materials science

Ice discs slingshot across a metal surface all on their own

fluid dynamics, Leidenfrost effect, materials science, Physics, reverse Leidenfrost effect, Science / Mike M. / August 14, 2025

VA Tech experiment was inspired by Death Valley’s mysterious “sailing stones” at Racetrack Playa.

Graduate student Jack Tapocik sets up ice on an engineered surface in the VA Tech lab of Jonathan Boreyko. Credit: Alex Parrish/Virginia Tech

Scientists have figured out how to make frozen discs of ice self-propel across a patterned metal surface, according to a new paper published in the journal ACS Applied Materials and Interfaces. It’s the latest breakthrough to come out of the Virginia Tech lab of mechanical engineer Jonathan Boreyko.

A few years ago, Boreyko’s lab experimentally demonstrated a three-phase Leidenfrost effect in water vapor, liquid water, and ice. The Leidenfrost effect is what happens when you dash a few drops of water onto a very hot, sizzling skillet. The drops levitate, sliding around the pan with wild abandon. If the surface is at least 400° Fahrenheit (well above the boiling point of water), cushions of water vapor, or steam, form underneath them, keeping them levitated. The effect also works with other liquids, including oils and alcohol, but the temperature at which it manifests will be different.

Boreyko’s lab discovered that this effect can also be achieved in ice simply by placing a thin, flat disc of ice on a heated aluminum surface. When the plate was heated above 150° C (302° F), the ice did not levitate on a vapor the way liquid water does. Instead, there was a significantly higher threshold of 550° Celsius (1,022° F) for levitation of the ice to occur. Unless that critical threshold is reached, the meltwater below the ice just keeps boiling in direct contact with the surface. Cross that critical point and you will get a three-phase Leidenfrost effect.

The key is a temperature differential in the meltwater just beneath the ice disc. The bottom of the meltwater is boiling, but the top of the meltwater sticks to the ice. It takes a lot to maintain such an extreme difference in temperature, and doing so consumes most of the heat from the aluminum surface, which is why it’s harder to achieve levitation of an ice disc. Ice can suppress the Leidenfrost effect even at very high temperatures (up to 550° C), which means that using ice particles instead of liquid droplets would be better for many applications involving spray quenching: rapid cooling in nuclear power plants, for example, firefighting, or rapid heat quenching when shaping metals.

This time around, Boreyko et al. have turned their attention to what the authors term “a more viscous analog” to a Leidenfrost ratchet, a form of droplet self-propulsion. “What’s different here is we’re no longer trying to levitate or even boil,” Boreyko told Ars. “Now we’re asking a more straightforward question: Is there a way to make ice move across the surface directionally as it is melting? Regular melting at room temperature. We’re not boiling, we’re not levitating, we’re not Leidenfrosting. We just want to know, can we make ice shoot across the surface if we design a surface in the right way?”

Mysterious moving boulders

The researchers were inspired by Death Valley’s famous “sailing stones” on Racetrack Playa. Watermelon-sized boulders are strewn throughout the dry lake bed, and they leave trails in the cracked earth as they slowly migrate a couple of hundred meters each season. Scientists didn’t figure out what was happening until 2014. Although co-author Ralph Lorenz (Johns Hopkins University) admitted he thought theirs would be “the most boring experiment ever” when they first set it up in 2011, two years later, the boulders did indeed begin to move while the playa was covered with a pond of water a few inches deep.

So Lorenz and his co-authors were finally able to identify the mechanism. The ground is too hard to absorb rainfall, and that water freezes when the temperature drops. When temperatures rise above freezing again, the ice starts to melt, creating ice rafts floating on the meltwater. And when the winds are sufficiently strong, they cause the ice rafts to drift along the surface.

A sailing stone in Death Valley's Racetrack Playa. — A sailing stone at Death Valley’s Racetrack Playa. Credit: Tahoenathan/CC BY-SA 3.0

“Nature had to have wind blowing to kind of push the boulder and the ice along the meltwater that was beneath the ice,” said Boreyko. “We thought, what if we could have a similar idea of melting ice moving directionally but use an engineered structure to make it happen spontaneously so we don’t have to have energy or wind or anything active to make it work?”

The team made their ice discs by pouring distilled water into thermally insulated polycarbonate Petrie dishes. This resulted in bottom-up freezing, which minimizes air bubbles in the ice. They then milled asymmetric grooves into uncoated aluminum plates in a herringbone pattern—essentially creating arrowhead-shaped channels—and then bonded them to hot plates heated to the desired temperature. Each ice disc was placed on the plate with rubber tongs, and the experiments were filmed from various angles to fully capture the disc behavior.

The herringbone pattern is the key. “The directionality is what really pushes the water,” Jack Tapocik, a graduate student in Boreyko’s lab, told Ars. “The herringbone doesn’t allow for water to flow backward, the water has to go forward, and that basically pushes the water and the ice together forward. We don’t have a treated surface, so the water just sits on top and the ice all moves as one unit.”

Boreyko draws an analogy to tubing on a river, except it’s the directional channels rather than gravity causing the flow. “You can see [in the video below] how it just follows the meltwater,” he said. “This is your classic entrainment mechanism where if the water flows that way and you’re floating on the water, you’re going to go the same way, too. It’s basically the same idea as what makes a Leidenfrost droplet also move one way: It has a vapor flow underneath. The only difference is that was a liquid drifting on a vapor flow, whereas now we have a solid drifting on a liquid flow. The densities and viscosities are different, but the idea is the same: You have a more dense phase that is drifting on the top of a lighter phase that is flowing directionally.”

Jonathan Boreyko/Virginia Tech

Next, the team repeated the experiment, this time coating the aluminum herringbone surface with water-repellant spray, hoping to speed up the disc propulsion. Instead, they found that the disc ended up sticking to the treated surface for a while before suddenly slingshotting across the metal plate.

“It’s a totally different concept with totally different physics behind it, and it’s so much cooler,” said Tapocik. “As the ice is melting on these coated surfaces, the water just doesn’t want to sit within the channels. It wants to sit on top because of the [hydrophobic] coating we have on there. The ice is directly sticking now to the surface, unlike before when it was floating. You get this elongated puddle in front. The easiest place [for the ice] to be is in the center of this giant, long puddle. So it re-centers, and that’s what moves it forward like a slingshot.”

Essentially, the water keeps expanding asymmetrically, and that difference in shape gives rise to a mismatch in surface tension because the amount of force that surface tension exerts on a body depends on curvature. The flatter puddle shape in front has less curvature than the smaller shape in back. As the video below shows, when the mismatch in surface tension becomes sufficiently strong, “It just rips the ice off the surface and flings it along,” said Boreyko. “In the future, we could try putting little things like magnets on top of the ice. We could probably put a boulder on it if we wanted to. The Death Valley effect would work with or without a boulder because it’s the floating ice raft that moves with the wind.”

Jonathan Boreyko/Virginia Tech

One potential application is energy harvesting. For example, one could pattern the metal surface in a circle rather than a straight line so the melting ice disk would continually rotate. Put magnets on the disk, and they would also rotate and generate power. One might even attach a turbine or gear to the rotating disc.

The effect might also provide a more energy-efficient means of defrosting, a longstanding research interest for Boreyko. “If you had a herringbone surface with a frosting problem, you could melt the frost, even partially, and use these directional flows to slingshot the ice off the surface,” he said. “That’s both faster and uses less energy than having to entirely melt the ice into pure water. We’re looking at potentially over a tenfold reduction in heating requirements if you only have to partially melt the ice.”

That said, “Most practical applications don’t start from knowing the application beforehand,” said Boreyko. “It starts from ‘Oh, that’s a really cool phenomenon. What’s going on here?’ It’s only downstream from that it turns out you can use this for better defrosting of heat exchangers for heat pumps. I just think it’s fun to say that we can make a little melting disk of ice very suddenly slingshot across the table. It’s a neat way to grab your attention and think more about melting and ice and how all this stuff works.”

DOI: ACS Applied Materials and Interfaces, 2025. 10.1021/acsami.5c08993 (About DOIs).

Jennifer is a senior writer at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. Jennifer lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban.

Ice discs slingshot across a metal surface all on their own Read More »

Misunderstood “photophoresis” effect could loft metal sheets to exosphere

engineering, materials science, photophoresis, Physics, Science / DJ Henderson / August 14, 2025

Photophoresis can generate a tiny bit of lift without any moving parts.

Image of a wooden stand holding a sealed glass bulb with a spinning set of vanes, each of which has a lit and dark side.

Most people would recognize the device in the image above, although they probably wouldn’t know it by its formal name: the Crookes radiometer. As its name implies, placing the radiometer in light produces a measurable change: the blades start spinning.

Unfortunately, many people misunderstand the physics of its operation (which we’ll return to shortly). The actual forces that drive the blades to spin, called photophoresis, can act on a variety of structures as long as they’re placed in a sufficiently low-density atmosphere. Now, a team of researchers has figured out that it may be possible to use the photophoretic effect to loft thin sheets of metal into the upper atmosphere of Earth and other planets. While their idea is to use it to send probes to the portion of the atmosphere that’s too high for balloons and too low for satellites, they have tested some working prototypes a bit closer to the Earth’s surface.

Photophoresis

It’s quite common—and quite wrong—to see explanations of the Crookes radiometer that involve radiation pressure. Supposedly, the dark sides of the blades absorb more photons, each of which carries a tiny bit of momentum, giving the dark side of the blades a consistent push. The problem with this explanation is that photons are bouncing off the silvery side, which imparts even more momentum. If the device were spinning due to radiation pressure, it would be turning in the opposite direction than it actually does.

An excess of the absorbed photons on the dark side is key to understanding how it works, though. Photophoresis operates through the temperature difference that develops between the warm, light-absorbing dark side of the blade and the cooler silvered side.

Any gas molecule that bumps into the dark side will likely pick up some of the excess thermal energy from it and move away from the blade faster than it arrived. At the sorts of atmospheric pressures we normally experience, these molecules don’t get very far before they bump into other gas molecules, which keeps any significant differences from developing.

But a Crookes radiometer is in a sealed glass container with a far lower air pressure. This allows the gas molecules to speed off much farther from the dark surface of the blade before they run into anything, creating an area of somewhat lower pressure at its surface. That causes gas near the surface of the shiny side to rush around and fill this lower-pressure area, imparting the force that starts the blades turning.

It’s pretty impressively inefficient in that sort of configuration, though. So people have spent a lot of time trying to design alternative configurations that can generate a bit more force. One idea with a lot of research traction is a setup that involves two thin metal sheets—one light, one dark—arranged parallel to each other. Both sheets would be heavily perforated to cut down on weight. And a subset of them would have a short pipe connecting holes on the top and bottom sheet. (This has picked up the nickname “nanocardboard.”)

These pipes would serve several purposes. One is to simply link the two sheets into a single unit. Another is to act as an insulator, keeping heat from moving from the dark sheet to the light one, and thus enhancing the temperature gradient. Finally, they provide a direct path for air to move from the top of the light-colored sheet to the bottom of the dark one, giving a bit of directed thrust to help keep the sheets aloft.

Optimization

As you might imagine, there are a lot of free parameters you can tweak: the size of the gap between the sheets, the density of perforations in them, the number of those holes that are connected by a pipe, and so on. So a small team of researchers developed a system to model different configurations and attempt to optimize for lift. (We’ll get to their motivations for doing so a bit later.)

Starting with a disk of nanocardboard, “The inputs to the model are the geometric, optical and thermal properties of the disk, ambient gas conditions, and external radiative heat fluxes on the disk,” as the researchers describe it. “The outputs are the conductive heat fluxes on the two membranes, the membrane temperatures, and the net photophoretic lofting force on the structure.” In general, the ambient gas conditions needed to generate lift are similar to the ones inside the Crookes radiometer: well below the air pressure at sea level.

The model suggested that three trends should influence any final designs. The first is that the density of perforations is a balance. At relatively low elevations (meaning a denser atmosphere), many perforations increase the stress on large sheets, but they decrease the stress for small items at high elevations. The other thing is that, rather than increasing with surface area, lift tends to drop because the sheets are more likely to equilibrate to the prevailing temperatures. A square millimeter of nanocardboard produces over 10 times more lift per surface area than a 10-square-centimeter piece of the same material.

Finally, the researchers calculate that the lift is at its maximum in the mesosphere, the area just above the stratosphere (50–100 kilometers above Earth’s surface).

Light and lifting

The researchers then built a few sheets of nanocardboard to test the output of their model. The actual products, primarily made of chromium, aluminum, and aluminum oxide, were incredibly light, weighing only a gram for a square meter of material. When illuminated by a laser or white LED, they generated measurable force on a testing device, provided the atmosphere was kept sufficiently sparse. With an exposure equivalent to sunlight, the device generated more than it weighed.

It’s a really nice demonstration that we can take a relatively obscure and weak physical effect and design devices that can levitate in the upper atmosphere, powered by nothing more than sunlight—which is pretty cool.

But the researchers have a goal beyond that. The mesophere turns out to be a really difficult part of the atmosphere to study. It’s not dense enough to support balloons or aircraft, but it still has enough gas to make quick work of any satellites. So the researchers really want to turn one of these devices into an instrument-carrying aircraft. Unfortunately, that would mean adding the structural components needed to hold instruments, along with the instruments themselves. And even in the mesosphere, where lift is optimal, these things do not generate much in the way of lift.

Plus, there’s the issue of getting them there, given that they won’t generate enough lift in the lower atmosphere, so they’ll have to be carried into the upper stratosphere by something else and then be released gently enough to not damage their fragile structure. And then, unless you’re lofting them during the polar summer, they will likely come floating back down at night.

None of this is to say this is an impossible dream. But there are definitely a lot of very large hurdles between the work and practical applications on Earth—much less on Mars, where the authors suggest the system could also be used to explore the mesosphere. But even if that doesn’t end up being realistic, this is still a pretty neat bit of physics.

John is Ars Technica’s science editor. He has a Bachelor of Arts in Biochemistry from Columbia University, and a Ph.D. in Molecular and Cell Biology from the University of California, Berkeley. When physically separated from his keyboard, he tends to seek out a bicycle, or a scenic location for communing with his hiking boots.

Misunderstood “photophoresis” effect could loft metal sheets to exosphere Read More »

New adhesive surface modeled on a remora works underwater

Biology, Biomechanics, engineering, materials science, Science / Kelly Newman / August 9, 2025

It was tested for its ability to adhere to the inside of the digestive tract.

Most adhesives can’t stick to wet surfaces because water and other fluids disrupt the adhesive’s bonding mechanisms. This problem, though, has been beautifully solved by evolution in remora suckerfish, which use an adhesive disk on top of their heads to attach to animals like dolphins, sharks, and even manta rays.

A team of MIT scientists has now taken a close look at these remora disks and reverse-engineered them. “Basically, we looked at nature for inspiration,” says Giovanni Traverso, a professor at MIT Department of Mechanical Engineering and senior author of the study.

Sticking Variety

Remora adhesive disks are an evolutionary adaptation of the fish’s first dorsal fin, the one that in other species sits on top of the body, just behind the head and gill covers. The disk rests on an intercalary backbone—a bone structure that most likely evolved from parts of the spine. This bony structure supports lamellae, specialized bony plates with tiny backward-facing spikes called spinules. The entire disk is covered with soft tissue compartments that are open at the top. “This makes the remora fish adhere very securely to soft-bodied, fast-moving marine hosts,” Traverso says.

A remora attaches to the host by pressing itself against the skin, which pushes the water out of these compartments, creating a low-pressure zone. Then, the spinules mechanically interlock with the host’s surface, making the whole thing work a bit like a combination of a suction cup and Velcro. When the fish wants to detach from a host, it lifts the disk, letting water back into the compartments to remove the suction. Once released, it can simply swim away.

What impressed the scientists the most, though, was the versatility of those disks. Reef-associated species of remora like Phtheirichthys lineatus are generalists and stick to various hosts, including other fish, sharks, or turtles. Other species living in the open sea are more specialized and attach to cetaceans, swordfish, or marlins. While most remoras attach to the external tissue of their hosts, R. albescens sticks within the oral cavities and gill chamber of manta rays.

a close up of a fish, showing its head covered by an oval-shaped pad that has lots of transverse ridges. — A close-up of the adhesive pad of a remora. Credit: Stephen Frink

To learn what makes all these different disks so good at sticking underwater, the team first examined their anatomy in detail. It turned out that the difference between the disks was mostly in the positioning of lamellae. Generalist species have a mix of parallel and angled lamellae, while remoras sticking to fast-swimming hosts have them mostly parallel. R. albescens, on the other hand, doesn’t have a dominant lamellae orientation pattern but has them positioned at a very wide variety of angles.

The researchers wanted to make an adhesive device that would work for a wide range of applications, including maritime exploration or underwater manufacturing. Their initial goal, though, was designing a drug delivery platform that could reliably stick to the inside walls of the gastrointestinal tract. So, they chose R. albescens disks as their starting point, since that species already attaches internally to its host. They termed their device an Mechanical Underwater Soft Adhesion System (MUSAS).

However, they didn’t just opt for a biomimetic, copy-and-paste design. “There were things we did differently,” Traverso says.

Upgrading nature

The first key difference was deployment. MUSAS was supposed to travel down the GI tract to reach its destination, so the first challenge was making it fit into a pill. The team chose the size 000 capsule, which at 26 millimeters in length and 9.5 millimeters in diameter, is the largest Food and Drug Administration-approved ingestible form. MUSAS had a supporting structure—just like remora disks, but made with stainless steel. The angled lamellae with spinules fashioned after those on R. albescens were made of a shape memory nickel-titanium alloy. The role of remora’s soft tissues, which provide the suction by dividing the disk into compartments, was played by an elastomer.

MUSAS, would be swallowed in a folded form within its huge pill. “The capsule is tuned to dissolve in specific pH environment, which is how we determine the target location—for example the small intestine has a slightly different pH than the stomach”, says Ziliang Kang, an MIT researcher in Traverso’s group and lead author of the study. Once released, the shape memory alloy in MUSAS lamellae-like structures would unfold in response to body temperature and the whole thing would stick to the wall of the target organ, be it the esophagus, the stomach, or the intestines.

The mechanism of sticking was also a bit different from that of remoras. “The fish can swim and actively press itself against the surface it wants to stick to. MUSAS can’t do that, so instead we relied on the peristaltic movements within the GI tract to exert the necessary force,” Traverso explains. When the muscles contract, MUSAS would be pressed against the wall and attach to it. And it was expected to stay there for quite some time.

The team ran a series of experiments to evaluate MUSAS performance in a few different scenarios. The drug-delivery platform application was tested on pig organ samples. MUSAS stayed in the sample GI tract for an average of nine days, with the longest sticking time reaching three and a half weeks. MUSAS managed to stay in place despite food and fluids going through the samples.

Even when the team poked the devices with a pipette to test what they called “resisting dynamic interference,” MUSAS just slid a little but remained firmly attached. Other experiments included using MUSAS to attach temperature sensors to external tissues of live fish and putting sensors that could detect reflux events in the GI tract of live pigs.

Branching out

The team is working on making MUSAS compatible with a wider range of drugs and mRNA vaccines. “We also think about using this for stimulating tissues,” Traverso says. The solution he has in mind would use MUSAS to deliver electrical pulses to the walls of the GI tract, which Traverso’s lab has shown can activate appetite-regulating hormones. But the team also wants to go beyond strictly medical applications.

The team demonstrated that MUSAS is really strong as an adhesive. When it sticks to a surface, it can hold a weight over a thousand times greater than its own. This puts MUSAS more or less on par with some of the best adhesives we have, such as polyurethane glues or epoxy resins. What’s more, this sticking strength was measured when MUSAS was attached to soft, uneven, wet surfaces. “On a rigid, even surface, the force-to-weight ratio should be even higher,” Kang claims. And this, Kang thinks, makes scaled-up variants of MUSAS a good match for underwater manufacturing.

“The first scenario I see is using MUSAS as grippers attached to robotic arms moving around soft objects,” Kang explains. Currently, this is done using vacuum systems that simply suck onto a fabric or other surface. The problem is that these solutions are rather complex and heavy. Scaled-up MUSAS should be able to achieve the same thing passively, cutting cost and weight. The second idea Kang has is using MUSAS in robots designed to perform maintenance jobs beneath the waterline on boats or ships. “We are really trying to see what is possible,” Traverso says.

Nature, 2025. DOI: 10.1038/s41586-025-09304-4

Jacek Krywko is a freelance science and technology writer who covers space exploration, artificial intelligence research, computer science, and all sorts of engineering wizardry.

New adhesive surface modeled on a remora works underwater Read More »

This aerogel and some sun could make saltwater drinkable

aerogel, chemistry, desalina, desalination, materials science, Science, solar power / Kelly Newman / July 26, 2025

Earth is about 71 percent water. An overwhelming 97 percent of that water is found in the oceans, leaving us with only 3 percent in the form of freshwater—and much of that is frozen in the form of glaciers. That leaves just 0.3 percent of that freshwater on the surface in lakes, swamps, springs, and our main sources of drinking water, rivers and streams.

Despite our planet’s famously blue appearance from space, thirsty aliens would be disappointed. Drinkable water is actually pretty scarce.

As if that doesn’t already sound unsettling, what little water we have is also threatened by climate change, urbanization, pollution, and a global population that continues to expand. Over 2 billion people live in regions where their only source of drinking water is contaminated. Pathogenic microbes in the water can cause cholera, diarrhea, dysentery, polio, and typhoid, which could be fatal in areas without access to vaccines or medical treatment.

Desalination of seawater is a possible solution, and one approach involves porous materials absorbing water that evaporates when heated by solar energy. The problem with most existing solar-powered evaporators is that they are difficult to scale up for larger populations. Performance decreases with size, because less water vapor can escape from materials with tiny pores and thick boundaries—but there is a way to overcome this.

Feeling salty

Researcher Xi Shen of the Hong Kong Polytechnic University wanted to figure out a way to improve these types of systems. He and his team have now created an aerogel that is far more efficient at turning over fresh water than previous methods of desalination.

“The key factors determining the evaporation performance of porous evaporators include heat localization, water transport, and vapor transport,” Shen said in a study recently published in ACS Energy Letters. “Significant advancements have been made in the structural design of evaporators to realize highly efficient thermal localization and water transport.”

Solar radiation is the only energy used to evaporate the water, which is why many attempts have been made to develop what are called photothermal materials. When sunlight hits these types of materials, they absorb light and convert it into heat energy, which can be used to speed up evaporation. Photothermal materials can be made of substances including polymers, metals, alloys, ceramics, or cements. Hydrogels have been used to successfully decontaminate and desalinate water before, but they are polymers designed to retain water, which negatively affects efficiency and stability, as opposed to aerogels, which are made of polymers that hold air. This is why Shen and his team decided to create a photothermal aerogel.

This aerogel and some sun could make saltwater drinkable Read More »

Researchers develop a battery cathode material that does it all

batteries, chemistry, lithium, materials science, Science / Kelly Newman / June 26, 2025

Battery electrode materials need to do a lot of things well. They need to be conductors to get charges to and from the ions that shuttle between the electrodes. They also need to have an open structure that allows the ions to move around before they reach a site where they can be stored. The storage of lots of ions also causes materials to expand, creating mechanical stresses that can cause the structure of the electrode material to gradually decay.

Because it’s hard to get all of these properties from a single material, many electrodes are composite materials, with one chemical used to allow ions into and out of the electrode, another to store them, and possibly a third that provides high conductivity. Unfortunately, this can create new problems, with breakdowns at the interfaces between materials slowly degrading the battery’s capacity.

Now, a team of researchers is proposing a material that seemingly does it all. It’s reasonably conductive, it allows lithium ions to move around and find storage sites, and it’s made of cheap and common elements. Perhaps best of all, it undergoes self-healing, smoothing out damage across charge/discharge cycles.

High capacity

The research team, primarily based in China, set out to limit the complexity of cathodes. “Conventional composite cathode designs, which typically incorporate a cathode active material, catholyte, and electronic conducting additive, are often limited by the substantial volume fraction of electrochemically inactive components,” the researchers wrote. The solution, they reasoned, was to create an all-in-one material that gets rid of most of these materials.

A number of papers had reported good luck with chlorine-based chemicals, which allowed ions to move readily through the material but didn’t conduct electricity very well. So the researchers experimented with pre-loading one of these materials with lithium. And they focused on iron chloride since it’s a very cheap material.

Researchers develop a battery cathode material that does it all Read More »

Infrared contact lenses let you see in the dark

Color blindness, infrared, materials science, nanoparticles, Neuroscience, optics, Science, Vision, visual perception / Mike M. / May 22, 2025

A new perspective

illustration showing the Preparation procedures for infrared contacts. — Preparation procedures for infrared contacts. Credit: Sheng Wang/CC BY-SA

The team tested their lenses on humans by asking subjects to detect flashing signals, akin to Morse code, in the infrared, and to identify the direction of incoming infrared light. The subjects could only perform those tasks while wearing the special contact lenses.

The authors were intrigued to find that both mice and humans were better able to discriminate infrared light compared to visible light when their eyes were closed, which they attribute to the fact that infrared light can penetrate the eyelid more effectively than visible light. They also tweaked the nanoparticles so that they could color-code different infrared wavelengths, thereby enabling wearers to perceive more details in the infrared, an adaptation that could help color-blind people perceive more wavelengths.

There are some limitations. The contact lenses are so close to the retina that they can’t really capture fine details very well, because the converted light particles tend to scatter. The team made a wearable glass version of their nanoparticle technology so wearers could get higher resolution in the infrared. And right now the lenses can only detect infrared light projected from an LED; increasing the sensitivity of the nanoparticles to pick up lower levels of infrared would address this issue.

Still, it’s a significant step. “Our research opens up the potential for non-invasive wearable devices to give people super-vision,” said co-author Tian Xue, a neuroscientist at the University of Science and Technology of China. “There are many potential applications right away for this material. For example, flickering infrared light could be used to transmit information in security, rescue, encryption, or anti-counterfeiting settings. In the future, by working together with materials scientists and optical experts, we hope to make a contact lens with more precise spatial resolution and higher sensitivity.”

Cell, 2025. DOI: 10.1016/j.cell.2025.04.019 (About DOIs).

Infrared contact lenses let you see in the dark Read More »

The seemingly indestructible fists of the mantis shrimp can take a punch

Biomechanics, Mantis shrimp, materials science, Science / Kelly Newman / February 23, 2025

To find out how much force a mantis shrimp’s dactyl clubs can possibly withstand, the researchers tested live shrimp by having them strike a piezoelectric sensor like they would smash a shell. They also fired ultrasonic and hypersonic lasers at pieces of dactyl clubs from their specimens so they could see how the clubs defended against sound waves.

By tracking how sound waves propagated on the surface of the dactyl club, the researchers could determine which regions of the club diffused the most waves. It was the second layer, the impact surface, that handled the highest levels of stress. The periodic surface was almost as effective. Together, they made the dactyl clubs nearly immune to the stresses they generate.

There are few other examples that the protective structures of the mantis shrimp can be compared to. On the prey side, evidence has been found that the scales on some moths’ wings absorb sound waves from predatory bats to keep them from echolocation to find them.

Understanding how mantis shrimp defend themselves from extreme force could inspire new technology. The structures in their dactyl clubs could influence the designs of military and athletic protective gear in the future.

“Shrimp impacts contain frequencies in the ultrasonic range, which has led to shrimp-inspired solutions that point to ultrasonic filtering as a key [protective] mechanism,” the team said in the same study.

Maybe someday, a new bike helmet model might have been inspired by a creature that is no more than seven inches long but literally doesn’t crack under pressure.

Science, 2025. DOI: 10.1126/science.adq7100

The seemingly indestructible fists of the mantis shrimp can take a punch Read More »

Researchers figure out how to get fresh lithium into batteries

batteries, chemistry, materials science, Science / Shannon Garcia / February 22, 2025

In their testing, they use a couple of unusual electrode materials, such as a chromium oxide (Cr₈O₂₁) and an organic polymer (a sulfurized polyacrylonitrile). Both of these have significant weight advantages over the typical materials used in today’s batteries, although the resulting batteries typically lasted less than 500 cycles before dropping to 80 percent of their original capacity.

But the striking experiment came when they used LiSO₂CF₃ to rejuvenate a battery that had been manufactured as normal but had lost capacity due to heavy use. Treating a lithium-iron phosphate battery that had lost 15 percent of its original capacity restored almost all of what was lost, allowing it to hold over 99 percent of its original charge. They also ran a battery for repeated cycles with rejuvenation every few thousand cycles. At just short of 12,000 cycles, it still could be restored to 96 percent of its original capacity.

Before you get too excited, there are a couple of things worth noting about lithium-iron phosphate cells. The first is that, relative to their charge capacity, they’re a bit heavy, so they tend to be used in large, stationary batteries like the ones in grid-scale storage. They’re also long-lived on their own; with careful management, they can take over 8,000 cycles before they drop to 80 percent of their initial capacity. It’s not clear whether similar rejuvenation is possible in the battery chemistries typically used for the sorts of devices that most of us own.

The final caution is that the battery needs to be modified so that fresh electrolytes can be pumped in and the gases released by the breakdown of the LiSO₂CF₃ removed. It’s safest if this sort of access is built into the battery from the start, rather than provided by modifying it much later, as was done here. And the piping needed would put a small dent in the battery’s capacity per volume if so.

All that said, the treatment demonstrated here would replenish even a well-managed battery closer to its original capacity. And it would largely restore the capacity of something that hadn’t been carefully managed. And that would allow us to get far more out of the initial expense of battery manufacturing. Meaning it might make sense for batteries destined for a large storage facility, where lots of them could potentially be treated at the same time.

Nature, 2025. DOI: 10.1038/s41586-024-08465-y (About DOIs).

Researchers figure out how to get fresh lithium into batteries Read More »

The physics of ugly Christmas sweaters

knitting, materials science, Physics, Science, ugly Christmas sweaters / Shannon Garcia / December 27, 2024

In 2018, a team of French physicists developed a rudimentary mathematical model to describe the deformation of a common type of knit. Their work was inspired when co-author Frédéric Lechenault watched his pregnant wife knitting baby booties and blankets, and he noted how the items would return to their original shape even after being stretched. With a few colleagues, he was able to boil the mechanics down to a few simple equations, adaptable to different stitch patterns. It all comes down to three factors: the “bendiness” of the yarn, the length of the yarn, and how many crossing points are in each stitch.

A simpler stitch

A simplified model of how yarns interact Credit: J. Crassous/University of Rennes

One of the co-authors of that 2018 paper, Samuel Poincloux of Aoyama Gakuin University in Japan, also co-authored this latest study with two other colleagues, Jérôme Crassous (University of Rennes in France) and Audrey Steinberger (University of Lyon). This time around, Poincloux was interested in the knotty problem of predicting the rest shape of a knitted fabric, given the yarn’s length by stitch—an open question dating back at least to a 1959 paper.

It’s the complex geometry of all the friction-producing contact zones between the slender elastic fibers that makes such a system too difficult to model precisely, because the contact zones can rotate or change shape as the fabric moves. Poincloux and his cohorts came up with their own more simplified model.

The team performed experiments with a Jersey stitch knit (aka a stockinette), a widely used and simple knit consisting of a single yarn (in this case, a nylon thread) forming interlocked loops. They also ran numerical simulations modeled on discrete elastic rods coupled with dry contacts with a specific friction coefficient to form meshes.

The results: Even when there were no external stresses applied to the fabric, the friction between the threads served as a stabilizing factor. And there was no single form of equilibrium for a knitted sweater’s resting shape; rather, there were multiple metastable states that were dependent on the fabric’s history—the different ways it had been folded, stretched, or rumpled. In short, “Knitted fabrics do not have a unique shape when no forces are applied, contrary to the relatively common belief in textile literature,” said Crassous.

DOI: Physical Review Letters, 2024. 10.1103/PhysRevLett.133.248201 (About DOIs).

The physics of ugly Christmas sweaters Read More »

Generating power with a thin, flexible thermoelectric film

cooling, materials science, power, Science, thermoelectric material / DJ Henderson / December 12, 2024

The No. 1 nuisance with smartphones and smartwatches is that we need to charge them every day. As warm-blooded creatures, however, we generate heat all the time, and that heat can be converted into electricity for some of the electronic gadgetry we carry.

Flexible thermoelectric devices, or F-TEDs, can convert thermal energy into electric power. The problem is that F-TEDs weren’t actually flexible enough to comfortably wear or efficient enough to power even a smartwatch. They were also very expensive to make.

But now, a team of Australian researchers thinks they finally achieved a breakthrough that might take F-TEDs off the ground.

“The power generated by the flexible thermoelectric film we have created would not be enough to charge a smartphone but should be enough to keep a smartwatch going,” said Zhi-Gang Chen, a professor at Queensland University of Technology in Brisbane, Australia. Does that mean we have reached a point where it would be possible to make a thermoelectric Apple Watch band that could keep the watch charged all the time? “It would take some industrial engineering and optimization, but we can definitely achieve a smartwatch band like that,” Chen said.

Manufacturing heaven

Thermoelectric generators producing enough power to run something like an Apple Watch were, so far, made with rigid bulk materials. The obvious problem with them was that nobody would want to wear a metal slab on their wrist or run a power cable from anywhere else to their watch. Flexible thermoelectric devices, on the other hand, were perfectly wearable but offered efficiencies that made them good for low-power health-monitoring electronics rather than more power-hungry hardware like smartwatches.

Back in 2021, generating 35 microwatts per square centimeter in a wristband worn during a typical walk outside was impressive enough to land your research paper in Nature. Today, Chen and his colleagues made a flexible thermoelectric device that performed over 34 times better at room temperature. “To the best of our knowledge, we hold a current record in this field,” Chen says.

Generating power with a thin, flexible thermoelectric film Read More »

What makes baseball’s “magic mud” so special?

adhesion, baseball, geomaterials, materials science, Non-newtonian fluids, Physics, rheology, Science, soft matter / Beth Washington / November 7, 2024

“Magic mud” composition and microstructure: (top right) a clean baseball surface; (bottom right) a mudded baseball. Credit: S. Pradeep et al., 2024

Pradeep et al. found that magic mud’s particles are primarily silt and clay, with a bit of sand and organic material. The stickiness comes from the clay, silt, and organic matter, while the sand makes it gritty. So the mud “has the properties of skin cream,” they wrote. “This allows it to be held in the hand like a solid but also spread easily to penetrate pores and make a very thin coating on the baseball.”

When the mud dries on the baseball, however, the residue left behind is not like skin cream. That’s due to the angular sand particles bonded to the baseball by the clay, which can increase surface friction by as much as a factor of two. Meanwhile, the finer particles double the adhesion. “The relative proportions of cohesive particulates, frictional sand, and water conspire to make a material that flows like skin cream but grips like sandpaper,” they wrote.

Despite its relatively mundane components, the magic mud nonetheless shows remarkable mechanical behaviors that the authors think would make it useful in other practical applications. For instance, it might replace synthetic materials as an effective lubricant, provided the gritty sand particles are removed. Or it could be used as a friction agent to improve traction on slippery surfaces, provided one could define the optimal fraction of sand content that wouldn’t diminish its spreadability. Or it might be used as a binding agent in locally sourced geomaterials for construction.

“As for the future of Rubbing Mud in Major League Baseball, unraveling the mystery of its behavior does not and should not necessarily lead to a synthetic replacement,” the authors concluded. “We rather believe the opposite; Rubbing Mud is a nature-based material that is replenished by the tides, and only small quantities are needed for great effect. In a world that is turning toward green solutions, this seemingly antiquated baseball tradition provides a glimpse of a future of Earth-inspired materials science.”

DOI: PNAS, 2024. 10.1073/pnas.241351412 (About DOIs).

What makes baseball’s “magic mud” so special? Read More »

“Impact printing” is a cement-free alternative to 3D-printed structures

3D printing, construction, materials science, Science / Mike M. / October 29, 2024

Recently, construction company ICON announced that it is close to completing the world’s largest 3D-printed neighborhood in Georgetown, Texas. This isn’t the only 3D-printed housing project. Hundreds of 3D-printed homes are under construction in the US and Europe, and more such housing projects are in the pipeline.

There are many factors fueling the growth of 3D printing in the construction industry. It reduces the construction time; a home that could take months to build can be constructed within days or weeks with a 3D printer. Compared to traditional methods, 3D printing also reduces the amount of material that ends up as waste during construction. These advantages lead to reduced labor and material costs, making 3D printing an attractive choice for construction companies.

A team of researchers from the Swiss Federal Institute of Technology (ETH) Zurich, however, claims to have developed a robotic construction method that is even better than 3D printing. They call it impact printing, and instead of typical construction materials, it uses Earth-based materials such as sand, silt, clay, and gravel to make homes. According to the researchers, impact printing is less carbon-intensive and much more sustainable and affordable than 3D printing.

This is because Earth-based materials are abundant, recyclable, available at low costs, and can even be excavated at the construction site. “We developed a robotic tool and a method that could take common material, which is the excavated material on construction sites, and turn it back into usable building products, at low cost and efficiently, with significantly less CO₂ than existing industrialized building methods, including 3D printing,” said Lauren Vasey, one of the researchers and an SNSF Bridge Fellow at ETH Zurich.

How does impact printing work?

Excavated materials can’t be used directly for construction. So before beginning the impact printing process, researchers prepare a mix of Earth-based materials that has a balance of fine and coarse particles, ensuring both ease of use and structural strength. Fine materials like clay act as a binder, helping the particles stick together, while coarser materials like sand or gravel make the mix more stable and strong. This optimized mix is designed such that it can move easily through the robotic system without getting stuck or causing blockages.

“Impact printing” is a cement-free alternative to 3D-printed structures Read More »