Levitation in the Classroom

Part One

By Jacob Breman on June 24, 2020

Update: Check out Part 2!

Waves show up in nature all over the place, but the most interesting ones are invisible. One of the many challenges I face in teaching 7th grade science is creating up-close and personal connections for students as they try to wrap their minds around the models we convince them to believe in even though they can’t directly hold and look at the phenomena the models address. Sound waves have been one of those difficult areas. You can show the compression wave in a slinky, draw air molecules with regions of compression and rarefaction, and watch cool youtube videos, but I may have found something better: an acoustic levitator

The device is called TinyLev by its makers, and it uses two arrays of ultrasonic transducers (speakers) to create a standing wave. A standing wave has nodes where the air is not moving which allow low density materials to become trapped there. I was able to build my own by referencing this instructables.

I would say it is fairly affordable project. As a classroom science teacher, I am always on the lookout for cost effective ways to do cool things in class. I have a friend with a 3-D printer, so I got him to print out the support structure. You could use a multimeter to figure out the polarity of the transducers if you don’t have an oscilloscope. The main issue there is that you want to make sure you have all the transducers wired in the same direction electrically otherwise they will destructively interfere with each other. I found the project pretty straight forward and ended up watching the first video many, many times as I went along.

I was able to levitate small pieces of styrofoam, soap droplets, a small bug, and water droplets. The density and size of the material is important when it comes to acoustic levitation. If the object is too large, it will be too big to just be located at a node and will experience the oscillating air pressure and get kicked out. If the object is too dense, it will simply fall through the standing wave. The nice thing about the ultrasonic is that you don’t hear it very well. Imagine a hundred audible speakers generating the same tone the whole time the levitator was operating! I do hear whining when I get close to my levitator, but I am guessing that is caused by secondary vibrations at a lower frequency.

I have not been able to try out my levitator with students yet, but hope to this year. Here are the things I plan to do with it:

  1. If you have some dry ice or LN2 around, you can use the water vapor to visualize the standing compression wave.
  2. See how many nodes you can find and figure out the wavelength based on the number of objects you can trap.
  3. After trapping an object, shift the phase of the standing wave to see if you can move the trapped particle up and down (or left and right).

I need to remember to follow this post up with some pictures of my acoustic levitator in action when I get a chance to get back into my classroom . . . stay tuned!

Update: Check out Part 2!


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