Droplets bouncing off surfaces are an everyday phenomenon, like raindrops bouncing off lotus leaves or water drops sizzling in a hot pan, levitating and sliding around—aka the Leidenfrost effect. There is also an inverse Leidenfrost effect, first described in 1969, that involves a hot object such as a droplet levitating above a cold surface. Understanding the mechanisms behind these phenomena is crucial to a broad range of practical applications, such as self-cleaning, anti-icing, anti-fogging, surface charge printing, or droplet-based logic systems.
Droplets usually only bounce if the surface is superheated or engineered in some way to reduce stickiness. Physicists from the City University of Hong Kong have figured out how to achieve this bouncing behavior of hot oil droplets off almost any surface, according to a new paper published in the journal Newton.
As we've reported previously, in 1756, a German scientist named Johann Gottlob Leidenfrost reported his observation of the unusual phenomenon. Normally, he noted, 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 skitter across the surface. It's called 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° 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.
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. In 2019, an international team of scientists finally identified the source of the accompanying cracking sound Leidenfrost reported. The scientists found that it depends on the size of the droplet; it's the larger drops that explode with that telltale crack. You can even achieve the Leidenfrost effect with ice, as physicists at Virginia Tech demonstrated in 2022.
The Hong Kong physicists were interested in hot droplets striking cold surfaces. Prior research showed there was less of a bouncing effect in such cases involving heated water droplets, with the droplets sticking to the surface instead thanks to various factors such as reduced droplet surface tension. The Hong Kong team discovered they could achieve enhanced bouncing by using hot droplets of less volatile liquids—namely, n-hexadecane, soybean oil, and silicon oil, which have lower saturation pressures compared to water.
Follow the bouncing droplet
The researchers tested these hot droplets (as well as burning and normal temperature droplets) on various solid, cold surfaces, including scratched glass, smooth glass, acrylic surfaces, surfaces with liquid-repellant coatings from candle soot, and surfaces coated with nanoparticles with varying "wettability" (i.e., how well particles stick to the surface). They captured the droplet behavior with both high-speed and thermal cameras, augmented with computer modeling.
The room-temperature droplets stuck to all the surfaces as expected, but the hot and burning droplets bounced. The team found that the bottom of a hot droplet cools faster than the top as it approaches a room-temperature surface, which causes hotter liquid within the droplet to flow from the edges toward the bottom. The air that is dragged to the bottom with it forms a thin cushion there and prevents the droplet from making contact with the surface, bouncing off instead. They dubbed the behavior "self-lubricated bouncing."
"It is now clear that droplet-bouncing strategies are not isolated to engineering the substrate and that the thermophysical properties of droplets themselves are critical," Jonathan B. Boreyko of Virginia Tech, who was not involved in the research, wrote in an accompanying commentary.
Future applications include improving the combustion efficiency of fuels or developing better fire-retardant coatings. "If burning droplets can't stick to surfaces, they won't be able to ignite new materials and allow fires to propagate," co-author Pingan Zhu said. "Our study could help protect flammable materials like textiles from burning droplets. Confining fires to a smaller area and slowing their spread could give firefighters more time to put them out."
DOI: Newton, 2025. 10.1016/j.newton.2025.100014 (About DOIs).
