Typically when a solid powder dissolves in a liquid, lots of small bubbles are produced. Credit: University of California, BerkeleyĬrawford found that the hot chocolate effect, as he called it, comes down to bubbles. Frank Crawford, the physicist who described what he called the hot chocolate effect, plays a musical instrument he invented. Most of the basic physics behind the acoustic phenomenon was worked out in the early 1980s when Frank Crawford, a physicist at Lawrence Berkeley National Laboratory in California, made the same observation as Fitzpatrick while mixing a cup of cocoa. The technique is already being used by academic and industry researchers to complement other analytical tools such as mass spectrometry. They can even exploit sound to track the pH of an acid–base reaction without a chemical indicator or litmus paper. Through a technique he calls broadband acoustic resonance dissolution spectroscopy, or BARDS, he and his colleagues have harnessed the effect to check the purity of foods and to determine the beach from which a sample of sand came. Some 20 years since his chance observation, Fitzpatrick-now a research scientist at University College Cork in Ireland-has developed a way to turn the kitchen curiosity into a valuable analytical instrument. “I always wanted to see if it was a reproducible effect.” “It was in the back of my mind every time I made a cup of coffee,” he says. But unlike most who hear it, Fitzpatrick, an analytical chemist, was in a position to investigate the underlying mechanism. Then, as he continued to stir, the tap-tap gradually rose in pitch again until it sounded like it did at the start.įitzpatrick wasn’t the first to notice the acoustic dip. The sound of the spoon tapping against the glass as he stirred in the salt rapidly switched from a high-pitched clatter to a much deeper banging noise. One day as he mixed his homemade solution, he heard a curious thing. Among his remedies was gargling with salt water. Credit: Pxfuel, PD-USĪs a PhD student in rainy Dublin in the late 1990s, Dara Fitzpatrick suffered from frequent tonsillitis. As one stirs coffee or hot chocolate, the frequency of the sound of the spoon tapping against the side of the cup changes.

Salt soundwaves