In 1756, a German scientist named Johann Gottlob Leidenfrost reported his observation of an unusual phenomenon. Normally, water splashed onto a very hot pan sizzles and evaporates very quickly. But if the pan's temperature is well above water's boiling point, "gleaming drops resembling quicksilver" will form and will skitter across the surface. It's known as the "Leidenfrost effect" in his honor.

In the ensuing 250 years, physicists came up with a viable explanation for why this occurs. If the surface is at least 400 degrees Fahrenheit (well above the boiling point of water), cushions of water vapor, or steam, form underneath them, keeping them levitated. The Leidenfrost effect also works with other liquids, including oils and alcohol, but the temperature at which it manifests will be different. In a 2009 Mythbusters episode, for instance, the hosts demonstrated how someone could wet their hand and dip it ever so briefly into molten lead without injury, thanks to this effect.

But nobody had been able to identify the source of the accompanying cracking sound Leidenfrost reported. Now, an international team of scientists has filled in that last remaining gap in our knowledge with a recent paper by Mathai et al. in Science Advances.

The answer: it depends on the size of the droplet. Smaller drops will skitter off the surface and evaporate, while larger drops explode with that telltale crack. "This answers the 250-year-old question of what produces this cracking sound," said co-author Varghese Mathai, a postdoctoral researcher at Brown University. "We couldn't find any prior attempts in the literature to explain the source of the crack sound, so it's a fundamental question answered." The insights gained could one day make it possible to control the effect for application in cooling systems or particle transport or techniques for transporting and depositing particles for microelectronic fabrication.

The phenomenon continues to fascinate physicists. For instance, in 2018, French physicists discovered that the drops aren't just riding along on a cushion of steam; as long as they are not too big, they also propel themselves. That's because of an imbalance in the fluid flow inside the Leidenfrost drops, acting like a small internal motor. Large drops showed a balanced flow, but as the drops evaporated, becoming smaller (about half a millimeter in diameter) and more spherical, an imbalance of forces developed. This caused the drops to roll like a wheel, helped along by a kind of "ratchet" effect from a downward tilt in the same direction the fluid in the droplet flowed. The French physicists dubbed their discovery a "Leidenfrost wheel."

For their recent study, Mathai et al. wanted to figure out where the cracking sound came from. So they set up an array of high-speed cameras and microphones to monitor individual drops of ethanol as they dropped onto a surface heated above the Leidenfrost threshold. Leidenfrost drops gradually shrink before bouncing off the surface and evaporating. That occurs because they become so light that the vapor flowing around them has a much stronger impact, effectively launching them into the air.

Chinese scientists had previously observed the same thing in their experiments, but only for smaller drops below a certain diameter. [corrected] Mathai et al. found that If a drop starts out bigger (a bit over a millimeter in diameter or more), they don't shrink to a small enough size to fly off the surface. Instead, they move down toward the hot surface, emitting a loud crack when they make contact.

The culprit in this case is particle contaminants. Any liquid will have them, but larger drops will start out with a higher concentration of contaminants. As the droplets shrink, the concentration of contaminants increases. Larger droplets end with such a high concentration that the particles slowly form a kind of shell around the droplet. That shell interferes with the vapor cushion holding the drop aloft, and it explodes when it hits the surface. And as the level of contaminants increases, so does the average size of the drops when they emit the cracking sound.

DOI: Science Advances, 2019. 10.1126/sciadv.aav8081 (About DOIs).