Vibrate a pool of water and above a critical frequency, a pattern of standing waves will form on the surface. These are known as Faraday waves after Michael Faraday, who studied the phenomenon in the early half of the nineteenth century. The kaleidoscopic view of them you see here comes from photographer Linden Gledhill, who used a high-speed camera and an LED ring light reflecting off the water to capture the changing motions of the waves. The wave patterns oscillate at half the frequency of the driving vibration, and, as the driving frequency changes, the wave patterns shift dramatically. Higher frequencies create more complicated patterns. (Image and video credit: L. Gledhill)
Every year brings faster high-speed cameras and better quality imaging, so the Slow Mo Guys like to occasionally revisit topics they’ve done before, like paint vibrated on a speaker. The physics involved here are fantastic, so I’ll revisit the topic, too! In this version, Gav and Dan are using a pretty beefy speaker at a relatively high volume, so the paint gets a strong acceleration. As they note, the paint colors mix to brown almost immediately. In the high-speed footage, we can see why.
Watch how the individual strands of paint behave. As they fly upward, they stretch out and get thinner. That stretching has a side effect: it makes the paint spin. This is angular momentum of the paint being conserved. Just like a spinning ice skater who pulls his arms in, the paint spins faster as it gets thinner. This provides a lot of the mixing. Just look at how the different colors twist together! (Image and video credit: The Slow Mo Guys)
The Slow Mo Guys have a history of personal sacrifice in the name of cool high-speed footage, and their Super Slow Show is no exception. In a recent segment, both Dan and Gav were knocked flat by giant swinging balloons of paint, and, as you might expect, the splashes are spectacular. The speed is just right for some of the paint to form nice sheets before momentum pulls them into long ligaments. Eventually, that momentum overcomes surface tension’s ability to keep the paint together, and the paint separates into droplets, which, as you see below, rain down on the hapless victims. (Video and image credit: The Slow Mo Guys)
Vortex rings may look relatively calm, but they are concentrated regions of intensely spinning flow, as this poor jellyfish demonstrates. The rings form when a high-speed fluid gets pushed suddenly (and briefly) into a slower fluid. In the case of this bubble ring, a burst of air is pushed by a diver into relatively still water. The vorticity caused by the two areas of fluid trying to move past one another forms the ring. Like a spinning ice skater who pulls his arms inward, the narrow core of the vortex spins fast due to the conservation of angular momentum. Meanwhile, the bubble ring moves upward due to its buoyancy, pulling nearby water in as it goes. This catches the hapless jellyfish (who relies on vortex rings itself) and gives it quite a spin. But. don’t worry, the photographer confirmed that the jelly was okay after its ride. (Video credit: V. de Valles; via Ashlyn N.)
Thanks for joining us this week as we explored nearly two centuries’ worth of scientific discoveries around vibration, fluid dynamics, and quantum mechanics. For those who’d like to learn more about these and related topics, we’ve compiled some helpful resources below.
One of my favorite things to do while we were sailing along the Galapagos was watching the blue-footed boobies hunt. Like the gannets shown above, boobies are plunge divers. They circle overhead until they spot their prey, then they fold their wings and dive headfirst into the water, impacting at speeds of more than 20 m/s (~45 mph). It’s absolutely incredible to watch. The physics involved are impressive, too, especially considering how badly a human would be injured diving at their speeds!
Fluid dynamically speaking, there are three important phases to the birds’ entry. The first is the impact phase, which lasts from initial contact until the bird’s head is underwater. In the second phase, an air cavity forms behind the head and around the neck as it enters the water. Finally, when the chest – the widest point of the bird – hits the water, the bird reaches the submerged phase.
Mechanically, the most interesting part is the air cavity phase. During this time, the bird’s head is slowing down due to high hydrodynamic drag from the water, but the rest of the bird is still moving fast. That means the bird’s slender neck experiences strong compressive forces, which would tend to make it buckle. Researchers at Virginia Techexamined this very problem and found that the birds’ sizing – its head shape, neck length, and so forth – combined with their typical diving speeds kept these birds well away from the conditions that would cause their necks to buckle. With the added stabilization from the birds’ neck muscles, they estimated that gannets and other plunge divers might be able to safely dive at speeds twice what would kill a human! Check out the BBC video below to see high-speed footage of gannets diving. (Image credits: G. Lecoeur; B. Chang et al.; research credits: B. Chang et al., pdf; video credit: BBC)
Tomorrow will be the final day of Galapagos Week. Catch up on previous posts here.
In their newest video, the Slow Mo Guys recreated one of my favorite effects: vibration-driven droplet ejection. For this, they use a Chinese spouting bowl, which has handles that the player rubs after partially filling the bowl with water. By rubbing, a user excites a vibrational mode in the bowl. Watch the GIFs above and you can actually see the bowl deforming steadily back and forth. This is the fundamental mode, and it’s the same kind of vibration you’d get from, say, ringing a bell.
Without a high-speed camera, the bowl’s vibration is pretty hard to see, but it’s readily apparent from the water’s behavior in the bowl. In the video, Gav and Dan comment that the ripples (actually Faraday waves) on the water always start from the same four spots. That’s a direct result of the bowl’s movement; we see the waves starting from the points where the bowl is moving the most, the antinodes. In theory, at least, you could see different generation points if you manage to excite one of the bowl’s higher harmonics. The best part, of course, is that, once the vibration has reached a high enough amplitude, the droplets spontaneously start jumping from the water surface! (Video and image credits: The Slow Mo Guys; submitted by effyeah-artandfilm)
Flying fragments can be a big danger in explosions. Shown above are two shadowgraph images of 1 gram explosives originally packed in solid containers. Each explosion produced a visible spherical shock wave, about 1 meter across in both pictures. On the left side, the container has fragmented into large pieces, each of which travels near to but less than the speed of sound. On the right, the fragments are much smaller, but many of them are traveling at supersonic speeds ahead of the main shock wave. If you look closely, you can even see faint Mach cones extending from each fragment. In a real, full-scale explosion, these shards would strike like a hail of bullets ahead of the blast wave. (Image credit: G. Settles)
The mantis shrimp is a tiny, clown-colored juggernaut of underwater physics. Some species have modified claws that serve as clubs for punching their prey, and the mantis shrimp swings that club fast – its acceleration is comparable to a bullet’s! Moving that quickly in water causes a drastic drop in local pressure, low enough to form a cavitation bubble. Such low-pressure bubbles themselves are not particularly dangerous, but their collapse is incredibly violent, especially near a solid surface, like the shell of the shrimp’s prey. Collapsing cavitation bubbles can send out shock waves, shatter glass, and even generate light. In the case of the mantis shrimp, it’s more than enough to stun, if not outright kill, its prey. (Video credit: Physics Girl)
In a recent video, Warped Perception filmed a model rocket engine firing underwater. Firstly, it’s no surprise that the engine would still operate underwater (after its wax waterproofing). The solid propellant inside the engine is a mixture of fuel and oxidizer, so it has all the oxygen it needs. Fluid dynamically speaking, though, this high-speed footage is just gorgeous.
Ignition starts at about 3:22 with some cavitation as the exhaust gases start flowing. Notice how that initial bubble dimples the surface when it rises (3:48). At the same time, the expanding exhaust on the right side of the tank is forcing the water level higher on that side, triggering an overflow starting at about 3:55. At this point, the splashes start to obscure the engine somewhat, but that’s okay. Watch that sheet of liquid; it develops a thicker rim edge and starts forming ligaments around 4:10. Thanks to surface tension and the Plateau-Rayleigh instability, those ligaments start breaking into droplets (4:20). A couple seconds later, holes form in the liquid sheet, triggering a larger breakdown. By 4:45, you can see smoke-filled bubbles getting swept along by the splash, and larger holes are nucleating in that sheet.
The second set of fireworks comes around 5:42, when the parachute ejection charge triggers. That second explosive triggers a big cavitation bubble and shock wave that utterly destroys the tank. If you look closely, you can see the cavitation bubble collapse and rebound as the pressure tries to adjust, but by that point, the tank is already falling. Really spectacular stuff! (Video and image credit: Warped Perception)
