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Sound waves levitate chemistry

Merging droplets in air for precise control and unobstructed analysis

by Sam Lemonick
June 5, 2020 | A version of this story appeared in Volume 98, Issue 22


Photo of two droplets suspended in acoustic levitator and a photo of them merging and reacting.
Credit: Ralf Kaiser
Droplets of sodium hydroxycarbonate and acetic acid are suspended on sound waves (left), then merged to react (right).

Researchers have built a levitation device to help them make precise measurements of chemical reactions (Anal. Chem. 2020, DOI: 10.1021/acs.analchem.0c00929). Levitating droplets on sound waves allows Stephen J. Brotton and Ralf I. Kaiser of the University of Hawaiʻi at Mānoa to precisely control the amount and temperature of reagents, eliminates the effects that a flask’s surface might have on a reaction, and gives an unobstructed view of the reaction for analysis via spectroscopy. Bringing two substances together like this is chemistry at its most basic, Brotton says. They adapted an ultrasonic levitator to create pockets of different air pressure that will hold droplets, one above the other. By changing the sound waves from the levitator, Brotton and Kaiser induced the upper droplet to stretch and merge with the droplet below. They demonstrated an acid-base reaction, spontaneous combustion of rocket propellants, and coalescence of a liquid droplet and solid crystals. They measured reactions using Raman, Fourier-transform infrared, and ultraviolet−visible spectroscopies. Other techniques for merging droplets rely on lasers or electromagnetic forces, but Kaiser says those methods only work with certain chemicals, while their acoustic device is material agnostic. The equipment could be used to study reactions in the atmosphere or on other planets, or for medical chemistry using very small sample sizes. But since the levitator relies on air pressure, it cannot be used to study reactions in a vacuum.


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