Dropping water from a plastic pipette onto a pool of oil electrically charges the drop. Then, as it evaporates, it shrinks and concentrates the charges closer and closer. Eventually, the strength of the electrical charge overcomes surface tension, making the drop form a cone-shaped edge that jets out tiny, highly-charged microdrops. Afterward, the drop returns to its spherical shape… until shrinkage builds up the charge density again. This microjetting behavior can carry on for hours! (Video and image credit: M. Lin et al.; research preprint: M. Lin et al.)
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)
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)
Convection cells like these are all around us — in the clouds, on the Sun, and in our pans — but we rarely get to watch them in action. Convection results from densities differing in different areas of a fluid. Under gravity’s influence, having a dense fluid over a lighter one is unstable; the dense fluid will always sink and the lighter one will rise. When that motion has to take place across a large surface area, we often end up with cells like the ones seen here.
Convection cells in an alcohol-paint mixture.
What drives the density differences in the fluid? That depends. Often there’s a temperature difference that drives warmer fluid to rise and cool fluid to sink. But that’s not always the source of convection. Evaporating a volatile chemical — like alcohol — out of a mixture can also create the density differences needed for convection. That may be the source of the convection we see here in a mixture of paint and alcohol. (Video and image credit: W. Zhu; via Nikon Small World in Motion)
When surface tension varies along an interface, fluids move from regions of low surface tension to higher surface tension, a behavior known as the Marangoni effect. Here, a drop of (dyed) water is placed on glycerol. The two fluids are miscible, but water has much a lower viscosity and density yet a higher surface tension. The drop’s interface quickly becomes unstable; viscous fingers form along the edge as the less viscous water pushes into the more viscous glycerol. Eventually, the surface-tension-driven Marangoni flow breaks those fingers off into lip-like daughter drops. The researchers also show how the interplay between viscosity and surface tension affects the size of fingers that form by varying the water/glycerol concentration. (Image and video credit: A. Hooshanginejad et al.)
In recent years, Arctic permafrost has thawed at a surprisingly fast pace. Much of that is, of course, due to the rapid warming caused by climate change. But some of that phenomenon lives underground, where water’s unusual properties cause convection in gaps between rocks, sediment, and soil.
Water is densest not as ice but as water. This is why ice cubes float in your glass. Water’s densest form is actually a liquid at 4 degrees Celsius. For water-logged Arctic soils, this means that the densest layer is not at the frozen depth but at a higher, shallower depth. This places a dense liquid-infused layer over a lighter one, a recipe for unstable convection.
Illustration of underground convection and permafrost thaw. On the left: temperature and density of the water in Arctic soil varies with depth. The temperature gets colder the deeper you go, but because water is densest at 4 degrees Celsius, the density is greatest at a shallower depth than the freezing interface. As a result of this unstable configuration (dense water over less dense water), convection can occur (right).
In a recent numerical simulation, researchers found that this underground convection caused permafrost to thaw much more quickly than it would due to heat conduction alone. In fact, the effects appeared in as little as one month, so in a single summer, this convection could have a big effect on the thaw depth. (Image credit: top – Florence D., figure – M. Magnani et al.; research credit: M. Magnani et al.)
For this latest experimental film, artist Roman De Giuli provides a glimpse of the unique fluid art machine he’s built over the last 3.5 years. With 10 channels driven by peristasltic tube pumps and stepper motors, his “printer” drips up to 10 colors on a paint-covered, tilted canvas to create these beautiful images. As he says in his description of the invention, the set-up produces paint layering that’s almost impossible to create by hand. Fluid dynamically speaking, we’re seeing gravity currents like a lava flow or avalanche that are mixing together viscously. There’s also some added effects from density differences between different layered paint colors. Artistically, this machine offers an infinite palette of visual opportunities; financially, though, De Giuli admits its an absolute beast at consuming paint! (Image and video credit: R. De Giuli)
Eagles, butterflies, and whales don’t appear to have much in common, but a new study shows that they — along with over 400 other flying and swimming animals of all sizes — flap with a frequency determined by a simple equation. Their beat frequency is proportional to the square root of their mass divided by their wing area. As you can see in the graph below, this scaling collapses pretty much all of the data onto a single line:
Illustration of the predicted relationship between size and wing frequency (black line) shown alongside various insects, birds, bats, penguins, and whales. The swimming animals also fall on the line, once adjustments are made for the difference in density between air and water.
It’s surprising to find such a consistent relationship among animals of such vastly different sizes and types. The next big question for researchers will be unpicking exactly why and how animals evolved to use such a consistent pattern between their size and their wing(/fin) frequency. (Image credit: top – E. Ward, graph – J. Jensen et al.; research credit: J. Jensen et al.; via Physics World)
An ultrasoft viscoelastic fluid drips in this research poster from the Gallery of Soft Matter. Complex materials like this one have stretchy, elastic behaviors typical of a solid along with the flowing, viscous properties of a fluid. Here, gravity overcomes the material’s elasticity, leaving it to sag and flow. As that happens, the fluid must slide past air, and the density difference between the two fluids creates the small distortions seen on the liquid sheet. This is an example of a Rayleigh-Taylor instability. (Image credit: J. Hwang et al.)
We’re surrounded daily by convection — a buoyancy-driven flow — but most of the time it’s invisible to us. In this video, Steve Mould shows off what convection really looks like with some of his excellent tabletop demos. The first half of the video gives profile views of turbulent convection, with chaotic and unsteady patterns. When he switches to oil instead of water, the higher viscosity (and lower Reynolds number) offer a more structured, laminar look. And finally, he shows a little non-temperature-dependent convection with a mixture of Tia Maria and cream, which convects due to evaporation changing the density. (Image and video credit: S. Mould; submitted by Eric W.)
