The Saudi Arabian oasis of Jubbah sits in the bed of an ancient lake. It’s protected from the westerly winds that sculpt the surrounding dunes by the wind shadow of the mountain Jabel Umm Sinman. The long, skinny shape of the settlement reveals the shape of the mountain’s wake! (Image credit: NASA; via NASA Earth Observatory)
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Swapping Emulsions
Chemically speaking, oil and water don’t mix. But with a little fluid mechanical effort, it’s possible to make them an emulsion — a mixture of oil droplets in water or water droplets in oil. Researchers in the Netherlands discovered that the viscosity of these emulsions depends critically on which of those mixtures you have.
To create their emulsions, the team used a tank consisting of two concentric cylinders. When the inner cylinder spins, it creates a well-understood flow field between the inner and outer cylinder. By varying the ratio of oil to water in the tank, they could explore a wide range of emulsions. They found that the emulsion’s viscosity changed dramatically when the emulsion shifted from oil droplets in water to water droplets in oil, something known as a catastrophic phase inversion. During this switch the viscosity dropped from 3 times higher than pure water to 2 times lower! (Image credit: A_Different_Perspective; research credit: D. Bakhuis et al.; via APS Physics; submitted by Kam-Yung Soh)
Protecting From Storm Surge
The most dangerous and destructive part of a tropical cyclone isn’t the wind or rain; it’s the storm surge of water moving inland. This landward shift of ocean takes place because of a cyclone’s strong winds, which drive the water via shear. The depth storm surges reach depends on the wind speed and direction, shape of the shoreline, and many other factors, making exact predictions difficult.
Fortunately, engineers can — with enough foresight and investment — build structures and networks to help protect developed land from storm surge flooding. (Image and video credit: Practical Engineering)
Decelerating Jets
For more than a century, scientists have been fascinated by the jet that forms after a drop impacts a liquid. In this study, researchers tracked fluorescent particles in the fluid to understand the velocity and acceleration of flow inside the jet. They found that, within the first 10ms after the jet appears, it decelerates at up to 20 times the gravitational acceleration. That’s much too fast for gravity to cause, pointing instead to the critical importance of surface tension in dictating the behavior of these fast-moving jets. (Image and research credit: C. van Rijn et al.; via APS Physics; submitted by Kam-Yung Soh)
Whiskey Webs
Unlike scotch whisky, when American bourbon whiskeys are diluted, they form unique web-like evaporation patterns. These differences arise in part from the way the liquors are aged: scotch is aged in re-used barrels, whereas bourbons require aging in a new, charred American white oak barrel*.
During aging, the whiskey picks up water-insoluble chemicals from the barrel. When water is added to the bourbon, it helps transport those insoluble components to the surface of a droplet, where they form a monolayer of fatty acid chains (Image 2; in green). As evaporation continues and the droplet gets smaller, the molecules at the shrinking surface collapse inward, forming the rigid web structure we see left behind. The patterns that form act as a kind of fingerprint for the bourbon. Check out some of the brand-to-brand variations over at the researchers’ Whiskey Webs site. (Image and research credit: S. Williams et al.; via Physics Today)
* In case you were wondering, this is actually a legal requirement in order to be considered bourbon. Bourbons must also be made from a grain mixture that is >50% corn.
Visualizing Radiation
Radiation is invisible, but it’s not too difficult to build an apparatus that lets you see it. This video shows the ghostly aftermath of passing radiation in a cloud chamber, one of the first set-ups used to study radiation. The chamber contains a radioactive source and chilled isopropyl alcohol. The alcohol forms a supersaturated vapor — essentially a cloud in waiting — inside the chamber.
When a radioactive particle gets emitted from the source, it streaks through the chamber, colliding with atoms and ionizing them. Those ions then serve as nucleation sites where alcohol condenses into droplets. It’s these condensation trails that we see bloom and decay in the particle’s wake. (Image and video credit: L. Gledhill)
Meltwater Tracking Via Seal
Monitoring meltwater from Antarctic glaciers is critical for understanding our changing climate, but such remote and inaccessible regions are tough to collect data in. So researchers are turning to local workers to help them gather data. By collecting and analyzing data from seal tags, researchers have mapped new seasonal variations in meltwater flows around Pine Island Glacier. Although the seals are somewhat tough collaborators — they rarely swim exactly where the researchers would like them to — their winter activities are revealing data researchers could never have collected on their own. (Image credit: Y. Rzhemovskiy; research credit: Y. Zheng et al.; via Gizmodo)
Freezing Splats
In fluid physics, there’s often a tug of war between different effects. For droplets falling onto a surface colder than their freezing point, the hydrodynamics of impact, sudden heat transfer, and solidification processes all compete to determine how quickly and in what form droplets freeze.
The images above form a series based on changing the height from which the droplet falls. Each image is divided into two synchronized parts. On the left, we see a visible light, top-down view of the freezing droplet; on the right, we see an infrared view of freezing. As the height of impact increases, the shape of the frozen drop becomes more elaborate, moving from a flat splat with a small conical tip all the way to one with a concentric double-ring in its center. (Image and research credit: M. Hu et al.)
Fallstreak Holes
Occasionally clouds appear to have a hole in them; these are known as fallstreak holes or hole-punch clouds. To form, the water droplets in the cloud must be supercooled; in other words, they must be colder than their freezing point but still in liquid form. When disturbed — say, by the temperature drop caused by flowing over an airplane wing — the supercooled water droplets will suddenly freeze. This typically kicks off a chain reaction in which many droplets freeze and the heavy ice crystals fall out of the sky, leaving behind a void in the cloud. Because airplanes are particularly good at creating these fallstreak holes, they’re often seen near busy airports. (Image credit: J. Stevens/NASA; via NASA Earth Observatory)
“Radiolarians”
“Radiolarians” is a short film by artist Roman De Giuli using ink, alcohol, and oil. Much of the fluid motion involves break-up into droplets. The effects appear to rely heavily on Marangoni bursting, the physics of which you can learn about in this previous post. (Image and video credit: R. De Giuli)
