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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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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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.)
Why Icy Giants Have Strange Magnetic Fields
When Voyager 2 visited Uranus and Neptune, scientists were puzzled by the icy giants’ disorderly magnetic fields. Contrary to expectations, neither planet had a well-defined north and south magnetic pole, indicating that the planets’ thick, icy interiors must not convect the way Earth’s mantle does. Years later, other researchers suggested that the icy giants’ magnetic fields could come from a single thin, convecting layer in the planet, but how that would look remained unclear. Now a scientist thinks he has an answer.
When simulating a mixture of water, methane, and ammonia under icy giant temperature and pressure conditions, he saw the chemicals split themselves into two layers — a water-hydrogen mix capable of convection and a hydrocarbon-rich, stagnant lower layer. Such phase separation, he argues, matches both the icy giants’ gravitational fields and their odd magnetic fields. To test whether the model holds up, we’ll need another spacecraft — one equipped with a Doppler imager — to visit Uranus and/or Neptune to measure the predicted layers firsthand. (Image credit: NASA; research credit: B. Militzer; via Physics World)
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Blooming in Blue
Summers in the Barents Sea — a shallow region off the northern coasts of Norway and Russia — trigger phytoplankton blooms like the one in this satellite image. The blue shade of the bloom suggests the work of coccolithophores, a type of plankton armored in white calcium carbonate. This type of plankton thrives in the warm, stratified waters of the late summer. Earlier in the year, the water tends to be nutrient-rich and well-mixed, conditions which favor diatom plankton species instead. Their blooms appear greener in satellite images. (Image credit: W. Liang; via NASA Earth Observatory)
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Tracking Coastal Sediment Loss
Shorelines rely on an influx of sediment to counter what’s lost to erosion by waves and currents. But tracking that sediment flux is challenging in coastal regions where salt, waves, and storms batter delicate instruments. Instead, researchers have turned to remote sensing through high-resolution satellites like Landsat to monitor these areas. Researchers built an algorithm to analyze coastal imagery, validated with local sediment measurements; once built, they deployed it in a free tool that lets anyone build a 40-year timeline of a coastal area’s sediment history.
Looking at thousands of sites around the world, the team found coastal sediment is on the decline, especially along sandy and muddy coastlines. Where has the sediment gone? It’s likely that human-built infrastructure — both on coasts and upstream along rivers — is disrupting the natural flow of sediments that would replenish these regions. (Image credit: NASA; research credit: W. Teng et al.; via Eos)
How Cooling Towers Work
Power plants (and other industrial settings) often need to cool water to control plant temperatures. This usually requires cooling towers like the iconic curved towers seen at nuclear power plants. Towers like these use little to no moving parts — instead relying cleverly on heat transfer, buoyancy, and thermodynamics — to move and cool massive amounts of water. Grady breaks them down in terms of operation, structural engineering, and fluid/thermal dynamics in this Practical Engineering video. Grady’s videos are always great, but I especially love how this one tackles a highly visible piece of infrastructure from multiple engineering perspectives. (Video and image credit: Practical Engineering)
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A New Mantle Viscosity Shift
The rough picture of Earth’s interior — a crust, mantle, and core — is well-known, but the details of its inner structure are more difficult to pin down. A recent study analyzed seismic wave data with a machine learning algorithm to identify regions of the mantle where waves slowed down. These shifts in seismic wave speed occur in areas where the mantle’s viscosity changes; a higher viscosity makes waves travel slower.
The team found seismic wave speed shifts at depths of 400 and 650 kilometers, corresponding to known viscosity changes. But they found shifts at 1050 and 1500 kilometers, as well — the first time anyone has shown a global viscosity shift at those depths. Their analysis suggests a higher viscosity in this mid-mantle transition zone, which could affect how tectonic plates, which rely on these slow mantle flows, move. (Image credit: NASA; research credit: K. O’Farrell and Y. Wang; via Eos)
