Stir hot chocolate powder into milk or water, and you can recreate this bizarre acoustic phenomenon. Once the powder is mixed in, tapping the side of the cup creates a low pitch that steadily rises as you continue tapping. This is known as the hot chocolate, or allossonic, effect. When you stir, it creates tiny bubbles in the fluid, which changes the effective speed of sound. As the bubbles pop, the speed of sound goes up and the pitch of your tapping gets higher! Stirring the cup up again (even without adding more powder) should lower the pitch once more. (Video credit: C. Kalelkar)
Category: Phenomena
Cavitation-Induced Microjets
In cavitation, tiny bubbles of vapor form and collapse in a liquid, often sending shock waves ricocheting. In most occurrences beyond the lab, cavitation bubbles aren’t a solo act; many bubbles can form and interact. This video takes a look at some of the effects of those interactions. When close together, two cavitation bubbles can act to focus the flow during collapse, generating a microjet strong enough to penetrate into nearby surfaces. Researchers hope this technique may one day be used for needle-free injections. (Image, video, and submission credit: A. Mishra et al.)
How To: Ink Flow Photography
Ever wanted to try your hand at making these cool billowing ink photos? Photographer Jason Parnell-Brookes has a detailed tutorial over at PetaPixel laying out the necessary tools and set-up. I haven’t tried this out myself, but I hope to! How about you? (Image credit: J. Parnell-Brookes; submitted by clogwog)
A Colorful Fire Tornado
This one definitely belongs in the do-not-try-this-yourself category, but this Slow Mo Guys video of a colorful fire tornado is pretty spectacular. Using an array of different fuels and a ring of box fans, Gav sets up a vortex of flame that transitions smoothly from red all the way to blue. As he points out in the video, the translucency of the vortex is so good that you can see how the two sides of the vortex rotate! (Video credit: The Slow Mo Guys)
Breaking Compound Ligaments
When pulled, viscous liquids stretch into ligaments that thin and then break into droplets. In this video, researchers investigate how these ligaments break up, depending on their composition. The initial views show the break-up of a water-glycerol ligament (Image 1) and an oil ligament (Image 2). By placing a water droplet inside oil, the researchers got quite different results, including oil-encapsulated droplets (Image 3). The technique could be useful for making compound droplets, even with more than two components. (Image and video credit: V. Thiévenaz and A. Sauret)
Insects Taking Flight
As awkward as they look sometimes, insects are amazing fliers. In this video from Ant Lab, we see all kinds of insects taking flight. Some, like the mantis, execute flying leaps to get in the air, whereas weevils begin flapping from a tripod stance. Watching these videos I’m always struck by how flexible insect wings are. They flex far more than I would imagine. And these insects have a lot of excess lift. Just check out that carrion beetle taking off despite being covered in mites! (Image and video credit: Ant Lab)
Filming the Brinicle
It may have been 10 years since the BBC filmed the first timelapse of a growing brinicle, but the footage is just as amazing now as it was then! This video gives you the behind-the-scenes story of what it took to capture this natural wonder under the Antarctic ice. It’s incredible to see the shots of sinking brine streaming off the brinicles, too. The difference in density (and thus refractive index) of the brine and the ocean water is substantial enough that your eye can actually pick them out as separate fluids. I once went snorkeling in an area with similarly varied salinity and it was completely bizarre watching everything suddenly go wavy and blurry as I swam. (Image and video credit: BBC)
The Bubbly Escape
Sometimes experiments don’t work as planned and, instead of answers, they lead to more questions. In this video, we see an experiment looking at an air bubble trapped beneath a cone. It’s the same situation you get by holding a mug upside-down in a sink full of water but with inclined walls. As the cone moves downward, it squeezes the trapped air bubble. A film of air gets pushed along the walls of the cone, eventually forming finger-like bubbles that wrap around the edge of the cone and get entrained into the vortex ring outside the cone.
Clearly, there is some kind of instability that drives the air bubble to form these fingers rather than spreading uniformly. But the big question is which one? Is this a density-driven Rayleigh-Taylor instability caused by air getting pushed into water? Or is it a Saffman-Taylor instability causes by the less viscous air forcing its way into the more viscous water? What do you think? (Image and submission credit: U. Jain)
Ice and Dunes
Although dunes are usually associated with scorching climates, they can form in any desert, including in the frozen steppes of western Mongolia. This sunrise photo, taken by an astronaut aboard the ISS, shows Ulaagchinii Khar Nuur. The ice-covered Khar Nuur Lake surrounds two islands, Big and Small Avgash, and cold dunes form textured streaks on either side. The low sun angle accentuates the dunes, making every rippling crest clear. (Image credit: NASA; via NASA Earth Observatory)
Ink-Based Propulsion
In this video, Steve Mould explores an interesting phenomenon: propulsion via ballpoint pen ink. Placing ink on one side of a leaf or piece of paper turns it into a boat with a dramatic dye-filled wake. It’s not 100% clear what’s happening here, though I agree with Steve that there are likely several effects contributing.
Firstly, there’s the Marangoni effect, the flow that happens from an area of low surface tension to high surface tension. This is what propels a soap boat as well as many water-walking insects. I think this is a big one here, and not just because the ink has surfactants. As any component of the ballpoint ink spreads, its varying concentration is going to trigger this effect.
Secondly, there’s a rocket effect. Rockets operate on a fairly simple principle: throw mass out the back in order to go forward. These dye boats are also doing this to some extent.
And finally there’s some chemistry going on. Some kind of reaction seems to be taking place between one or more of the ink components and the water in order to create the semi-solid layer of dye. Presumably this is why the dye doesn’t simply dissolve as it does in some of Steve’s other experiments.
I figure some of my readers who are better versed in interfacial dynamics, rheology, and surface chemistry than I am will have some more insights. What do you think is going on here? (Video and image credit: S. Mould)
