Ultrasonic levitation in Earth sciences
And a good STEM project
To analyze a crystallization event in a liquid, you typically have to put the crystal on (or in) something. Most of the time that isn’t a problem, but what if you were required to look at the shape of the droplet? Or a live bug that you don’t want crawling away? The water droplet changes shape as soon as it touches a glass microscope slide, and the bug will escape unless you pin it down and kill it. In general, I am referring to challenges associated with a sample holder impacting a signal or the ability to measure a signal.
In my case, I often do Raman spectroscopy and X-ray diffraction. For both techniques, the sample needs to be held still using some type of holder. But this is problematic because the signal that is measured is coming from both the sample and sample holder. Sometimes that’s easy to separate, and sometimes it’s not. The analyses would be so much easier if the data only contained the sample’s signal with no sample holder interfering with the measurement.
Enter acoustic levitation. Using sound waves, the sample can be held motionless in mid-air while data is being collected: no more background subtraction or superimposed spectra/diffraction patterns. Just the signal from measuring a parameter associated with the sample itself. I no longer need to second guess if the sample holder is influencing my data analysis.
This is an under-utilized analytical technique, most likely because of the cost to administer it. Acoustic levitators have conventionally been expensive or difficult to build. A recent paper from Marzo et. al (2017) has changed that by presenting the design of a new levitator using off-the-shelf parts and 3D-printed components. I recreated this levitator, and I put the tool through a series of real-life examples in my research. I was amazed at how easy it was to build, and what sorts of new experiments I could do.
What is Acoustic Levitation?
Acoustic levitation, and in this case ultrasonic levitation, uses sound waves to hold an object in place. It does this with a series of ultrasonic speakers (like those backup sensors on cars) pointing at each other. The waves are programmed so that when they interact, they constructively interfere. Ever heard of a “rogue wave” in the ocean? It’s when ocean waves interfere with each other to produce a giant wave that seems to defy the overall pattern of nearby waves. Maybe a more analogous phenomenon would be a seiche, where water oscillates back and forth in a lake/bay/bathtub to produce standing waves. The same process can happen with any kind of wave, including sound waves.
Between the waves are nodes, basically where the wave bends up-and-down, like a harmonic on a guitar string. This node is where objects can be held in place by the acoustic pressure above and below the node (or left and right as in the title animation). If the waves are strong enough, you can suspend a small object on the node. The video blow shows acoustic levitator with an array of ultrasonic speakers, and a bit of styrofoam that can easily be placed on a node, and it just stays there. It spins because the styrofoam is not round, so the sound waves coming from different directions hit it and make it spin.
A video with proof that I finally got my acoustic levitator made! It suspends particles and liquids in mid-air, and testing it out using some red styrofoam.
Real example: evaporation of salty water
One of the most fun parts of making your own scientific tools is using them in real experiments. The next video shows an experiment at the Advanced Photon Source where I performed the X-ray scattering studies. The goal is to figure out how molecules begin to organize themselves in the liquid before crystals form, and what the structure of this salty water is as the water is lost to the air. Seems like this crystallization process should already be well understood, but my results were not what was expected and are a lot different from what conventional wisdom would dictate. This kind of work will eventually allow scientists to recreate the conditions that marine water and brine lakes were like millions of years ago (looking at waters still trapped inside crystals), or what water is like on other planets and moons, like Mars and Europa. Knowing the molecular structure of these waters will lead to better predictions on what kinds of life can exist in these strange places. But a discussion of that will have to wait for another time. Beyond fundamentally profound science questions related to the origin of life, this technique could also be useful in industries where crystallization phenomena are crucial, such as in mineral processing, chemical manufacturing, pharmaceuticals, and elsewhere.
Fun times at the Advanced Photon Source. This video shows the acoustic levitator in action collecting data for X-ray pair distribution function analysis. Adding multiple droplets of brine helps to stabilize the levitated droplets.
The need for levitation is simple. There must be 100% certainty that even the tiniest of changes in the data are a direct result of how the liquid structure is changing. The molecular changes are so subtle that they can be easily lost to noise, and the critical details would have been missed. Levitation gives confidence in the data.
Evaporation of super salty water is slow. Even a single drop on a glass slide takes hours to days to evaporate. I can’t wait that long — not because I’m impatient, but because my experiment time at the national lab will be over before I’m finished collecting one data set! Or, my spectrometer would drift, and my data won’t be calibrated anymore. I could misinterpret changes in the spectrum as being changes in my sample when, in reality, it is my instrument that is changing. With levitation, the whole droplet is tiny, and the entire droplet is exposed to air, which increases its surface area for faster evaporation. What took 5 micro-L of saturated saltwater 5 hours to completely evaporate on a glass slide, takes less than 10 minutes when levitated. I can do more experiments faster, with increased reproducibility.
The results of the salt crystallization study should be published soon, so you’ll see how measuring the tiniest details can affect how we interpret natural processes.
Aaron Celestian is Curator of Mineral Sciences at the Natural History Museum of Los Angeles County, Adjunct Associate Professor in the Department of Earth Sciences at the University of Southern California, Affiliate Research Scientist at NASA Jet Propulsion Lab, and member of the Origins and Habitability Lab at JPL. He researches how minerals interact with their environments and with living things, and how those minerals can be used to solve problems like climate change, pollution, and disease.