These hot oil droplets can bounce off any surface

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.