This high-speed video shows the cavitation that occurs when a bottle of water is struck. The impact accelerates the bottle downward, generating localized vacuums between the glass and the liquid. These are cavitation bubbles, which expand until the pressure of the water surrounding them is too great. This outside pressure triggers an implosion of the bubble, which collapses until the pressure within the bubble makes it expand again. These rapid oscillations in pressure can often shatter the glass bottle. Cavitation can also generate extremely high temperatures and even trigger luminescence. It’s used by both pistol shrimp and mantis shrimp to hunt their prey. (Video credit: P. Taylor)
Videos
Bursting a Bubble
Though seemingly instantaneous to the naked eye, the bursting of a soap bubble is fascinating when slowed down. Here it is at about 2200 frames per second. Initially, the bubble is approximately spherical – its shape determined by a balance between surface tension, gravity, and pressure. The prick of a pinpoint disrupts the balance, and surface tension pulls the thin film away from the defect. The liquid sheet of the bubble retracts swiftly into a filament of fluid and a cloud of tiny droplets. (Video credit: soapbubble.dk)
Instability
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Many systems can exhibit unstable behaviors when perturbed. The classic example is a ball sitting on top of a hill; if you move the ball at all, it will fall down the hill due to gravity. There is no way to perturb the ball in such a way that it will return to the top of the hill; this makes the top of the hill an unstable point. In many dynamical systems, a very small perturbation may not be as obviously unstable as the ball atop the hill, especially at first. Often a perturbation will have a very small effect initially, but it can grow exponentially with time. That is the case in this video. Here a tank of fluid is being vibrated vertically with a constant amplitude. At first, the sloshing effect on the fluid interface is very small. But the vibration frequency sits in the unstable region of the parameter space, and the perturbation, which began as a small sloshing, grows very quickly. In a real system (as opposed to a mathematical one), this kind of unstable or unbounded growth very quickly leads to destruction. (Video credit: S. Srinivas)
Balloons in the Car
Destin from Smarter Every Day has just made a video on one of my favorite fluids brain teasers: what happens to a helium balloon when you accelerate in a car? Take a moment to think about the answer before watching or reading further…
Okay, so what happens? Contrary to what you may expect, hitting the accelerator with a balloon in the car will make it shift forward. This is a matter of buoyancy. As Destin demonstrates with the water bottle, when two fluids are accelerated forward, the denser one will shift backwards, which pushes the lighter one forward. Because the helium is lighter than the air filling the car, accelerating pushes the air backward (just as it does the pendulum and the car’s inhabitants) and that shifting of the air pushes the helium in the balloon forward. (Video credit: Smarter Every Day)
Rubens’ Table
Veritasium’s new video has an awesome demonstration featuring acoustics, standing waves, and combustion. It’s a two-dimensional take on the classic Rubens’ tube concept in which flammable gas is introduced into a chamber with a series of holes drilled across the top. Igniting the gas produces an array of flames, which is not especially interesting in itself, until a sound is added. When a note is played in the tube, the gas inside vibrates and, with the right geometry and frequency, can resonate, forming standing waves. The motion of the gas and the shape of the acoustic waves is visible in the flames. Extended into two-dimensions, this creates some very cool effects. (Video credit: Veritasium; via Ryan A.; submitted by jshoer)
“Becoming Harmonious”
Much as I try to keep from getting repetitious, this was just too neat to pass up. This new music video for The Glitch Mob’s “Becoming Harmonious” is built around the standing Faraday waves that form on a water-filled subwoofer. The vibration patterns, along with judicious use of strobe lighting, produce some fantastic and kaleidoscopic effects. (Video credit: The Glitch Mob/Susi Sie; submitted by @krekr)
Tiny Fliers
There’s an apocryphal story claiming that, aerodynamically speaking, honeybees should not be able to fly. Obviously, they can, but it’s true that a small, flapping creature and a large, fixed-wing aircraft will not generate lift exactly the same way. NYU professor Leif Ristroph has a lot of projects exploring flapping flight on smaller scales, as seen in this video. His oscillatory fliers and rotating flapping flight simulator have both been featured previously. Part of the beauty of these projects is their size; in a field that’s historically required giant wind tunnels and room-length wave tanks, Ristroph’s work provides insight into long-standing problems using apparatuses that fit on a countertop. (Video credit: Cool Hunting/L. Ristroph et al.)
What Sound Looks Like
NPR’s Skunk Bear Tumblr has a great new video on the schlieren visualization technique. The schlieren optical set-up is relatively simple but very powerful, as shown in the video. The technique is sensitive to variations in the refractive index of air; this bends light passing through the test area so that changes in fluid density appear as light and dark regions in the final image. Since air’s density changes with temperature and with compressibility, the technique gets used extensively to visualize buoyancy-driven flows and supersonic flows. Since sound waves are compression waves which change the air’s density as they travel, schlieren can capture them, too. (Video credit: A. Cole/NPR’s Skunk Bear)
“High Ball Stepper”
The recently released music video for Jack White’s “High Ball Stepper” is a fantastic marriage of science and art. The audio is paired with visuals based around vibration effects using both granular materials and fluids. There are many examples of Faraday waves, the rippling patterns formed when a fluid interface becomes unstable under vibration. There are also cymatic patterns and even finger-like protrusions formed by when shear-thickening non-Newtonian fluids get agitated. (Video credit: J. White, B. Swank and J. Cathcart; submitted by Mike and Marius)
How Tsunamis Cross the Ocean
Last week an earthquake in Chile raised concerns over a possible tsunami in the Pacific. This animation shows a simulation of how waves would spread from the quake’s epicenter over the course of about 30 hours. In the open ocean, a tsunami wave can travel as fast as 800 kph (~500 mph), but due to its very long wavelength and small amplitude (< 1 m), such waves are almost unnoticeable to ships. It’s only near coastal areas, when the water shallows, that the wave train slows down and increases in height. Early in the video, the open ocean wave heights are only centimeters; note how, at the end of the video, the wave run-up heights along the coast are much larger, including the nearly 2 meter waves that impacted Chile. The power of the incoming waves in a tsunami are not their only danger, though; the force of the wave getting pulled back out to sea can also be incredibly destructive. (Video credit: NOAA/NWS/Pacific Tsunami Warning Center; via Wired)