The Semeru volcano rises in the background of this photo of Java’s Tumpak Sewa waterfall by Joan de la Malla. Rain that falls on the volcano slides down its flank and wanders through the jungle on its way to the spectacular 120-meter-high waterfall. From the clouds wreathing the mountain through the jungle’s drifting fogs to the mists of the falls, this portrait highlights the many forms water takes on its journey. (Image credit: J. de la Malla/WPOTY; via Colossal)
Month: January 2025
Peering Inside a Hailstone
In spring and summer, major thunderstorms can include dangerous and destructive hailstones. In Catalonia, a group of scientists collected hailstones after a record-breaking 2022 storm, finding some as large as 12 centimeters across. Using a dentist’s CT scanner, they looked at the interior of the hailstones, uncovering layers that reveal how the hail grew. In the past, researchers have studied hail by slicing the ice; that method gives them only a single cross-section through the hailstone, which gets destroyed in the process. In contrast, a CT scan revealed the full interior of the ice.
The scientists found that, even though hail often appears spherical, the nucleus of the hail is not always located in the center. They saw that the hail grew in uneven layers that varied in density, depending on the storm conditions the hail experienced. To get to the enormous sizes seen here, hailstones have to travel up and down repeatedly through a storm, building up layer by layer. From the hail’s interior structure, the team could also tell what orientation the hail took its final fall in; the ice along the bottom of the hailstone was bubble-free, indicating that it collected as water drops hit the surface and froze. (Image credit: T. Ribas; research credit: C. Barqué et al.; via New Scientist)
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Instabilities in Competition
When two liquid jets collide, they form a thin liquid sheet with a thicker rim. That rim breaks into threads and then droplets, forming a well-known fishbone pattern as the Plateau-Rayleigh instability breaks up the flow. This poster shows a twist on that set-up: here, the two colliding jets vary slightly in their velocities. That variability adds a second instability to the system, visible as the wavy pattern on the central liquid sheet. The sheet’s rim still breaks apart in the usual fishbone pattern, but the growing waves in the center of the sheet eventually that structure apart as well. (Image credit: S. Dighe et al.)
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Flow Behind Viscous Fingers
Nature is full of branching patterns: trees, lighting, rivers, and more. In fluid dynamics, our prototypical branching pattern is the Saffman-Taylor instability, created when a less viscous fluid is injected into a more viscous one in an confined space. Most attention in this problem has gone to the branching interface where the two fluids meet, but recently, a team has examined the flow away from the fingers by alternately injecting dyed and undyed fluid to visualize what goes on. That’s what we see here. Notice how the central dye rings, far from the branching fingers, still appear circular. Yet, even a few centimeters away from the fingers, the dye is starting to show ripples that correspond to the fingers. That’s an indication that the pressure gradient generated at the tips of the fingers is pretty far-reaching! (Image and research credit: S. Gowen et al.)
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Mapping the Oceans With Seals
Elephant seals are harbingers — canaries in the coal mine — for climate change. A long-running experiment tracks northern elephant seal populations using a combination of sensor tags and field measurements. With the miniaturization of sensors, a tagged seal can provide a wealth of data for scientists: foraging paths, temperature and salinity data, behavioral patterns, ecological data, and even information on the species around the seal. This video delves into this treasure trove, explaining how and what we’re learning from this species, especially as they navigate our changing climate. (Video and image credit: Science)
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Bubbling Up
By volume, Lake Baikal is the world’s largest lake, holding over 20% of the planet’s fresh water. It’s also a major carbon sink, holding large amounts of methane. That’s the gas trapped in the frozen bubbles seen here. Baikal’s ice is exceptionally clear, making long trails of frozen bubbles visible during the winter. (Image credit: K. Makeeva; via APOD)
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Tracking Meltwater Through Flex
Greenland’s ice sheet holds enough water to raise global sea levels by several meters. Each year meltwater from the sheet percolates through the ice, filling hidden pools and crevasses on its way to draining into the sea. Monitoring this journey directly is virtually impossible; too much goes on deep below the surface and the ice sheet is a precarious place for scientists to operate. So, instead, they’re monitoring the bedrock nearby.
Researchers used a network of Global Navigation Satellite System (GNSS) stations like the one above to track how the ground shifted and flexed as meltwater collected and moved. They found that the bedrock moved as much as 5 millimeters during the height of the summer melt. How quickly the ground relaxed back to its normal state depended on where the water went and how quickly it moved. Their results indicate that the water’s journey is not a short one: meltwater spends months collecting in subterranean pools on its way to the ocean — something that current climate models don’t account for. Overall, the team’s results indicate that there’s much more hidden meltwater than models predict and it spends a few months under the ice on its way to the sea. (Image credit: T. Nylen; research credit: J. Ran et al.; via Eos)
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The Underwater Effects of Volcanoes
Although volcanoes are typically located in or near the ocean, we’ve spent relatively little effort studying how eruptions affect the marine environment. A recent research voyage aimed to change that by studying the Patagonian Sea near the site of the 2008 Chaitén eruption. Marked by massive ashfalls that, when mixed with heavy rains, created huge mudslides, the 2008 eruption was the Chaitén volcano’s first in 9,000 years.
The researchers mapped the seafloor near the volcano, finding massive dunes shaped by strong currents. Using a remotely operated vehicle, the team surveyed and sampled the seafloor, collecting sediments reaching back some 15,000 years. They also located ash from the 2008 eruption over 24 kilometers from the volcano. With their data, they hope to understand both how the recent eruption changed the marine environment as well as how older eruptions affected the area. (Image credits: volcano – USGS, dunes – Schmidt Ocean Institute; see also Schmidt Ocean Institute; via Ars Technica)
Composite image showing the massive underwater dunes off the coast. P.S. – This Friday, January 24th from 12 to 1:30pm Eastern I’m moderating a panel discussion on the Traveling Gallery of Fluid Motion and how art and science can work together in public outreach. Register here to join. It’s free!
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Swimming Like a Ray
Manta rays are amazing and efficient swimmers — a necessity for any large animal that survives on tiny plankton. Researchers have built a new soft robot inspired by swimming mantas. Like its biological inspiration, the robot flaps its pectoral fins much as bird flaps its wings; this motion creates vortices that push water behind the robot, propelling it forward. For a downstroke, air inflates the robot’s body cavity, pushing the fins downward. When that air is released, its fins snap back up. With this simple and energy efficient stroke, researchers are able to control the robot’s swimming speed and depth, allowing it to maneuver around obstacles. Flapping faster helps the robot surface, and slower flapping allows it to sink. (Living manta rays also sink if they slow down.) Check out the robot in action below. (Image credit: J. Lanoy; video and research credit: H. Qing et al.; via Ars Technica)
The Mystery of the Binary Droplet
What goes on inside an evaporating droplet made up of more than one fluid? This is a perennially fascinating question with lots of permutations. In this one, researchers observed water-poor spots forming around the edges of an evaporating drop, almost as if the two chemicals within the drop are physically separating from one another (scientifically speaking, “undergoing phase separation“). To find out if this was really the case, they put particles into the drop and observed their behavior as the drop evaporated. What they found is that this is a flow behavior, not a phase one. The high concentration of hexanediol near the edge of the drop changes the value of surface tension between the center and edge of the drop. And that change is non-monotonic, meaning that there’s a minimum in the surface tension partway along the drop’s radius. That surface tension minimum is what creates the separated regions of flow. (Video and image credit: P. Dekker et al.; research pre-print: C. Diddens et al.)
