High-speed video is wonderful for appreciating fluid motion in ways we can’t on our own. In this video from Warped Perception, we see what happens when a vibrating tuning fork is lowered into water. The tines of the tuning fork create a spray of tiny droplets, reminiscent of what happens in ultrasonic atomization or when blowing through an immersed straw. The ejected droplets fall slowly back onto the disturbed surface; many of them bounce rather than coalescing. This is because the surface’s vibration pushes the drops aloft again before the air layer separating the drop from the surface has the time to drain away. (Video credit: Warped Perception)
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Paintball Collisions
In their latest video, the Slow Mo Guys collide paintballs in mid-air, creating some pretty great paint splashes. The high-speed video does a nice job of revealing some of the typical stages a splash goes through. Initially, the paint spreads in a liquid sheet. As it expands and (necessarily) thins, holes form and grow, driving the paint into string-like ligaments. These ligaments are also stretching and eventually break up into an spray of droplets, much like the jet dripping from your faucet does. If you’d like to see some more awesome high-speed liquid collisions, check out what happens when a solid projectile hits a falling drop and then look at when a laser pulse hits a droplet. (Image and video credit: The Slow Mo Guys; submitted by Omar M.)
Reader Question: Resonating Bottles
Reader shoebill-san asks:
why does it make that weird sound when i blow over a bottle? i did a science experiment in college where we looked at the resonance in a beaker at different water levels, is it like that? related?
Blowing across the top of a bottle creates what’s called Helmholtz resonance, where air inside the neck of the bottle actually vibrates up and down, like you see in the animation above. The stream of air from your mouth creates low pressure just outside the bottle, pulling some of the air out. That air will tend to overshoot, ultimately causing pressure in the bottle neck to drop lower. That vacuum will pull air back into the bottle, at which point the low pressure your blowing supplies pulls it back out, and so on. The actual sound you hear comes from those puffs of moving air. In reality, they move too fast to see; the animation comes from a high-speed video, and I highly recommend watching the full vid.
From your description, I’m not 100% sure what the experiment you did in college was, but I’m guessing it was some variation of the glass harp, where you rub a partially-filled glass and get an eerie sound that varies depending on how much water is in the glass. Like the bottle example above, that’s an example of resonance, but the two are different. In the bottle, it’s the air that’s resonating. For the glass harp, it’s the glass walls themselves that are resonating. The liquid inside just changes the pitch by slowing down the speed at which the glass’s walls vibrate. For a full and fantastic explanation of how that works, check out this video by Dan Quinn. (Image credit: N. Moore, source)
How Cycling Position Affects Aerodynamics
New FYFD video! How much does a rider’s position on the bike affect the drag they experience? To find out I teamed up with folks from the University of Colorado at Boulder and at SimScale to explore this topic using high-speed video, flow visualization, and computational fluid dynamics.
Check out the full video below, and if you need some more cycling science before the Tour de France gets rolling, you can find some of my previous cycling-related posts here. (Image and video credit: N. Sharp; CFD simulation – A. Arafat)
ETA: Please note that the video contained in this post was sponsored by SimScale.
Bottle Rocket Shock Diamonds
Mach diamonds or shock diamonds can often be seen in the exhaust of rocket engines. Here they’re shown in high-speed video of a bottle rocket’s launch. The rocket’s exhaust exits at a pressure that is higher than the surrounding atmosphere, which causes the exhaust to bulge outward and forms two expansion fans, seen in pink, to lower the pressure. The pressure actually drops too low, however, causing shock waves, seen in turquoise, to form in order to raise the exhaust’s pressure. This back-and-forth between shock waves and expansion fans continues, forming the diamond shapes we see. Each subsequent set gets weaker as the exhaust closes in on the right pressure, and ultimately the series of diamonds fades into turbulence. (Image credit: P. Peterson and P. Taylor, source)
Popping
Popcorn’s explosive pop looks pretty cool in high-speed video, but just watching it with a regular camera doesn’t show everything that’s going on. If we take a look at it through schlieren optics, the kernel’s pop looks even more extraordinary:
The schlieren technique reveals density differences in the gases around the corn–effectively allowing us to see what is invisible to the naked eye. The popcorn kernel acts like a pressure vessel until the expansion of steam inside causes its shell to rupture. The first hints of escaping steam send droplets of oil shooting upward. The kernel may hop as steam pours out the rupture point, causing the turbulent billowing seen in the animation above. As the heat causes legs of starch to expand out of the kernel, they can push off the ground and propel the popcorn higher. As for the eponymous popping sound, that is the result of escaping water vapor, not the actual rupture or rebound of the kernel! See more of the invisible world surrounding a popping kernel in the video below. (Image credits: Warped Perception, source; Bell Labs Ireland, source; WP video via Gizmodo; BLI video submitted by Kevin)
The Blue Whirl
We wrote earlier this year about the discovery of a new type of fire whirl – the blue whirl – but now the authors have published video of the blue whirl in action! The blue whirl was discovered while investigating the use of fire whirls to more efficiently burn off oil spilled atop water. A tightly spinning yellow fire whirl produces less soot than a non-vortex burn; the blue whirl is even more efficient, producing little to no soot at all. Much remains to be learned about this new type of fire vortex, but in the meantime, enjoy some high-speed video of the blue whirl, particularly from 1:50 onward. (Video credit: M. Gollner et al.)
Starting a Lighter
Lots of fluids are transparent, which makes it hard for us to appreciate their motion. One technique for making these invisible motions visible is schlieren photography, which makes differences in density visible. Here it’s combined with high-speed video to show what happens when you use a lighter (minus the spark!). When the fuel starts flowing, it’s unstable and turbulent, but after that initial start-up, you can see the jet settle into a smooth and laminar flow. Wisps of fuel diffuse away from the jet as the fluid disperses. As the valve shuts off, the flow becomes unstable again, and the remains of the lighter fluid diffuse away. (Video credit: The Missing Detail)
Flamethrowing
Humans have long been fascinated by staring into flames, and the Slow Mo Guys carry on the grand tradition here with 4K, high-speed video of a flamethrower. Like firebreathers, a flamethrower’s fire is the result of a spray of tiny, volatile droplets of fuel. Once ignited, the spray becomes a turbulent jet of flames. Turbulent flows are known for having both large and small-scale structure, and there’s some really great close-ups showing this around the 2:00 mark. Also watch the edges of the flame, where the nearby air has gotten hot enough to shimmer. You can see how the trees in the background ripple and blur as the fire heats up the air and changes its density and refractive index. (Video credit: The Slow Mo Guys)
Jumping Off Water
Many insects and arachnids can walk on water by virtue of their hydrophobicity and small size. With their light weight and skinny legs, these invertebrates curve the air-water interface like a trampoline, with surface tension providing the elasticity that keeps them afloat. What’s truly incredible, though, is that many of these creatures, like water striders, can actually jump off the water surface.
The top animation shows high-speed video footage of a water strider leaping off the water. Notice how it distorts the air-water interface but doesn’t break the surface – it makes no splash.
The key is not to push too hard. If the insect exerts a force exceeding the limits of what surface tension can withstand, then its legs will break the water surface and it will lose energy to drag and viscous forces. The insect must generate its jumping force without exceeding a hard limit.
The water strider achieves this feat not by pushing downward but by rotating its middle and hind legs. Rotating its legs allows the insect to maintain contact with the water surface longer and continue deforming the interface as it jumps. This maximizes the momentum it transfers to the water, which, in turn, increases the insect’s take-off velocity. By studying and then emulating this mechanism, scientists were able to successfully create a tiny 68-mg water-jumping robot. (Image credits: J. Koh et al., sources, PDF)
This week FYFD is exploring the physics of walking on water, all leading up to a special webcast March 5th with guests from The Splash Lab.