There's rarely time to write about every cool science-y story that comes our way. So this year, we're once again running a special Twelve Days of Christmas series of posts, highlighting one science story that fell through the cracks in 2023, each day from December 25 through January 5. Today: how applying magnetic forces to individual "micro-roller" particles spurs collective motion, producing some pretty counter-intuitive results.

Engineering researchers at Lehigh University have discovered that sometimes sand can actually flow uphill. (credit: Lehigh University)

We intuitively understand that the sand pouring through an hourglass, for example, forms a neat roughly pyramid-shaped pile at the bottom, in which the grains near the surface flow over an underlying base of stationary particles. Avalanches and sand dunes exhibit similar dynamics. But scientists at Lehigh University in Pennsylvania have discovered that applying a magnetic torque can actually cause sand-like particles to collectively flow uphill in seeming defiance of gravity, according to a September paper published in the journal Nature Communications.

Sand is pretty fascinating stuff from a physics standpoint. It's an example of a granular material, since it acts both like a liquid and a solid. Dry sand collected in a bucket pours like a fluid, yet it can support the weight of a rock placed on top of it, like a solid, even though the rock is technically denser than the sand. So sand defies all those tidy equations describing various phases of matter, and the transition from flowing "liquid" to a rigid "solid" happens quite rapidly. It's as if the grains act as individuals in the fluid form, but are capable of suddenly banding together when solidarity is needed, achieving a weird kind of "strength in numbers" effect.

Enlarge / A hawk moth in flight. (credit: Gregory Dubus )

Different insects flap their wings in different manners. Understanding the variations between these modes of flight may help scientists design better and more efficient flying robots in the future. However, decoding insect flight is not as easy as it sounds.

Winged insects have been around for nearly 400 million years , and the evolution of flight in different insect species influences things like how insects flap their wings, what makes some insects highly maneuverable, and how their flight muscles work. A new study has used a mix of evolutionary analysis and robotic model wings to better understand how different flight modes operate.

Insects are the most skilled flyers

There are organisms other than insects that can fly . Scientists can also take inspiration from them, so what makes insect flight so special?

Enlarge / Timelapse photo of the "microflier" falling in its unfolded state, which makes it tumble chaotically in the wind. "Snapping" into a folded state results in a stable upright descent. (credit: Mark Stone/University of Washington)

University of Washington scientists have built a battery-free flying robot that stabilizes its descent by changing shape in mid-air—a design that was inspired by origami, according to a recent paper published in the journal Science Robotics. These microfliers weigh just 400 milligrams, and if there's a nice light breeze, they can travel the length of a football field when dropped by a drone from an altitude of 40 meters (131 feet).

Miniature robotics is a very active area of research. For instance, earlier this year, we reported on how engineers built a soft robot in the shape of a Lego minifig. The robot changes shape by "melting" into liquid form in response to a magnetic field, oozing between the bars of its cage before re-solidifying on the other side—just like the T-1000 in Terminator 2: Judgment Day . That robot belongs to a class known as magnetically actuated miniature machines, typically made of soft polymers (like elastomers or hydrogels) embedded with ferromagnetic particles that have programmed magnetization profiles. These kinds of robots can swim, climb, roll, walk, and jump, as well as change their shape simply by altering the corresponding magnetic field.

As for flying robots, back in 2017, we reported on Dutch scientists who built a flying robot capable of executing the impressive aerodynamic feats flying insects like bees, dragonflies, and fruit flies, particularly when said insects seek to evade predators or the swatting motion of a human hand. Even though the robot was much larger than the average insect, it could hover and fly in any direction (up, down, forward, backward, and sideways), as well as perform banked turns and 360-degree flips, akin to loops or barrel rolls. It also boasted excellent power efficiency, capable of hovering for five minutes or flying more than a kilometer on a single charge.

Enlarge / The world’s smallest 3D-printed wineglass in silica glass (left) and an optical resonator for fiber optic telecommunication, photographed with scanning electron microscopy. The rim of the glass is smaller than the width of a human hair. (credit: KTH Royal Institute of Technology)

A team of Swedish scientists has developed a novel 3D-printing technique for silica glass that streamlines a complicated energy-intensive process. As a proof of concept, they 3D-printed the world's smallest wineglass (made of actual glass) with a rim smaller than the width of one human hair, as well as an optical resonator for fiber optic telecommunications systems—one of several potential applications for 3D-printed silica glass components. They described their new method in a recent paper in the journal Nature Communications.

“The backbone of the Internet is based on optical fibers made of glass," said co-author Kristinn Gylfason of the KTH Royal Institute of Technology in Stockholm. "In those systems, all kinds of filters and couplers are needed that can now be 3D printed by our technique. This opens many new possibilities.”

Silica glass (i.e., amorphous silicon dioxide) is one material that remains challenging for 3D printing, particularly at the microscale, according to the authors, though several methods seek to address that challenge, including stereolithography, direct ink writing, and digital light processing. Even those have only been able to achieve feature sizes on the order of several tens of micrometers, apart from one 2021 study that reported nanoscale resolution.

Enlarge / MIT engineers built a battery-free, wireless underwater camera that could help scientists explore unknown regions of the ocean, track pollution, or monitor the effects of climate change. (credit: Adam Glanzman)

MIT engineers have built a wireless, battery-free underwater camera, capable of harvesting energy by itself while consuming very little power, according to a new paper published in the journal Nature Communications. The system can take color photos of remote submerged objects—even in dark settings— and convey the data wirelessly for real-time monitoring of underwater environments, aiding the discovery of new rare species or monitoring ocean currents, pollution, or commercial and military operations.

We already have various methods of taking underwater images, but according to the authors, "Most of the ocean and marine organisms have not been observed yet." That's partly because most existing methods require being tethered to ships, underwater drones, or power plants for both power and communication. Those methods that don't use tethering must incorporate battery power, which limits their lifetime. While it's possible in principle to harvest energy from ocean waves, underwater currents, or even sunlight, adding the necessary equipment to do so would result in a much bulkier and more expensive underwater camera.

So the MIT team set about developing a solution for a battery-free, wireless imaging method. The design goal was to minimize the hardware required as much as possible. Since they wanted to keep power consumption to a minimum,  for instance, the MIT team used cheap off-the-shelf imaging sensors. The trade-off is that such sensors only produce grayscale images. The team also needed to develop a low-power flash as well, since most underwater environments don't get much natural light.

Enlarge (credit: Getty Images )

Earth Day was April 22nd, and its usual message—take care of our planet—has been given added urgency by the challenges highlighted in the latest IPCC report. This year, Ars is taking a look at the technologies we normally cover, from cars to chipmaking, and finding out how we can boost their sustainability and minimize their climate impact.

Gone are the days of going to Blockbuster to pick out a film for a night in. Physical media like CDs, DVDs, Blu-ray discs, Sony’s weird PlayStation Portable UMDs, and countless other formats have been thoroughly dethroned thanks to a barrage of streaming services like Netflix— itself ailing at the moment—Amazon Prime, and Spotify.

For the first time in the past 17 years, CDs saw an increase in sales—of 1.1 percent , or 40.59 million units in 2021, compared to 40.16 million units the year prior. In 2021, people purchased 1.2 billion pieces of physical video media, compared to 6.1 billion a decade prior. Meanwhile, according to the Recording Industry Association of America, revenue from music streaming grew 13.4 percent to $10.1 billion in 2020.