Sound and vibration can be powerful tools for controlling liquids. In this animation, a water/glycerin drop violently bursts into a cloud of droplets when it is vibrated vertically 1000 times per second by a piezoelectric actuator. This vibration shakes the drop with accelerations of 150 g. Initially, the amplitude is small enough to simply create ripples around the drop’s circumference. As it increases, the drop deforms more at the edges and starts to eject droplets there. When the vibration hits a critical amplitude, the entire drop explodes into droplets. The technique is called vibration-induced droplet bursting, and its near-instantaneous ability to atomize drops makes it a candidate for applications like spray cooling microprocessors or spray coating a solid surface. (Video credit: B. Vukasinovic, source)
Tag: vibration
Crowns On Impact
Dropping a partially-filled test tube of water against a table makes the meniscus at the air-water interface invert into a jet of liquid. In some cases, the impact is strong enough to generate splashing crowns of water around the base of the jet. These crowns come in two forms – one with many splashes layered upon one another and the other with only a few splashes and a faster jet.
The many-layered splash crowns come from the pressure wave that reflects back and forth from the bottom of the tube to the surface and back. This pressure wave moves at the speed of sound and vibrates the water surface, creating the many splashes. The same reflected pressure wave occurs in the second type of splash crown, but it gets disrupted by cavitation bubbles that form in the water (visible in the lower left image). Instead the splash crowns form from the shock waves generated when the cavitation bubbles collapse. (Image credits: A. Kiyama et al.)
Resonating with the Windows Down
Ever roll down your window a bit while driving and immediately hear a terrible, rhythmic noise? That awful whum-whum-whum is–oddly enough–an example of the same physics that allows you to make an open bottle whistle by blowing over it. Fluid dynamicists call it Helmholtz resonance. Air flowing over the bottle neck or around the car makes the air inside the container vibrate with a frequency that depends on the bottle or car’s characteristics. That vibration generates noise that we hear as a hum or whistle for a bottle or a lower frequency whum-whum for a car window.
The images above show flow past different open windows on a car. Air flow remains relatively steady past the side-view mirror and front window of a modern car, so the noise from opening the front window is not usually too bad. But flow separation and reconnection near the rear window of a car creates very unsteady airflow there which exacerbates this resonance issue. This is why lowering the rear window usually causes more noise. Fortunately, the solution is relatively simple: open more than one window and it disrupts the resonance! (Image credit: Car and Driver; submitted by Simon H.)
Buzzing Straws
Many woodwind instruments owe their sound to the vibration caused when air moves past parts of them. As Nick Moore demonstrates in this video, you can create a simple version of this effect with a slit drinking straw. The buzzing the straw produces when air passes through is a sort of aeroelasticity – it’s a combination of aerodynamic and structural forces that drive the behavior. Low pressure created by the fast-moving flow tends to draw the straw together, but once flow is stopped, the elasticity of the straw makes it rebound open, allowing air to flow again. Even more elaborate vibrations are possible when the straw is elastic. (Video credit: N. Moore)
Bouncing Droplets
Droplets bouncing on a pool form a beautiful and fascinating system, as recently featured by Physics Girl, Veritasium, and Smarter Every Day. The Lutetium Project – a consortium of French physics, graphic design, and music students – have their own take on the subject with beautiful short videos constructed from experimental research footage. With simple text explanations and lovely original music, they combine science, art, and outreach brilliantly. Also check out their quantum walker video and be sure to subscribe to their channel (in English or French) for more! (Video credit: The Lutetium Project; submitted by @g_durey)
Avoiding Coalescence
If you watch closely as you go about your day, you may notice drops of water sometimes bounce off a pool of water instead of coalescing. Fluid dynamicists have been fascinated by this behavior since the 1800s, but it was Couder et al. who explained that these droplets can bounce indefinitely as long as the thin air layer separating the drop and pool is refreshed by vibrating the pool. In this video, Destin teams up with astronaut Don Pettit to film the phenomenon in beautiful high-speed. My favorite part of the video starts around 8:18, where Destin shows Don’s experiments with this effect in microgravity. It turns out that the cello produces just the right frequencies to create a cascade of bouncing water droplets, much like a Tibetan singing bowl turned back on itself! (Video credit: Smarter Every Day; submitted by Destin and effyeahjoebiden)
Shaking in the Wind
Sitting at a traffic stop on a windy day, you may have noticed the beam holding the traffic lights shaking steadily up and down. This phenomenon is called vortex-induced vibration. When the wind flows over the beam, it looks something like the flow animation shown above. Airflow follows the shape of the beam until near the backside, where the air separates from the surface and creates a vortex that sloughs off into the beam’s wake. These vortices form asymmetrically on the beam – first on one side, then the other. This creates unequal pressures on either side of the beam, and those pressure differences create a force that moves the beam. Because vortices are being steadily shed off the beam, it will keep moving back and forth as long as the wind is strong enough. (Image credits: traffic light – L. Sennick, source; cylinder – Aphex82/Wikimedia)
Making Droplets
If you’ve ever wondered how fluid dynamicists create those tiny perpetually bouncing droplets they study, wonder no further. A typical method, shown here, is to use a simple toothpick. First, you take a shallow container of silicone oil and vibrate it vertically. Then you dip the tip of the toothpick into the oil and pull it out, stretching the oil into a long filament. When it detaches from the toothpick, a droplet will start to form at the tip of the filament as it falls back toward the pool. But the bouncing of the surface is enough to keep the new drop from coalescing back into the pool, leaving the little drop to bounce along on its liquid trampoline. (Image credit: S. Lapointe)
Inside a Humidifier
After this, you may never look at a humidifier the same way again. Ultrasonic humidifiers generate tiny droplets using piezoelectric transducers. When the humidifier is on, the ultrasonic vibrations of the piezoelectric transducer create a pressure wave that forces the water above into a hill with a string of liquid droplets extending upward. For a sense of the scale, the gray bars shown in each image above represent 1mm. The super-fine droplets the humidifier produces come from cavitation of these larger drops, as shown in image c). Image d) shows snapshots of the formation of the droplet string over a matter of milliseconds. (Image credit: S. J. Kim et al., original poster)
Quantum Droplets
Over the past decade, fluid dynamicists have been investigating tiny droplets bouncing on a vibrating fluid. This seemingly simple experiment has remarkable depth, including the ability to recreate quantum behaviors in a classical system. In this video, some of the researchers demonstrate their experimental techniques, including how they vary the frame rate relative to the bouncing of the drops. At the right frame rate, this sampling makes the droplets appear to glide along with their ripples, giving us a look at a system that is simultaneously a particle (drop) and wave (ripple). (Video credit: D. Harris et al.)
