Acoustic levitation of water droplets


These water droplets are not falling; they’re actually stuck in place. What we’re seeing is the effects of an acoustic levitator. The device was initially developed by NASA to simulate microgravity. Now it’s being used by the pharmaceutical industry do develop better drugs.

The two parts of the apparatus seen in the image above are both speakers. They put out a sound at about 22 kHz, which is beyond the human range of hearing. When precisely aligned they interfere with each other and create a standing wave. The droplets are trapped in the nodes of that wave.

So are these guys just playing around with the fancy lab equipment? Nope. The levitation is being used to evaporate water from a drug without the substance touching the sides of a container. This prevents the formation of crystals in the solution. But we like it for the novelty and would love to see someone put one of these together in their home workshop.

Don’t miss the mystical demo in the clip after the break.

62 thoughts on “Acoustic levitation of water droplets

  1. There was speculation ancient Egyptians used similar concepts to raise the blocks of the pyramids. Apparently they found a bunch of giant rocks that if enough people pounded on them would resonate at a magic frequency and had potential to do what is seen here. Not sure I buy into the idea and I believe it was dis-proven later but that is how I learned about this . Figured someone might enjoy it.

      1. Right, I said it had been dis-proven it was more for the giggle factor and interesting that scientist who suggested it realized this could be done like 15 years ago because they thought out side the box. My goal was to make you smile , not convince you this is right.

      2. Maybe they used a levitator to levitate the levitator which was levitated by a levitator which was . . . .
        and then they ran out of rocks to build any more pyramids because the levitated the hell out of all of them, so they went on to invent beer instead.

    1. Awesome levitation device.
      The Egyptian pyramids were made by geopolimeric concrete using the primary components found in Nile river sludge and ground rocks. Almost no egyptologyst wishes to review his “magical” methods of pyramid raising. If one looks at the fragments of the pyramid blocs it becomes evident that the “rock” is homogeneous and contains organic fibers. No rock is homogenic and cannot have natural fibers (palm tree wovings) inside of it. Its simple concrete of its times. Its a shame that the “experts” are unwilling even to discuss those facts.

      1. eh…the Jury’s still out on the ‘concrete’ hypothesis…the reason it’s not accepted by the mainstream yet is because there isn’t much evidence for it.

        I haven’t been able to find a source on your claim the bricks have it’s of palm tree in them, either…

        1. I thought they used straw, possibly from the barley they were using to make the beer with.
          But maybe I’m just getting my hopes up.

          Question for thread origin dude. Have you touched the pyramids? I Haven’t, been the couple of friends I have that have been their did and it had a profound impact on them. They never mentioned fibers or anything other than Solid (structurally and aesthetically ).

      1. Good try. But transducers like that are designed to be coupled to water or other dense materials, which is how its being used in the link you provided. If you attempt to run them in air, there’s a serious impedance mismatch, since air is much less dense. I hear this results in less than 1% of the vibrational energy actually being transferred to the air, and the rest being reflected back into the transducer; quickly resulting in its destruction at any significant power level. I’ve yet to see any hobbyist attempt at high-power ultrasound in air.

        1. Yes and no ;-) It’s all to do with impedance matching, like you say, and this is achieved through suitable horn design (of which I know absolutely nothing). If you look at the horns he’s using, they start at a very small diameter then widen rapidly to a large disc – this has the effect of increasing the amplitude of vibration from the transducer itself, resulting in a better impedance match to air. The transducer itself will probably still be just a piezo stack like the tank transducer I was playing with, but it will be at much higher power (judging by the size, I’d guess a good few hundred watts).

  2. Would this be dangerous at all to duplicate? Your ears may not tell your brain about a 22KHz sound but it is still a physical vibration and I would imagine it must be a pretty powerfull one to have an effect like this. Could it damage your hearing without you even knowing it is happening? Would it be like staring into a bright UV source?

    1. I think this setup use some sort of ultra directionnal ultra-sound horn (as the one featured here once to destroy objects ;)). But yes ultra-sonic vibration even if not heard can destroy permanently your ears. So be careful, you can find many schematic for ultra-sonic detector and use it to monitor the pressure level outside of your setup.

    2. The average raindrop is about 0.1 mL
      1L of water weighs 1kg
      so 0.1mL weighs 0.1 grams

      potential energy = mgh.(formula is only valid when close to earth)
      Let’s assume that h is 1 meter and m is 0.1 grams
      then the potential 0.0001*9.81*1.0=0.000981 joules

      To me that doesn’t sound like much energy is required in the standing wave of a column air to keep a drop from falling due to gravity. But I’m no physicist.

      1. I guess an interesting over-kill “heavy water” project would be to make the “resonance” column pencil size and chain a few of these together to make a water led display?

      2. Truth,
        That formula tells you the energy required to raise one raindrop to a height of 1 meter, not the force required to keep it there. That force is exactly equal to the weight of the raindrop.

      3. That would be the amount of energy being imparted to the droplet, not how much energy is contained within the wave. Acoustic levitation devices operate at sound pressure levels in excess of 150dB. Studies in the area of the effects of ultrasonics on humans indicate that at 20kHz, permanent hearing loss can be measured after exposure to an SPL of 120dB. Now, I would venture that 22kHz probably requires more power to do harm than 20kHz, but considering this device operates at better than a THOUSAND times as much sound intensity, tinkering with this at home without earplugs and a sound level meter is about as dangerous as building a 500mW near-UV laser out of a blu-ray drive and using it without goggles.

      4. Great demo. Is 22 kHz or generally ultrasound necessary to hold the droplet or a lower frequency would be sufficient?

        Assuming the 0.000981 joules is a correct value what does it mean in terms of effectively radiated ultrasound or sound from the speaker? For example, would a standing wave created by 0.5 W speaker on 16 kHz be sufficient?

  3. We created a levitating drop not long ago in our university lab with help of a ultrasonic welding machine. But I’m not sure if it’s a good idea to misuse one of these machines to create a standing wave. Not much later the relatively new ultrasonic head died in normal use.
    I also heared about the possibilty of using some parts of old polaroid cameras as ultrasonic speakers.

      1. “Science isn’t about why, it’s about why not. You ask: why is so much of our science dangerous? I say: why not marry safe science if you love it so much. In fact, why not invent a special safety door that won’t hit you in the butt on the way out, because you are fired.”

        -Cave Johnson

  4. So, a few possibilities. Though I fear the devil might be in the details when it comes to designing an impedance matching horn/plate with decent efficiency.

    A shame there isn’t something midway in difficulty, power, and price between this and common piezo tweeters. I wonder if a properly designed electrostatic speaker can produce large amounts of ultrasound.

  5. This was my absolute FAVORITE display at Huntsville and I kept sneaking back all week to play with it at space camp. Theirs had kitty litter and random little plastic bits you could pour in with a scooper that would float and spin. Hated their liquid rocket demo though. That thing was way too loud lol. Long live the acoustic levitator. As per the Egyptologists/alien hunters, check out “Singing Rocks”. I saw some in Georgia when I lived there, but I am pretty sure there are more around the country. It was not overwhelming, but eerie. I imagine the natives revered the area as it would have freaked me out a bit too :) Update: after a quick google, it appears they can be ringing rocks, too. Enjoy and thanks for the memories of the SCAL :)

    1. I’m guessing the best you could do is what is shown . . . the technology is getting water droplets into the harmonic sweet spots of sound waves. Even if you change the wave, it’s still in the shape of a sound wave. [not sure if that made sense]

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