How We Get the Smell of Rain →
They found that at the right velocity on the right kind of soil (sandy clay works, but sand doesn’t) a falling water drop can trap tiny air bubbles under it. Those bubbles capture molecules in the soil. As the water drop deforms, the bubbles scoot up through the drop and jet out into the air, like champagne bubbles or spray from a crashing wave.
If the drop falls too slowly, it is absorbed; too fast, and it splatters without the bubbles emerging. “The sweet spot has to do with the velocity of the droplet and the qualities of the soil,” said Cullen R. Buie, an assistant professor of mechanical engineering. He and a postdoctoral researcher, Youngsoo Joung, reported on their work in Nature Communications.