Destin from Smarter Every Day has a great new video exploring acoustic levitation. With carefully placed speakers, you can create a standing wave with sound that’s capable of levitating lightweight objects against the force of gravity. Around 4:00, Destin demonstrates this with colored water droplets, which is where the real fireworks start. As he turns up the volume on the speakers, the big droplets explode. This happens when surface tension can no longer hold the drop together. But the high-speed footage offers other clues about what’s going on. Notice how the drops flatten out as the sound volume increases. If you look back to the standing wave animation at 1:33, you’ll notice that just to either side of the nodes (the spots that don’t move), the wave is still oscillating back and forth a little bit. As you increase the sound volume, that standing wave gets stretched to a larger amplitude, which means that those little oscillations just to either side of the node get stronger (and steeper), too. This change in acoustic pressure squishes the drops into pancakes as the fluid tries to stay right at the node. Eventually the droplet is just too flattened for surface tension to keep it together and it bursts into smaller droplets. (Video credit: Smarter Every Day; submitted by Matthew P.)
Tag: acoustic levitation
Acoustic Levitation in Three Dimensions
Acoustic sound is a form of pressure wave propagating through air or another fluid. Place a speaker opposite a plate, and its sound will reflect off the surface. The original pressure wave and its reflection form a standing wave. With intense enough sound waves, the acoustic radiation pressure can be large enough to counter the force of gravity on an object, causing it to levitate. We’ve shown you several examples of acoustic levitation before, including squished and vibrating droplets and applications for container-free mixing. Today’s video, however, shows the first acoustic levitation system capable of manipulating objects in three dimensions, an important step in developing the technology for application. (Video credit: Y. Ochiai et al.; via NatGeo)
Shaping and Levitating Droplets
Opposing ultrasonic speakers can be used to trap and levitate droplets against gravity using acoustic pressure. Changes to field strength can do things like bring separate objects together or flatten droplets. The squished shape of the droplet is the result of a balance between acoustic pressure trying to flatten the drop and surface tension, which tries to pull the drop into a sphere. If the acoustic field strength changes with a frequency that is a harmonic of the drop’s resonant frequency, the drop will oscillate in a star-like shape dependent on the harmonic. The video above demonstrates this for many harmonic frequencies. It also shows how alterations to the drop’s surface tension (by adding water at 2:19) can trigger the instability. Finally, if the field strength is increased even further, the drop’s behavior becomes chaotic as the acoustic pressure overwhelms surface tension’s ability to hold the drop together. Like all of this week’s videos, this video is a submission to the 2103 Gallery of Fluid Motion. (Video credit: W. Ran and S. Fredericks)
Levitation By Sound
Levitation is an effect usually associated with electromagnetic forces, but it’s possible with sound as well. This acoustic levitation is achieved by using the pressure from sound waves to balance gravity’s effect. By manipulating the sound, it’s possible to bring separate objects together while continuing to levitate them. The behavior is demonstrated in the video above by combining solid sodium with a drop of water for what any high school chemist will tell you is a spectacular reaction. (Though, if that’s too small-scale for you, there’s also this video.) (Video credit: D. Foresti et al; via SciAm)
Acoustic Levitation
Researchers at Argonne National Laboratory are using acoustic levitation of droplets to further pharmaceuticals. By placing two precisely aligned speakers opposite one another, a standing wave can be created. At nodes along the standing wave, there is no net transfer of energy, but the acoustic pressure is sufficient to cancel the effect of gravity, allowing light objects like droplets to levitate. This is why, in the video, you see the droplets are placed at equally spaced distances and if one is slightly off the node, it vibrates noticeably. The benefit of this levitation to pharmaceutical research comes at the molecular level; drugs formed from solutions kept in a solid container are likely to be crystalline in structure and thus less efficiently absorbed by the body. If the drug can instead be kept in an amorphous state by evaporating the solution without a container, then the resulting drug may be effective at a lower dosage than its crystalline counterpart. (Video credit: Argonne National Laboratory, via Laughing Squid, submitted by @__pj)
Ultrasonic Levitation of Drops
This video shows an ultrasonically levitated 3 mm drop of propylene glycol changing shape. A couple of things are happening here. Firstly, the drop is suspended due to the acoustic radiation pressure from intense ultrasonic sound waves being produced by a transducer vibrating at 30kHz. Then the power input to the ultrasonic transducer is increased, which strengthens the acoustic field, and this is what causes the drop to flatten. Currently, acoustic levitation is used for containerless processing of very pure materials or chemicals. As with many methods for levitation, it is currently restricted to objects of relatively light weight. (Video credit: J. R. Saylor et al, Clemson University)
