The team initially tried to use a simple home coffee machine for their experiments but eventually partnered with Coffeelab, a major roaster in Poland, and CoffeeMachineSale, the largest global distributor of roasting gear. This brought industrial-grade equipment and much professional coffee expertise to the project: state-of-the-art grinders, for instance, and a cafe-grade espresso machine, tricked out with a pressure sensor, flow meter, and a set of scales. The entire setup was connected to laboratory laptops via a microchip and controlled with custom software that allowed the scientists to precisely monitor pressure, mass, and water flowing through the coffee.
The scientists measured the total dissolved solids to determine the rate at which coffee is dissolved, comparing brews without a channel to those with artificially induced channels. They found that, indeed, channeling adversely affected extraction yields. However, channeling does not have an impact on the rate at which water flows through the espresso puck.
“That is mostly due to the structural rearrangement of coffee grounds under pressure,” Lisicki said. “When the dry coffee puck is hit with water under high pressure—as high as 10 times the atmospheric pressure, so roughly the pressure 100 meters below the sea surface—it compacts and swells up. So even though water can find a preferential path, there is still significant resistance limiting the flow.”
The team is now factoring their results into numerical and theoretical models of porous bed extraction. They are also compiling an atlas of the different kinds of espresso pucks based on micro-CT imaging of the coffee.
“What we have found can help the coffee industry brew with more knowledge,” said Myck. “Many people follow procedures based on unconfirmed intuitions or claims which prove to have confirmation. What’s more, we have really interesting data regarding pressure-induced flow in coffee, the results of which have been a surprise to us as well. Our approach may let us finally understand the magic that happens inside your coffee machine.”
Dancing sea turtles, the discovery of an Egyptian pharaoh’s tomb, perfectly boiled eggs, and more.
X-ray image of the PHerc.172 scroll Credit: Vesuvius Challenge
It’s a regrettable reality that there is never time to cover all the interesting scientific stories we come across each month. In the past, we’ve featured year-end roundups of cool science stories we (almost) missed. This year, we’re experimenting with a monthly collection. February’s list includes dancing sea turtles, the secret to a perfectly boiled egg, the latest breakthrough in deciphering the Herculaneum scrolls, the discovery of an Egyptian pharaoh’s tomb, and more.
Dancing sea turtles
There is growing evidence that certain migratory animal species (turtles, birds, some species of fish) are able to exploit the Earth’s magnetic field for navigation, using it both as a compass to determine direction and as a kind of “map” to track their geographical position while migrating. A paper published in the journal Nature offers evidence of a possible mechanism for this unusual ability, at least in loggerhead sea turtles, who perform an energetic “dance” when they follow magnetic fields to a tasty snack.
Sea turtles make impressive 8,000-mile migrations across oceans and tend to return to the same feeding and nesting sites. The authors believe they achieve this through their ability to remember the magnetic signature of those areas and store them in a mental map. To test that hypothesis, the scientists placed juvenile sea turtles into two large tanks of water outfitted with large coils to create magnetic signatures at specific locations within the tanks. One tank features such a location that had food; the other had a similar location without food.
They found that the sea turtles in the first tank performed distinctive “dancing” moves when they arrived at the area associated with food: tilting their bodies, dog-paddling, spinning in place, or raising their head near or above the surface of the water. When they ran a second experiment using different radio frequencies, they found that the change interfered with the turtles’ internal compass, and they could not orient themselves while swimming. The authors concluded that this is compelling evidence that the sea turtles can distinguish between magnetic fields, possibly relying on complex chemical reactions, i.e., “magnetoreception.” The map sense, however, likely relies on a different mechanism.
Archaeologists found a simple tomb near Luxor and identified it as the 3,500-year-old burial site of King Thutmose II. Credit: Egypt’s Ministry of Tourism and Antiquities
Thutmose II was the fourth pharaoh of the Tutankhamun (18th) dynasty. He reigned only about 13 years and married his half-sister Hatshepsut (who went on to become the sixth pharaoh in the dynasty). Archaeologists have now confirmed that a tomb built underneath a waterfall in the mountains in Luxor and discovered in 2022 is the final resting place of Thutmose II. It’s the last of the 18th dynasty royal tombs to be found, more than a century after Tutankhamun’s tomb was found in 1922.
When it was first found, archaeologists thought the tomb might be that of a king’s wife, given its close proximity to Hatshepsut’s tomb and those of the wives of Thutmose III. But they found fragments of alabaster vases inscribed with Thutmose II’s name, along with scraps of religious burial texts and plaster fragments on the partially intact ceiling with traces of blue paint and yellow stars—typically only found in kings’ tombs. Something crucial was missing, however: the actual mummy and grave goods of Thutmose II.
It’s long been assumed that the king’s mummy was discovered in the 19th century at another site called Deir el-Bahari. But archaeologist Piers Litherland, who headed the British team that discovered the tomb, thinks that identification was in error. An inscription stated that Hatshepsut had the tomb’s contents relocated due to flooding. Litherland believes the pharaoh’s actual mummy is buried in a second tomb. Confirmation (or not) of his hypothesis won’t come until after archaeologists finish excavating what he thinks is the site of that second tomb, which is currently buried under multiple layers of rock and plaster.
Hidden images in Pollock paintings
“Troubled Queen” reveals a “hidden” figure, possibly a soldier. Credit: D.A. Morrissette et al., CNS Spectrums 2025
Physicists have long been fascinated by the drip paintings of “splatter master” Jackson Pollock, pondering the presence of fractal patterns (or lack thereof), as well as the presence of curls and coils in his work and whether the artist deliberately exploited a well-known fluid dynamics effect to achieve them—or deliberately avoided them. Now psychiatrists are getting into the game, arguing in a paper published in CNS Spectrums that Pollock—known to incorporate images into his early pre-drip paintings—also used many of the same images repeatedly in his later abstract drip paintings.
People have long claimed to see images in those drip paintings, but the phenomenon is usually dismissed by art critics as a trick of human perception, much like the fractal edges of Rorschach ink blots can fool the eye and mind. The authors of this latest paper analyzed Pollock’s early painting “Troubled Queen” and found multiple images incorporated into the painting, which they believe establishes a basis for their argument that Pollock also incorporated such images into his later drip painting, albeit possibly subconsciously.
“Seeing an image once in a drip painting could be random,” said co-author Stephen M. Stahl of the University of California, San Diego. “Seeing the same image twice in different paintings could be a coincidence. Seeing it three or more times—as is the case for booze bottles, monkeys and gorillas, elephants, and many other subjects and objects in Pollock’s paintings—makes those images very unlikely to be randomly provoked perceptions without any basis in reality.”
Soap opera in the maze: Geometry matters in Marangoni flows.
Every fall, the American Physical Society exhibits a Gallery of Fluid Motion, which recognizes the innate artistry of images and videos derived from fluid dynamics research. Several years ago, physicists at the University of California, Santa Barbara (UCSB) submitted an entry featuring a pool of red dye, propelled by a few drops of soap acting as a surfactant, that seemed to “know” how to solve a maze whose corridors were filled with milk. This is unusual since one would expect the dye to diffuse more uniformly. The team has now solved that puzzle, according to a paper published in Physical Review Letters.
The key factor is surface tension, specifically a phenomenon known as the Marangoni effect, which also drives the “coffee ring effect” and the “tears of wine” phenomenon. If you spread a thin film of water on your kitchen counter and place a single drop of alcohol in the center, you’ll see the water flow outward, away from the alcohol. The difference in their alcohol concentrations creates a surface tension gradient, driving the flow.
In the case of the UCSB experiment, the soap reduces local surface tension around the red dye to set the dye in motion. There are also already surfactants in the milk that work in combination with the soapy surfactant to “solve” the maze. The milk surfactants create varying points of resistance as the dye makes its way through the maze. A dead end or a small space will have more resistance, redirecting the dye toward routes with less resistance—and ultimately to the maze’s exit. “That means the added surfactant instantly knows the layout of the maze,” said co-author Paolo Luzzatto-Fegiz.
There’s more than one way to boil an egg, whether one likes it hard-boiled, soft-boiled, or somewhere in between. The challenge is that eggs have what physicists call a “two-phase” structure: The yolk cooks at 65° Celsius, while the white (albumen) cooks at 85° Celsius. This often results in overcooked yolks or undercooked whites when conventional methods are used. Physicists at the Italian National Research Council think they’ve cracked the case: The perfectly cooked egg is best achieved via a painstaking process called “periodic cooking,” according to a paper in the journal Communications Engineering.
They started with a few fluid dynamics simulations to develop a method and then tested that method in the laboratory. The process involves transferring a cooking egg every two minutes—for 32 minutes—between a pot of boiling water (100° Celsius) and a bowl of cold water (30° Celsius). They compared their periodically cooked eggs with traditionally prepared hard-boiled and soft-boiled eggs, as well as eggs prepared using sous vide. The periodically cooked eggs ended up with soft yolks (typical of sous vide eggs) and a solidified egg white with a consistency between sous vide and soft-boiled eggs. Chemical analysis showed the periodically cooked eggs also contained more healthy polyphenols. “Periodic cooking clearly stood out as the most advantageous cooking method in terms of egg nutritional content,” the authors concluded.
X-ray scans and AI reveal the inside of an ancient scroll. Credit: Vesuvius Challenge
The Vesuvius Challenge is an ongoing project that employs “digital unwrapping” and crowd-sourced machine learning to decipher the first letters from previously unreadable ancient scrolls found in an ancient Roman villa at Herculaneum. The 660-plus scrolls stayed buried under volcanic mud until they were excavated in the 1700s from a single room that archaeologists believe held the personal working library of an Epicurean philosopher named Philodemus. The badly singed, rolled-up scrolls were so fragile that it was long believed they would never be readable, as even touching them could cause them to crumble.
In 2023, the Vesuvius Challenge made its first award for deciphering the first letters, and last year, the project awarded the grand prize of $700,000 for producing the first readable text. The latest breakthrough is the successful generation of the first X-ray image of the inside of a scroll (PHerc. 172) housed in Oxford University’s Bodleian Libraries—a collaboration with the Vesuvius Challenge. The scroll’s ink has a unique chemical composition, possibly containing lead, which means it shows up more clearly in X-ray scans than other Herculaneum scrolls that have been scanned.
The machine learning aspect of this latest breakthrough focused primarily on detecting the presence of ink, not deciphering the characters or text. Oxford scholars are currently working to interpret the text. The first word to be translated was the Greek word for “disgust,” which appears twice in nearby columns of text. Meanwhile, the Vesuvius Challenge collaborators continue to work to further refine the image to make the characters even more legible and hope to digitally “unroll” the scroll all the way to the end, where the text likely indicates the title of the work.
What ancient Egyptian mummies smell like
Mummified bodies in the exhibition area of the Egyptian Museum in Cairo. Credit: Emma Paolin
Much of what we know about ancient Egyptian embalming methods for mummification comes from ancient texts, but there are very few details about the specific spices, oils, resins, and other ingredients used. Science can help tease out the secret ingredients. For instance, a 2018 study analyzed organic residues from a mummy’s wrappings with gas chromatography-mass spectrometry and found that the wrappings were saturated with a mixture of plant oil, an aromatic plant extract, a gum or sugar, and heated conifer resin. Researchers at University College London have now identified the distinctive smells associated with Egyptian mummies—predominantly”woody,” “spicy,” and “sweet,” according to a paper published in the Journal of the American Chemical Society.
The team coupled gas chromatography with mass spectrometry to measure chemical molecules emitted by nine mummified bodies on display at the Egyptian Museum in Cairo and then asked a panel of trained human “sniffers” to describe the samples smells, rating them by quality, intensity, and pleasantness. This enabled them to identify whether a given odor molecule came from the mummy itself, conservation products, pesticides, or the body’s natural deterioration. The work offers additional clues into the materials used in mummification, as well as making it possible for the museum to create interactive “smellscapes” in future displays so visitors can experience the scents as well as the sights of ancient Egyptian mummies.
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.
Deposit morphologies for a settling particle. When increasing either the injection volume or the settling height, the deposit radius increases. Credit: M. Souzy et al., 2025
They used spherical borosilicate glass beads of varying diameters to represent the grains of salt and loaded different fixed volumes of beads into cylindrical tubes. Then they slid open the tube’s bottom to release the beads, capturing how they fell and settled with a Nikon D300 camera placed at the top of the tank. The tank was illuminated from below by a uniform LED light screen and diffuser to get an even background.
The physicists found that gravity will pull a single particle to the bottom of the tank, creating a small wake drag that affects the flow of water around it. That perturbation becomes much more complicated when many large particles are released at once, each with its own wake that affects its neighbors. So, the falling particles start to shift horizontally, distributing the falling particles in an expanding circular pattern.
Particles released from a smaller height fall faster and form a pattern with a clean central region. Those released from a greater height take longer to fall to the bottom, and the cloud of particles expands radially until the particles are far enough apart not to be influenced by the wakes of neighboring particles such that they no longer form a cloud. In that case, you end up with a homogeneous salt ring deposit.
“These are the main physical ingredients, and despite its apparent simplicity, this phenomenon encompasses a wide range of physical concepts such as sedimentation, non-creeping flow, long-range interactions between multiple bodies, and wake entrainment,” said Souzy. “Things get even more interesting once you realize larger particles are more radially shifted than small ones, which means you can sort particles by size just by dropping them into a water tank. It was a great overall experience, because we soon realized our simple observation of daily life conceals a rich variety of physical mechanisms.”
Those phenomena are just as relevant outside the kitchen, according to the authors, most notably in such geophysical and industrial contexts as “the discharge of dredged materials and industrial waste into rivers lakes and oceans,” they wrote. “In scenarios involving contaminated waste, comprehending the behavior of both the solid waste and the interacting fluid is crucial.”
Bronze Age combat, moral philosophy and Reddit’s AITA, Mondrian’s fractal tree, and seven other fascinating papers.
There is rarely time to write about every cool science paper that comes our way; many worthy candidates sadly fall through the cracks over the course of the year. But as 2024 comes to a close, we’ve gathered ten of our favorite such papers at the intersection of science and culture as a special treat, covering a broad range of topics: from reenacting Bronze Age spear combat and applying network theory to the music of Johann Sebastian Bach, to Spider-Man inspired web-slinging tech and a mathematical connection between a turbulent phase transition and your morning cup of coffee. Enjoy!
Reenacting Bronze Age spear combat
An experiment with experienced fighters who spar freely using different styles. Credit: Valerio Gentile/CC BY
The European Bronze Age saw the rise of institutionalized warfare, evidenced by the many spearheads and similar weaponry archaeologists have unearthed. But how might these artifacts be used in actual combat? Dutch researchers decided to find out by constructing replicas of Bronze Age shields and spears and using them in realistic combat scenarios. They described their findings in an October paper published in the Journal of Archaeological Science.
There have been a couple of prior experimental studies on bronze spears, but per Valerio Gentile (now at the University of Gottingen) and coauthors, practical research to date has been quite narrow in scope, focusing on throwing weapons against static shields. Coauthors C.J. van Dijk of the National Military Museum in the Netherlands and independent researcher O. Ter Mors each had more than a decade of experience teaching traditional martial arts, specializing in medieval polearms and one-handed weapons. So they were ideal candidates for testing the replica spears and shields.
Of course, there is no direct information on prehistoric fighting styles, so van Dijk and Mors relied on basic biomechanics of combat movements with similar weapons detailed in historic manuals. They ran three versions of the experiment: one focused on engagement and controlled collisions, another on delivering wounding body blows, and the third on free sparring. They then studied wear marks left on the spearheads and found they matched the marks found on similar genuine weapons excavated from Bronze Age sites. They also gleaned helpful clues to the skills required to use such weapons.
Shimmer Wall, The Franklin Institute, Philadelphia, Pennsylvania. Credit: Ned Kahn
Environmental artist and sculptor Ned Kahn is famous for his kinematic building facades, inspired by his own background in science. An exterior wall on the Children’s Museum of Pittsburgh, for instance, consists of hundreds of flaps that move in response to wind, creating distinctive visual patterns. Kahn used the same method to create his Shimmer Wall at Philadelphia’s Franklin Institute, as well as several other similar projects.
Physicists at Sorbonne Universite in Paris have studied videos of Kahn’s kinetic facades and conducted experiments to measure the underlying physical mechanisms, outlined in a November paper published in the journal Physical Review Fluids. The authors analyzed 18 YouTube videos taken of six of Kahn’s kinematic facades, working with Kahn and building management to get the dimensions of the moving plates, scaling up from the video footage to get further information on spatial dimensions.
They also conducted their own wind tunnel experiments, using strings of pendulum plates. Their measurements confirmed that the kinetic patterns were propagating waves to create the flickering visual effects. The plates’ movement is driven primarily by their natural resonant frequencies at low speeds, and by pressure fluctuations from the wind at higher speeds.
Trajectories in time traced out by turbulent puffs as they move along a simulated pipe and in experiments, with blue regions indicate puff “traffic jams.” Credit: Grégoire Lemoult et al., 2024
Physicists have been studying turbulence for centuries, particularly the transitional period where flows shift from predictably smooth (laminar flow) to highly turbulent. That transition is marked by localized turbulent patches known as “puffs,” which often form in fluids flowing through a pipe or channel. In an October paper published in the journal Nature Physics, physicists used statistical mechanics to reveal an unexpected connection between the process of brewing coffee and the behavior of those puffs.
Traditional mathematical models of percolation date back to the 1940s. Directed percolation is when the flow occurs in a specific direction, akin to how water moves through freshly ground coffee beans, flowing down in the direction of gravity. There’s a sweet spot for the perfect cuppa, where the rate of flow is sufficiently slow to absorb most of the flavor from the beans, but also fast enough not to back up in the filter. That sweet spot in your coffee brewing process corresponds to the aforementioned laminar-turbulent transition in pipes.
Physicist Nigel Goldenfeld of the University of California, San Diego, and his coauthors used pressure sensors to monitor the formation of puffs in a pipe, focusing on how puff-to-puff interactions influenced each other’s motion. Next, they tried to mathematically model the relevant phase transitions to predict puff behavior. They found that the puffs behave much like cars moving on a freeway during rush hour: they are prone to traffic jams—i.e., when a turbulent patch matches the width of the pipe, causing other puffs to build up behind it—that form and dissipate on their own. And they tend to “melt” at the laminar-turbulent transition point.
In a network representation of music, notes are represented by nodes, and transition between notes are represented by directed edges connecting the nodes. Credit: S. Kulkarni et al., 2024
When you listen to music, does your ability to remember or anticipate the piece tell you anything about its structure? Physicists at the University of Pennsylvania developed a model based on network theory to do just that, describing their work in a February paper published in the journal Physical Review Research. Johann Sebastian Bach’s works were an ideal choice given the highly mathematical structure, plus the composer was so prolific, across so many very different kinds of musical compositions—preludes, fugues, chorales, toccatas, concertos, suites, and cantatas—as to allow for useful comparisons.
First, the authors built a simple “true” network for each composition, in which individual notes served as “nodes” and the transitions from note to note served as “edges” connecting them. Then they calculated the amount of information in each network. They found it was possible to tell the difference between compositional forms based on their information content (entropy). The more complex toccatas and fugues had the highest entropy, while simpler chorales had the lowest.
Next, the team wanted to quantify how effectively this information was communicated to the listener, a task made more difficult by the innate subjectivity of human perception. They developed a fuzzier “inferred” network model for this purpose, capturing an essential aspect of our perception: we find a balance between accuracy and cost, simplifying some details so as to make it easier for our brains to process incoming information like music.
The results: There were fewer differences between the true and inferred networks for Bach’s compositions than for randomly generated networks, suggesting that clustering and the frequent repetition of transitions (represented by thicker edges) in Bach networks were key to effectively communicating information to the listener. The next step is to build a multi-layered network model that incorporates elements like rhythm, timbre, chords, or counterpoint (a Bach specialty).
Count me among the many people practically addicted to Reddit’s “Am I the Asshole” (AITA) forum. It’s such a fascinating window into the intricacies of how flawed human beings navigate different relationships, whether personal or professional. That’s also what makes it a fantastic source of illustrative common-place dilemmas of moral decision-making for philosophers like Daniel Yudkin of the University of Pennsylvania. Relational context matters, as Yudkin and several co-authors ably demonstrated in a PsyArXiv preprint earlier this year.
For their study, Yudkin et al. compiled a dataset of nearly 370,000 AITA posts, along with over 11 million comments, posted between 2018 and 2021. They used machine learning to analyze the language used to sort all those posts into different categories. They relied on an existing taxonomy identifying six basic areas of moral concern: fairness/proportionality, feelings, harm/offense, honesty, relational obligation, and social norms.
Yudkin et al. identified 29 of the most common dilemmas in the AITA dataset and grouped them according to moral theme. Two of the most common were relational transgression and relational omission (failure to do what was expected), followed by behavioral over-reaction and unintended harm. Cheating and deliberate misrepresentation/dishonesty were the moral dilemmas rated most negatively in the dataset—even more so than intentional harm. Being judgmental was also evaluated very negatively, as it was often perceived as being self-righteous or hypocritical. The least negatively evaluated dilemmas were relational omissions.
As for relational context, cheating and broken promise dilemmas typically involved romantic partners like boyfriends rather than one’s mother, for example, while mother-related dilemmas more frequently fell under relational omission. Essentially, “people tend to disappoint their mothers but be disappointed by their boyfriends,” the authors wrote. Less close relationships, by contrast, tend to be governed by “norms of politeness and procedural fairness.” Hence, Yudkin et al. prefer to think of morality “less as a set of abstract principles and more as a ‘relational toolkit,’ guiding and constraining behavior according to the demands of the social situation.”
De grijze boom (Gray tree) by Piet Mondrian, 1911. Credit: Public domain
Leonardo da Vinci famously invented a so-called “rule of trees” as a guide to realistically depicting trees in artistic representations according to their geometric proportions. In essence, if you took all the branches of a given tree, folded them up and compressed them into something resembling a trunk, that trunk would have the same thickness from top to bottom. That rule in turn implies a fractal branching pattern, with a scaling exponent of about 2 describing the proportions between the diameters of nearby boughs and the number of boughs with a given diameter.
According to the authors of a preprint posted to the physics arXiv in February, however, recent biological research suggests a higher scaling exponent of 3 known as Murray’s Law, for the rule of trees. Their analysis of 16th century Islamic architecture, Japanese paintings from the Edo period, and 20th century European art showed fractal scaling between 1.5 and 2.5. However, when they analyzed an abstract tree painting by Piet Mondrian, they found it exhibited fractal scaling of 3, before mathematicians had formulated Murray’s Law, even though Mondrian’s tree did not feature explicit branching.
The findings intrigued physicist Richard Taylor of the University of Oregon, whose work over the last 20 years includes analyzing fractal patterns in the paintings of Jackson Pollock. “In particular, I thought the extension to Mondrian’s ‘trees’ was impressive,” he told Ars earlier this year. “I like that it establishes a connection between abstract and representational forms. It makes me wonder what would happen if the same idea were to be applied to Pollock’s poured branchings.”
Taylor himself published a 2022 paper about climate change and how nature’s stress-reducing fractals might disappear in the future. “If we are pessimistic for a moment, and assume that climate change will inevitably impact nature’s fractals, then our only future source of fractal aesthetics will be through art, design and architecture,” he said. “This brings a very practical element to studies like [this].”
A DNA study identified descendants of George Washington from unmarked remains. Credit: Public domain
DNA profiling is an incredibly useful tool in forensics, but the most common method—short tandem repeat (STR) analysis—typically doesn’t work when remains are especially degraded, especially if said remains have been preserved with embalming methods using formaldehyde. This includes the remains of US service members who died in such past conflicts as World War II, Korea, Vietnam, and the Cold War. That’s why scientists at the Armed Forces Medical Examiner System’s identification lab at the Dover Air Force Base have developed new DNA sequencing technologies.
They used those methods to identify the previously unmarked remains of descendants of George Washington, according to a March paper published in the journal iScience. The team tested three sets of remains and compared the results with those of a known living descendant, using methods for assessing paternal and maternal relationships, as well as a new method for next-generation sequencing data involving some 95,000 single-nucleotide polymorphisms (SNPs) in order to better predict more distant ancestry. The combined data confirmed that the remains belonged to Washington’s descendants and the new method should help do the same for the remains of as-yet-unidentified service members.
In related news, in July, forensic scientists successfully used descendant DNA to identify a victim of the 1921 Tulsa massacre in Oklahoma City, buried in a mass grave containing more than a hundred victims. C.L. Daniel was a World War I veteran, still in his 20s when he was killed. More than 120 such graves have been found since 2020, with DNA collected from around 30 sets of remains, but this is the first time those remains have been directly linked to the massacre. There are at least 17 other victims in the grave where Daniel’s remains were found.
stream of liquid silk quickly turns to a strong fiber that sticks to and lifts objects. Credit: Marco Lo Presti et al., 2024
Over the years, researchers in Tufts University’s Silklab have come up with all kinds of ingenious bio-inspired uses for the sticky fibers found in silk moth cocoons: adhesive glues, printable sensors, edible coatings, and light-collecting materials for solar cells, to name a few. Their latest innovation is a web-slinging technology inspired by Spider-Man’s ability to shoot webbing from his wrists, described in an October paper published in the journal Advanced Functional Materials.
Coauthor Marco Lo Presti was cleaning glassware with acetone in the lab one day when he noticed something that looked a lot like webbing forming on the bottom of a glass. He realized this could be the key to better replicating spider threads for the purpose of shooting the fibers from a device like Spider-Man—something actual spiders don’t do. (They spin the silk, find a surface, and draw out lines of silk to build webs.)
The team boiled silk moth cocoons in a solution to break them down into proteins called fibroin. The fibroin was then extruded through bore needles into a stream. Spiking the fibroin solution with just the right additives will cause it to solidify into fiber once it comes into contact with air. For the web-slinging technology, they added dopamine to the fibroin solution and then shot it through a needle in which the solution was surrounded by a layer of acetone, which triggered solidification.
The acetone quickly evaporated, leaving just the webbing attached to whatever object it happened it hit. The team tested the resulting fibers and found they could lift a steel bolt, a tube floating on water, a partially buried scalpel and a wooden block—all from as far away as 12 centimeters. Sure, natural spider silk is still about 1000 times stronger than these fibers, but it’s still a significant step forward that paves the way for future novel technological applications.
In 1181, astronomers in China and Japan recorded the appearance of a “guest star” that shone as bright as Saturn and was visible in the sky for six months. We now know it was a supernova (SN1181), one of only five such known events occurring in our Milky Way. Astronomers got a closer look at the remnant of that supernova and have determined the nature of strange filaments resembling dandelion petals that emanate from a “zombie star” at its center, according to an October paper published in The Astrophysical Journal Letters.
The Chinese and Japanese astronomers only recorded an approximate location for the unusual sighting, and for centuries no one managed to make a confirmed identification of a likely remnant from that supernova. Then, in 2021, astronomers measured the speed of expansion of a nebula known as Pa 30, which enabled them to determine its age: around 1,000 years, roughly coinciding with the recorded appearance of SN1181. PA 30 is an unusual remnant because of its zombie star—most likely itself a remnant of the original white dwarf that produced the supernova.
This latest study relied on data collected by Caltech’s Keck Cosmic Web Imager, a spectrograph at the Keck Observatory in Hawaii. One of the unique features of this instrument is that it can measure the motion of matter in a supernova and use that data to create something akin to a 3D movie of the explosion. The authors were able to create such a 3D map of P 30 and calculated that the zombie star’s filaments have ballistic motion, moving at approximately 1,000 kilometers per second.
Nor has that velocity changed since the explosion, enabling them to date that event almost exactly to 1181. And the findings raised fresh questions—namely, the ejected filament material is asymmetrical—which is unusual for a supernova remnant. The authors suggest that asymmetry may originate with the initial explosion.
There’s also a weird inner gap around the zombie star. Both will be the focus of further research.
Never underestimate the importance of marginalia in old manuscripts. Scholars from the University of Edinburgh and KU Leuven in Belgium can attest to that, having discovered a fragment of “lost” music from 16th-century pre-Reformation Scotland in a collection of worship texts. The team was even able to reconstruct the fragment and record it to get a sense of what music sounded like from that period in northeast Scotland, as detailed in a December paper published in the journal Music and Letters.
King James IV of Scotland commissioned the printing of several copies of The Aberdeen Breviary—a collection of prayers, hymns, readings, and psalms for daily worship—so that his subjects wouldn’t have to import such texts from England or Europe. One 1510 copy, known as the “Glamis copy,” is currently housed in the National Library of Scotland in Edinburgh. It was while examining handwritten annotations in this copy that the authors discovered the musical fragment on a page bound into the book—so it hadn’t been slipped between the pages at a later date.
The team figured out the piece was polyphonic, and then realized it was the tenor part from a harmonization for three or four voices of the hymn “Cultor Dei,” typically sung at night during Lent. (You can listen to a recording of the reconstructed composition here.) The authors also traced some of the history of this copy of The Aberdeen Breviary, including its use at one point by a rural chaplain at Aberdeen Cathedral, before a Scottish Catholic acquired it as a family heirloom.
“Identifying a piece of music is a real ‘Eureka’ moment for musicologists,” said coauthor David Coney of Edinburgh College of Art. “Better still, the fact that our tenor part is a harmony to a well-known melody means we can reconstruct the other missing parts. As a result, from just one line of music scrawled on a blank page, we can hear a hymn that had lain silent for nearly five centuries, a small but precious artifact of Scotland’s musical and religious traditions.”
Jennifer is a senior reporter 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.
There’s a common popular science demonstration involving “soap boats,” in which liquid soap poured onto the surface of water creates a propulsive flow driven by gradients in surface tension. But it doesn’t last very long since the soapy surfactants rapidly saturate the water surface, eliminating that surface tension. Using ethanol to create similar “cocktail boats” can significantly extend the effect because the alcohol evaporates rather than saturating the water.
That simple classroom demonstration could also be used to propel tiny robotic devices across liquid surfaces to carry out various environmental or industrial tasks, according to a preprint posted to the physics arXiv. The authors also exploited the so-called “Cheerios effect” as a means of self-assembly to create clusters of tiny ethanol-powered robots.
As previously reported, those who love their Cheerios for breakfast are well acquainted with how those last few tasty little “O”s tend to clump together in the bowl: either drifting to the center or to the outer edges. The “Cheerios effect is found throughout nature, such as in grains of pollen (or, alternatively, mosquito eggs or beetles) floating on top of a pond; small coins floating in a bowl of water; or fire ants clumping together to form life-saving rafts during floods. A 2005 paper in the American Journal of Physics outlined the underlying physics, identifying the culprit as a combination of buoyancy, surface tension, and the so-called “meniscus effect.”
It all adds up to a type of capillary action. Basically, the mass of the Cheerios is insufficient to break the milk’s surface tension. But it’s enough to put a tiny dent in the surface of the milk in the bowl, such that if two Cheerios are sufficiently close, the curved surface in the liquid (meniscus) will cause them to naturally drift toward each other. The “dents” merge and the “O”s clump together. Add another Cheerio into the mix, and it, too, will follow the curvature in the milk to drift toward its fellow “O”s.
Physicists made the first direct measurements of the various forces at work in the phenomenon in 2019. And they found one extra factor underlying the Cheerios effect: The disks tilted toward each other as they drifted closer in the water. So the disks pushed harder against the water’s surface, resulting in a pushback from the liquid. That’s what leads to an increase in the attraction between the two disks.
Enlarge/ Shark intestines are naturally occurring Tesla valves; scientists have figured out how to mimic their unique structure.
Sarah L. Keller/University of Washington
Scientists at the University of Washington have re-created the distinctive spiral shapes of shark intestines in 3D-printed pipes in order to study the unique fluid flow inside the spirals. Their prototypes kept fluids flowing in one preferred direction with no need for flaps to control that flow and performed significantly better than so-called “Tesla valves,” particularly when made of soft polymers, according to a new paper published in the Proceedings of the National Academy of Sciences.
As we’ve reported previously, in 1920, Serbian-born inventor Nikola Tesla designed and patented what he called a “valvular conduit“: a pipe whose internal design ensures that fluid will flow in one preferred direction, with no need for moving parts, making it ideal for microfluidics applications, among other uses. The key to Tesla’s ingenious valve design is a set of interconnected, asymmetric, tear-shaped loops.
In his patent application, Tesla described this series of 11 flow-control segments as being made of “enlargements, recessions, projections, baffles, or buckets which, while offering virtually no resistance to the passage of fluid in one direction, other than surface friction, constitute an almost impassable barrier to its flow in the opposite direction.” And because it achieves this with no moving parts, a Tesla valve is much more resistant to the wear and tear of frequent operation.
Tesla claimed that water would flow through his valve 200 times slower in one direction than another, which may have been an exaggeration. A team of scientists at New York University built a working Tesla valve in 2021, in accordance with the inventor’s design, and tested that claim by measuring the flow of water through the valve in both directions at various pressures. The scientists found the water only flowed about two times slower in the nonpreferred direction.
Flow rate proved to be a critical factor. The valve offered very little resistance at slow flow rates, but once that rate increased above a certain threshold, the valve’s resistance would increase as well, generating turbulent flows in the reverse direction, thereby “plugging” the pipe with vortices and disruptive currents. So it actually works more like a switch and can also help smooth out pulsing flows, akin to how AC/DC converters turn alternating currents into direct currents. That may even have been Tesla’s original intent in designing the valve, given that his biggest claim to fame is inventing both the AC motor and an AC/DC converter.
It helps to be a shark
Enlarge/ Different kinds of sharks have intestines with different spiral patterns that favor fluid flow in one direction.
Ido Levin
The Tesla valve also provides a useful model for how food moves through the digestive system of many species of shark. In 2020, Japanese researchers reconstructed micrographs of histological sections from a species of catshark into a three-dimensional model, offering a tantalizing glimpse of the anatomy of a scroll-type spiral intestine. The following year, scientists took CT scans of shark intestines and concluded that the intestines are naturally occurring Tesla valves.
That’s where the work of UW postdoc Ido Levin and his co-authors comes in. They had questions about the 2021 research in particular. “Flow asymmetry in a pipe with no moving flaps has tremendous technological potential, but the mechanism was puzzling,” said Levin. “It was not clear which parts of the shark’s intestinal structure contributed to the asymmetry and which served only to increase the surface area for nutrient uptake.”
Levin et al. 3D-printed several pipes with an internal helical structure mimicking that of shark intestines, varying certain geometrical parameters like the number of turns or the pitch angle of the helix. It was admittedly an idealized structure, so the team was delighted when the first batch, made from rigid materials, produced the hoped-for flow asymmetry. After further fine-tuning of the parameters, the rigid printed pipes produced flow asymmetries that matched or exceeded Tesla valves.
Enlarge/ Eight of the team’s 3D-printed prototypes with various interior helices.
Ido Levin/University of Washington
But the researchers weren’t done yet. “[Prior work] showed that if you connect these intestines in the same direction as a digestive tract, you get a faster flow of fluid than if you connect them the other way around. We thought this was very interesting from a physics perspective,” said Levin last year while presenting preliminary results at the 67th Annual Biophysical Society Meeting. “One of the theorems in physics actually states that if you take a pipe, and you flow fluid very slowly through it, you have the same flow if you invert it. So we were very surprised to see experiments that contradict the theory. But then you remember that the intestines are not made out of steel—they’re made of something soft, so while fluid flows through the pipe, it deforms it.”
That gave Levin et al. the idea to try making their pipes out of soft deformable polymers—the softest commercially available ones that could also be used for 3D printing. That batch of pipes performed seven times better on flow asymmetry than any prior measurements of Tesla valves. And since actual shark intestines are about 100 times softer than the polymers they used, the team thinks they can achieve even better performance, perhaps with hydrogels when they become more widely available as 3D printing continues to evolve. The biggest challenge, per the authors, is finding soft materials that can withstand high deformations.
Finally, because the pipes are three-dimensional, they can accommodate larger fluid volumes, opening up applications in larger commercial devices. “Chemists were already motivated to develop polymers that are simultaneously soft, strong and printable,” said co-author Alshakim Nelson, whose expertise lies in developing new types of polymers. “The potential use of these polymers to control flow in applications ranging from engineering to medicine strengthens that motivation.”
Enlarge/ Some cricket bowlers favor keeping the arm horizontal during delivery, the better to trick the batsmen.
Although the sport of cricket has been around for centuries in some form, the game strategy continues to evolve in the 21st century. Among the newer strategies employed by “bowlers”—the equivalent of the pitcher in baseball—is delivering the ball with the arm horizontally positioned close to the shoulder line, which has proven remarkably effective in “tricking” batsmen in their perception of the ball’s trajectory.
Scientists at Amity University Dubai in the United Arab Emirates were curious about the effectiveness of the approach, so they tested the aerodynamics of cricket balls in wind tunnel experiments. The team concluded that this style of bowling creates a high-speed spinning effect that shifts the ball’s trajectory mid-flight—an effect also seen in certain baseball pitches, according to a new paper published in the journal Physics of Fluids.
“The unique and unorthodox bowling styles demonstrated by cricketers have drawn significant attention, particularly emphasizing their proficiency with a new ball in early stages of a match,” said co-author Kizhakkelan Sudhakaran Siddharth, a mechanical engineer at Amity University Dubai. “Their bowling techniques frequently deceive batsmen, rendering these bowlers effective throughout all phases of a match in almost all formats of the game.”
As previously reported, any moving ball leaves a trail of air as it travels; the inevitable drag slows the ball down. The ball’s trajectory is affected by diameter and speed and by tiny irregularities on the surface. Baseballs, for example, are not completely smooth; they have stitching in a figure-eight pattern. Those stitches are bumpy enough to affect the airflow around the baseball as it’s thrown toward home plate. As a baseball moves, it creates a whirlpool of air around it, commonly known as the Magnus effect. The raised seams churn the air around the ball, creating high-pressure zones in various locations (depending on the pitch type) that can cause deviations in its trajectory.
Physicists have been enthusiastically studying baseballs since the 1940s, when Lyman Briggs became intrigued by whether a curveball actually curves. Initially, he enlisted the aid of the Washington Senators pitching staff at Griffith Stadium to measure the spin of a pitched ball; the idea was to determine how much the curve of a baseball depends on its spin and speed.
Briggs followed up with wind tunnel experiments at the National Bureau of Standards (now the National Institute of Standards and Technology) to make even more precise measurements since he could control most variables. He found that spin rather than speed was the key factor in causing a pitched ball to curve and that a curveball could dip up to 17.5 inches as it travels from the pitcher’s mound to home plate.
Enlarge/ The first recorded photo of a cricket match taken on July 25, 1857, by Roger Fenton.
Public domain
In 2018, we reported on a Utah State University study to explain the fastball’s unexpected twist in experiments using Little League baseballs. The USU scientists fired the balls one by one through a smoke-filled chamber. Two red sensors detected the balls as they zoomed past, triggering lasers that acted as flashbulbs. They then used particle image velocimetry to calculate airflow at any given spot around the ball. Conclusion: It all comes down to spin speed, spin axis, and the orientation of the ball, and there is no meaningful aerodynamical difference between a two-seam fastball and a four-seam fastball.
In 2022, two physicists developed a laser-guided speed measurement system to measure the change in speed of a baseball mid-flight and then used that measurement to calculate the acceleration, the various forces acting on the ball, and the lift and drag. They suggested their approach could also be used for other ball sports like cricket and soccer.
Enlarge/ The Armfield C15-15 Wake Survey Rake measured pressure downstream of the ball.
A.B. Faazil et al., 2024
Similarly, golf ball dimples reduce the drag flow by creating a turbulent boundary layer of air, while the ball’s spin generates lift by creating a higher air pressure area on the bottom of the ball than on the top. The surface patterns on volleyballs can also affect their trajectories. Conventional volleyballs have six panels, but more recent designs have eight panels, a hexagonal honeycomb pattern, or dimples. A 2019 study found that the surface panels on conventional volleyballs can give rise to unpredictable trajectories on float serves (which have no spin), and modifying the surface patterns could make for a more consistent flight.
From a physics standpoint, the float serve is similar to throwing a knuckleball in baseball, which is largely unaffected by the Magnus force because it has no spin. Its trajectory is determined entirely by how the seams affect the turbulent airflow around the baseball. The seams of a baseball can change the speed (velocity) of the air near the ball’s surface, speeding the ball up or slowing it down, depending on whether said seams are on the top or the bottom. The panels on conventional volleyballs have a similar effect.
Inertial confinement fusion is one method for generating energy through nuclear fusion, albeit one plagued by all manner of scientific challenges (although progress is being made). Researchers at LeHigh University are attempting to overcome one specific bugbear with this approach by conducting experiments with mayonnaise placed in a rotating figure-eight contraption. They described their most recent findings in a new paper published in the journal Physical Review E with an eye toward increasing energy yields from fusion.
The work builds on prior research in the LeHigh laboratory of mechanical engineer Arindam Banerjee, who focuses on investigating the dynamics of fluids and other materials in response to extremely high acceleration and centrifugal force. In this case, his team was exploring what’s known as the “instability threshold” of elastic/plastic materials. Scientists have debated whether this comes about because of initial conditions, or whether it’s the result of “more local catastrophic processes,” according to Banerjee. The question is relevant to a variety of fields, including geophysics, astrophysics, explosive welding, and yes, inertial confinement fusion.
The idea behind inertial confinement fusion is simple. To get two atoms to fuse together, you need to bring their nuclei into contact with each other. Both nuclei are positively charged, so they repel each other, which means that force is needed to convince two hydrogen nuclei to touch. In a hydrogen bomb, force is generated when a small fission bomb explodes, compressing a core of hydrogen. This fuses to create heavier elements, releasing a huge amount of energy.
Being killjoys, scientists prefer not to detonate nuclear weapons every time they want to study fusion or use it to generate electricity. Which brings us to inertial confinement fusion. In inertial confinement fusion, the hydrogen core consists of a spherical pellet of hydrogen ice inside a heavy metal casing. The casing is illuminated by powerful lasers, which burn off a large portion of the material. The reaction force from the vaporized material exploding outward causes the remaining shell to implode. The resulting shockwave compresses the center of the core of the hydrogen pellet so that it begins to fuse.
If confinement fusion ended there, the amount of energy released would be tiny. But the energy released due to the initial fusion burn in the center generates enough heat for the hydrogen on the outside of the pellet to reach the required temperature and pressure. So, in the end (at least in computer models), all of the hydrogen is consumed in a fiery death, and massive quantities of energy are released.
That’s the idea anyway. The problem is that hydrodynamic instabilities tend to form in the plasma state—Banerjee likens it to “two materials [that] penetrate one another like fingers” in the presence of gravity or any accelerating field—which in turn reduces energy yields. The technical term is a Rayleigh-Taylor instability, which occurs between two materials of different densities, where the density and pressure gradients move in opposite directions. Mayonnaise turns out to be an excellent analog for investigating this instability in accelerated solids, with no need for a lab setup with high temperature and pressure conditions, because it’s a non-Newtonian fluid.
“We use mayonnaise because it behaves like a solid, but when subjected to a pressure gradient, it starts to flow,” said Banerjee. “As with a traditional molten metal, if you put a stress on mayonnaise, it will start to deform, but if you remove the stress, it goes back to its original shape. So there’s an elastic phase followed by a stable plastic phase. The next phase is when it starts flowing, and that’s where the instability kicks in.”
More mayo, please
2019 video showcasing the rotating wheel Rayleigh Taylor instability experiment at Lehigh University.
His team’s 2019 experiments involved pouring Hellman’s Real Mayonnaise—no Miracle Whip for this crew—into a Plexiglass container and then creating wavelike perturbations in the mayo. One experiment involved placing the container on a rotating wheel in the shape of a figure eight and tracking the material with a high-speed camera, using an image processing algorithm to analyze the footage. Their results supported the claim that the instability threshold is dependent on initial conditions, namely amplitude and wavelength.
This latest paper sheds more light on the structural integrity of fusion capsules used in inertial confinement fusion, taking a closer look at the material properties, the amplitude and wavelength conditions, and the acceleration rate of such materials as they hit the Rayleigh-Taylor instability threshold. The more scientists know about the phase transition from the elastic to the stable phase, the better they can control the conditions and maintain either an elastic or plastic phase, avoiding the instability. Banerjee et al. were able to identify the conditions to maintain the elastic phase, which could inform the design of future pellets for inertial confinement fusion.
That said, the mayonnaise experiments are an analog, orders of magnitude away from the real-world conditions of nuclear fusion, which Banerjee readily acknowledges. He is nonetheless hopeful that future research will improve the predictability of just what happens within the pellets in their high-temperature, high-pressure environments. “We’re another cog in this giant wheel of researchers,” he said. “And we’re all working towards making inertial fusion cheaper and therefore, attainable.”
Light-scattering microparticles reveal the flow pattern for the reverse (sucking) mode of a sprinkler, showing vortices and complex flow patterns forming inside the central chamber. Credit: K. Wang et al., 2024
A typical lawn sprinkler features various nozzles arranged at angles on a rotating wheel; when water is pumped in, they release jets that cause the wheel to rotate. But what would happen if the water were sucked into the sprinkler instead? In which direction would the wheel turn then, or would it even turn at all? That’s the essence of the “reverse sprinkler” problem that physicists like Richard Feynman, among others, have grappled with since the 1940s. Now, applied mathematicians at New York University think they’ve cracked the conundrum, per a recent paper published in the journal Physical Review Letters—and the answer challenges conventional wisdom on the matter.
“Our study solves the problem by combining precision lab experiments with mathematical modeling that explains how a reverse sprinkler operates,” said co-author Leif Ristroph of NYU’s Courant Institute. “We found that the reverse sprinkler spins in the ‘reverse’ or opposite direction when taking in water as it does when ejecting it, and the cause is subtle and surprising.”
Ristroph’s lab frequently addresses these kinds of colorful real-world puzzles. For instance, back in 2018, Ristroph and colleagues fine-tuned the recipe for the perfect bubble based on experiments with soapy thin films. (You want a circular wand with a 1.5-inch perimeter, and you should gently blow at a consistent 6.9 cm/s.) In 2021, the Ristroph lab looked into the formation processes underlying so-called “stone forests” common in certain regions of China and Madagascar. These pointed rock formations, like the famed Stone Forest in China’s Yunnan Province, are the result of solids dissolving into liquids in the presence of gravity, which produces natural convective flows.
In 2021, his lab built a working Tesla valve, in accordance with the inventor’s design, and measured the flow of water through the valve in both directions at various pressures. They found the water flowed about two times slower in the nonpreferred direction. And in 2022, Ristroph studied the surpassingly complex aerodynamics of what makes a good paper airplane—specifically what is needed for smooth gliding. They found that paper airplane aerodynamics differ substantially from conventional aircraft, which rely on airfoils to generate lift.
Enlarge/ Illustration of a “reaction wheel” from Ernst Mach’s Mechanik (1883).
Public domain
The reverse sprinkler problem is associated with Feynman because he popularized the concept, but it actually dates back to a chapter in Ernst Mach’s 1883 textbook The Science of Mechanics (Die Mechanik in Ihrer Entwicklung Historisch-Kritisch Dargerstellt). Mach’s thought experiment languished in relative obscurity until a group of Princeton University physicists began debating the issue in the 1940s.
Feynman was a graduate student there at the time and threw himself into the debate with gusto, even devising an experiment in the cyclotron laboratory to test his hypothesis. (In true Feynman fashion, that experiment culminated with the explosion of a glass carboy used in the apparatus because of the high internal pressure.)
One might intuit that a reverse sprinkler would work just like a regular sprinkler, merely played backward, so to speak. But the physics turns out to be more complicated. “The answer is perfectly clear at first sight,” Feynman wrote in Surely You’re Joking, Mr. Feynman (1985). “The trouble was, some guy would think it was perfectly clear [that the rotation would be] one way, and another guy would think it was perfectly clear the other way.”
Enlarge/ Whether the toilet lid is up or down doesn’t make much difference in the spread of airborne bacterial and viral particles.
File this one under “Studies We Wish Had Let Us Remain Ignorant.” Scientists at the University of Arizona decided to investigate whether closing the toilet lid before flushing reduces cross-contamination of bathroom surfaces by airborne bacterial and viral particles via “toilet plumes.” The bad news is that putting a lid on it doesn’t result in any substantial reduction in contamination, according to their recent paper published in the American Journal of Infection Control. The good news: Adding a disinfectant to the toilet bowl before flushing and using disinfectant dispensers in the tank significantly reduce cross-contamination.
Regarding toilet plumes, we’re not just talking about large water droplets that splatter when a toilet is flushed. Even smaller droplets can form and be spread into the surrounding air, potentially carrying bacteria like E. coli or a virus (e.g., norovirus) if an infected person has previously used said toilet. Pathogens can linger in the bowl even after repeated flushes, just waiting for their chance to launch into the air and spread disease. That’s because larger droplets, in particular, can settle on surfaces before they dry, while smaller ones travel further on natural air currents.
The first experiments examining whether toilet plumes contained contaminated particles were done in the 1950s, and the notion that disease could be spread this way was popularized in a 1975 study. In 2022, physicists and engineers at the University of Colorado, Boulder, managed to visualize toilet plumes of tiny airborne particles ejected from toilets during a flush using a combination of green lasers and cameras. It made for some pretty vivid video footage:
Colorado researchers managed to visualize toilet plumes in 2022 using green lasers and strategically placed cameras.
“If it’s something you can’t see, it’s easy to represent it doesn’t exist,” study co-author John Grimaldi said at the time. They found that the ejected airborne particles could travel up to 6.6 feet per second, reaching heights of 4.9 feet above the toilet within 8 seconds. And if those particles were smaller (less than 5 microns), they could hang around in that air for over a minute.
More relevant to this latest paper, it’s been suggested that closing the lid before flushing could substantially reduce the airborne spread of contaminants. For example, in 2019, researchers at University College Cork deployed bioaerosol sensors in a shared lavatory for a week to monitor the number and size of contaminant particles. They concluded that flushing with the toilet lid down reduced airborne droplets between 30 and 60 percent. But this scenario also increased the diameter of the droplets and bacteria concentration. Leaving the lid down also means the airborne microdroplets are still detectable 16 minutes after flushing, 11 minutes longer than if one flushed with the lid up.
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.
Enlarge/ Engineering researchers at Lehigh University have discovered that sometimes sand can actually flow uphill.
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.
Nor can physicists precisely predict an avalanche. That’s partly because of the sheer number of grains of sand in even a small pile, each of which will interact with several of its immediate neighboring grains simultaneously—and those neighbors shift from one moment to the next. Not even a supercomputer can track the movements of individual grains over time, so the physics of flow in granular media remains a vital area of research.
But grains of sand that collectively flow uphill? That is simply bizarre behavior. Lehigh University engineer James Gilchrist manages the Laboratory for Particle Mixing and Self-Organization and stumbled upon this odd phenomenon while experimenting with “micro-rollers”: polymer particles coated in iron oxide (a process called micro-encapsulation). He was rotating a magnet under a vial of micro-rollers one day and noticed they started to pile uphill. Naturally he and his colleagues had to investigate further.
For their experiments, Gilchrist et al. attached neodymium magnets to a motorized wheel at 90-degree intervals, alternating the outward facing poles. The apparatus also included a sample holder and a USB microscope in a fixed position. The micro-rollers were prepared by suspending them in a glass vial containing ethanol and using a magnet to separate them from dust or any uncoated particles. Once the micro-rollers were clean, they were dried, suspended in fresh ethanol, and loaded onto the sample holder. A vibrating motor agitated the samples to produce flattened granular beds, and the motorized wheel was set in motion to apply magnetic torque. A gaussmeter measured the magnetic field strength relative to orientation.
Uphill granular flow of microrobotic microrollers. Credit: Lehigh University.
The results: each micro-roller began to rotate in response to the magnetic torque, creating pairs that briefly formed and then split, and increasing the magnetic force increased the particle cohesion. This in turn gave the micro-rollers more traction and enabled them to move more quickly, working in concert to counterintuitively flow uphill. In the absence of that magnetic torque, the miro-rollers flowed downhill normally. The torque-induced action was so unexpected that the researchers coined a new term to describe it: a “negative angle of repose” caused by a negative coefficient of friction.
“Up until now, no one would have used these terms,” said Gilchrist. “They didn’t exist. But to understand how these grains are flowing uphill, we calculated what the stresses are that cause them to move in that direction. If you have a negative angle of repose, then you must have cohesion to give a negative coefficient of friction. These granular flow equations were never derived to consider these things, but after calculating it, what came out is an apparent coefficient of friction that is negative.”
It’s an intriguing proof of principle that could one day lead to new ways to control how substances mix or separate, as well as potential swarming microrobotics applications. The scientists have already started building tiny staircases with laser cutters and videotaping the micro-rollers climbing up and down the other. One micro-roller can’t overcome the height of each step, but many working collectively can do so, per Gilchrist.
Enlarge/ Harvard University graduate student Yue Sun won a Milton Van Dyke Award for her video on the hydrodynamics of marbled paper.
Y. Sun/Harvard University et al.
Marbled paper is an art form that dates back at least to the 17th century, when European travelers to the Middle East brought back samples and bound them into albums. Its visually striking patterns arise from the complex hydrodynamics of paint interacting with water, inspiring a winning video entry in this year’s Gallery of Fluid Motion.
The American Physical Society’s Division of Fluid Dynamics sponsors the gallery each year as part of its annual meeting, featuring videos and posters submitted by scientists from all over the world. The objective is to highlight “the captivating science and often breathtaking beauty of fluid motion” and to “celebrate and appreciate the remarkable fluid dynamics phenomena unveiled by researchers and physicists.”
The three videos featured here are the winners of the Milton Van Dyke Awards, which also included three winning posters. There were three additional general video winners—on the atomization of impinging jets, the emergent collective motion of condensate droplets, and the swimming motion of a robotic eel—as well as three poster winners. You can view all the 2023 entries (winning and otherwise) here.
The hydrodynamics of marbling art
Harvard University graduate student Yue Sun was fascinated by the process and the resulting patterns of making marbled paper, particularly the randomness. “You don’t really know what you’re going to end up with until you have it printed,” she told Physics Magazine.
Although there are several different methods for marbling paper, the most common involves filling a shallow tray with water, then painstakingly applying different ink or paint colors to the water’s surface with an ink brush to cover the surface with concentric circles. Adding surfactants makes the colors float so that they can be stirred—perhaps with a very fine human hair—or fanned out by blowing on the circles of ink or paint with a straw. The final step is to lay paper on top to capture the colorful floating patterns. (Body marbling relies on a similar process, except the floating patterns are transferred onto a person’s skin.)
Sun was curious about the hydrodynamics at play and explored two key questions in the simulations for the video. Why does the paint or ink float despite being denser than the liquid bath? And why don’t the colors mix together to create new colors when agitated or stirred? The answer to the former is basically “surface tension,” while the latter does not occur because the bath is too viscous, so the diffusion of the paint or ink colors across color boundaries happens too slowly for mixing. Sun hopes to further improve her simulations of marbling in hopes of reverse-engineering some of her favorite patterns to determine which tools and movements were used to create them.
