scale processes of friction that we can see, feel or hear and the smaller, atomic properties of materials that produce those well-known behaviors.

Now, by scrutinizing atoms’ wily ways, scientists are devising new techniques to cut down on friction, going beyond known slippery surfaces like ice, Teflon and the banana peel of countless comedy gags. Some scientists have found ways to bring friction down to near-zero levels, a property known as superlubricity. Others are studying quantum effects that reduce friction.

Atomic acrobatics might help turn friction up and down at will, a useful ability since there are times when friction, a force working against the motion of a sliding or rolling object, is helpful. The frictional force of tires on asphalt, for example, lets a car turn without spinning out. But friction also saps the car’s speed, so that more energy is needed to keep the vehicle moving.

40
EEE / Plant-based fire retardants may offer a less toxic way to tame flames
« on: February 28, 2020, 09:09:24 PM »
Using compounds from plants, researchers are concocting a new generation of flame retardants, which one day could replace the fire-quenching chemicals added by manufacturers to furni
ture, electronics and other consumer products.

Many traditional synthetic flame retardants have come under fire for being linked to health problems like thyroid disruption and cancer (SN: 3/16/19, p. 14). And flame retardants that leach out of trash in landfills can persist in the environment for a long time (SN: 4/24/10, p. 12).

The scientists have not yet performed toxicity tests on the new plant-based creations. But “in general, things derived from plants are much less toxic … they’re usually degradable,” says Bob Howell, an organic chemist and polymer scientist at Central Michigan University in Mount Pleasant.
Howell’s team presented the work August 26 in San Diego at the American Chemical Society’s national meeting.

The raw ingredients for these plant-based flame retardants were gallic acid — found in nuts and tea leaves — and a substance in buckwheat called 3,5-Dihydroxybenzoic acid. Treating these compounds with a chemical called phosphoryl chloride converted them into flame-retardant chemicals named phosphorus esters. Since these plant-based ingredients are common, and the chemical treatment process is straightforward, it should be relatively easy to manufacture these flame retardants on a large scale, Howell says.

41
Inside a new microprocessor, the transistors — tiny electronic switches that collectively perform computations — are made with carbon nanotubes, rather than silicon. By devising techniques to overcome the nanoscale defects that often undermine individual nanotube transistors (SN: 7/19/17), researchers have created the first computer chip that uses thousands of these switches to run programs.

The prototype, described in the Aug. 29 Nature, is not yet as speedy or as small as commercial silicon devices. But carbon nanotube computer chips may ultimately give rise to a new generation of faster, more energy-efficient electronics.

This is “a very important milestone in the development of this technology,” says Qing Cao, a materials scientist at the University of Illinois at Urbana-Champaign not involved in the work.
The heart of every transistor is a semiconductor component, traditionally made of silicon, which can act either like an electrical conductor or an insulator. A transistor’s “on” and “off” states, where current is flowing through the semiconductor or not, encode the 1s and 0s of computer data (SN: 4/2/13). By building leaner, meaner silicon transistors, “we used to get exponential gains in computing every single year,” says Max Shulaker, an electrical engineer at MIT. But “now performance gains have started to level off,” he says. Silicon transistors can’t get much smaller and more efficient than they already are.

Because carbon nanotubes are almost atomically thin and ferry electricity so well, they make better semiconductors than silicon. In principle, carbon nanotube processors could run three times faster while consuming about one-third of the energy of their silicon predecessors, Shulaker says. But until now, carbon nanotubes have proved too finicky to construct complex computing systems.

One issue is that, when a network of carbon nanotubes is deposited onto a computer chip wafer, the tubes tend to bunch together in lumps that prevent the transistor from working. It’s “like trying to build a brick patio, with a giant boulder in the middle of it,” Shulaker says. His team solved that problem by spreading nanotubes on a chip, then using vibrations to gently shake unwanted bundles off the layer of nanotubes.

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Scientists have calculated that a hydrogen-rich compound could conduct electricity without resistance at temperatures up to about 200° Celsius — well above the 100° C boiling point of water. If that prediction is confirmed experimentally, the material would stand in stark contrast to all other known superconductors, which must be cooled below room temperature to work (SN: 12/15/15).

Superconductors’ need for cool conditions makes them difficult to use. So physicists are on a quest to find a superconductor that can stand the heat, which could revolutionize how electricity is transmitted and save vast amounts of energy.

The newly predicted superconductor — a compound of hydrogen, magnesium and lithium — comes with its own complications, however. It must be squeezed to extremely high pressure, nearly 2.5 million times the pressure of Earth’s atmosphere, physicist Hanyu Liu and colleagues, of Jilin University in Changchun, China, report in the Aug. 30 Physical Review Letters.

Scientists previously have used similar techniques to predict that a pressurized compound of lanthanum and hydrogen would be superconducting at higher temperatures than any yet known. That prediction seems likely to be correct: In 2018, physicist Russell Hemley and colleagues reported signs that the compound is superconducting up to a record-breaking −13° C (SN: 9/10/18).

If the new calculation is confirmed, the purported superconductor would smash Hemley and colleagues’ temperature record. “This is an important prediction using a level of theory that has proven quite accurate,” says Hemley, of the University of Illinois at Chicago, who was not involved in the research.

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EEE / A new magnetic swirl, or skyrmion, could upgrade data storage
« on: February 28, 2020, 09:08:00 PM »
Scientists have created a new version of the atomic whirlpools, in which the tiny magnetic fields of individual atoms in a material arrange into a swirl pattern. Known as antiferromagnetic skyrmions, the new structures have some advantages that could make them easier to work with than previously found varieties, researchers report September 2 in Nature Materials. If so, that development could bolster hopes for using skyrmions to store data and to create smaller, speedier hard drives (SN: 2/7/18).

Skyrmions previously have been created in materials known as ferromagnets, in which the tiny magnetic field of each atom aligns with its neighbors’. Those aligned fields are the source of ferromagnets’ ability to affix kids’ doodles to the fridge. But scientists hadn’t yet created skyrmions in antiferromagnets, where each atom’s magnetic field points opposite to its neighbor’s, cancelling out the magnetic field.

Antiferromagnets are difficult to work with. So the researchers created a synthetic version, layering magnetic materials so that the magnetization in one layer cancelled out the other layer, mimicking an antiferromagnet’s lack of a magnetic field. By tweaking the properties of each layer, the team optimized the conditions for producing skyrmions, and then imaged them using magnetic force microscopy.
“I’m really impressed,” says materials scientist Axel Hoffmann of the University of Illinois at Urbana-Champaign. “This was really a tour de force kind of effort.”

Scientists think skyrmions could improve on standard hard drives by packing more data into less space, but for that, skyrmions have to be small.  Larger versions of the magnetic whorls are hundreds of nanometers in size — and they won’t cut it. The researchers predict that, with fine-tuning, they could shrink the antiferromagnetic skyrmions down to 10 nanometers in diameter.

“This is the regime of skyrmion size that becomes really interesting,” says physicist Vincent Cros of Unité Mixte de Physique CNRS/Thales in Palaiseau, France, a coauthor of the study.

Skyrmions can also be moved around within a material via electric currents, allowing scientists to shuttle data from one place to another, for example, when it’s time to read out the data. That could avoid the need for the fragile moving parts found in traditional hard drives. But there’s a problem: Thanks to their swirling patterns, skyrmions tend to speed off at an angle to the input current, making them hard to control. But antiferromagnetic skyrmions, with their alternating orientations, are effectively pulled in two directions at once. That means they should travel straight ahead, relative to the current, and could be easier to manipulate.

44
EEE / Andrea Young uncovers the strange physics of 2-D materials
« on: February 28, 2020, 09:07:28 PM »
Speaking with Andrea Young feels like watching a racehorse holding itself back at the starting gate. We met on the campus of the University of California, Santa Barbara, where he’s a condensed matter physicist, to chat about his work on 2-D materials. His mind seems to be working faster than the conversation can flow. My sense is, once the reins are loosened — and he’s back in the lab — he’ll take off.

Young’s colleagues confirm that’s the case. “He’s a whirlwind,” says physicist Raymond Ashoori of MIT. When Young was a postdoc in his lab, Ashoori says, it felt like “an idea a minute.”

Young, 35, has a way with substances shaved to the thickness of a single atom, such as the sheets of carbon known as graphene. His research has revealed new states of matter, and advanced scientists’ understanding of the strange physics that arises when materials are sliced thin.
“Things change a lot when you change the number of dimensions,” Young says.

As a graduate student at Columbia University, Young helped create a new type of material that transformed how scientists study graphene. Along with physicists Cory Dean, Philip Kim and colleagues, Young devised a technique for layering graphene with other materials, in particular another compound that forms 2-D sheets called hexagonal boron nitride. The combination makes the sometimes-finicky graphene easier to work with. And the material’s electrons can be coaxed to behave in unusual ways, interacting strongly with one another, for example.  Reported in Nature Nanotechnology in 2010, the technique was quickly adopted by scientists around the world. “Everybody uses it now,” Ashoori says.

45
EEE / A new cooling technique relies on untwisting coiled fibers
« on: February 28, 2020, 09:07:00 PM »
Called twistocaloric cooling, the method involves unwinding tightly twisted strands of various materials. The technique was used to chill water by several degrees Celsius, scientists report in the Oct. 11 Science.

Cooling techniques like those used in traditional refrigerators rely on cycles of compressing and expanding gases. But those gases can contribute to global warming (SN: 10/25/16). So researchers have been looking for alternative cooling methods based on manipulating solid materials. Consider a rubber band: When stretched, it heats up, becoming warm to the touch. When released, it cools down. The same goes for twisting and untwisting.

To study this effect, a team of scientists from China, the United States and Brazil twisted fibers of rubber, fishing line and wires made of a nickel and titanium alloy. When twisted tightly enough, the various types of strands formed coils or even supercoils — coils of coils. Unwinding a stretched, supercoiled rubber fiber cooled its surface by as much as 15.5 degrees Celsius.

Unraveling cables made of several strands twisted together produced cooling as well. But simply cooling the strands isn’t particularly useful. So the researchers created a “twist fridge” that could chill water. Unwinding a three-ply, nickel-titanium cable while flowing water over it dropped the liquid’s temperature by nearly 8 degrees C, the team reports.

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