When a droplet of water falls on a hot pan, it dances across the surface, skimming on a thin layer of steam like a tiny hovercraft; this is known as the Leidenfrost effect. But now, researchers know what happens when a hot droplet falls on a cool surface. These new findings, publishing in the Cell Press journal Newton on March 3, demonstrate that hot and burning droplets can bounce off cool surfaces, propelled by a thin layer of air that forms beneath them. This phenomenon could inspire new strategies for slowing the spread of fires and improving engine efficiency.
"We started with a very fundamental question: What will happen when a burning droplet impacts a solid surface?" says senior author Pingan Zhu of City University of Hong Kong, China.
To better understand how heat affects droplet behavior, Zhu and his team used hexadecane, an oily liquid with fuel-like properties, for their experiment. They dropped room-temperature, heated (120°C/248°F), and burning droplets of the substance on various surfaces, some of which were smooth, scratched, or liquid-repellent. The room-temperature droplet stuck to all the surfaces upon contact, as the team expected. But the heated and burning ones bounced, suggesting that the heat was the key.
Using high-speed and thermal cameras along with computer models, the team captured the droplets in motion. They discovered that as a hot droplet approaches the room-temperature surface, the bottom cools faster than the top. This temperature difference stirs up circulation within the droplet, where hotter liquid flows from the edges toward the bottom, dragging air along with it. That air forms a thin, invisible cushion at the bottom of the droplet, preventing it from making contact with the surface and allowing it to bounce back instead. The authors note that while temperature distribution is key in this process, other factors may also play a role.
"Understanding why hot droplets bounce isn't just about curiosity—it could have real-world applications," says Zhu. "If burning droplets can't stick to surfaces, they won't be able to ignite new materials and allow fires to propagate."
By combining the bouncing behavior of hot droplets with liquid-repellent coatings, the team explored the potential applications of these findings. They demonstrated that burning droplets on liquid-repellent plastic films floated on a thin air cushion, preventing direct contact. This coating reduced the contact area of the droplet onto the surface by more than four times compared to bare plastic, which deformed and sustained fire damage when it came into contact with the burning droplet.
The team also studied this phenomenon in engines. They found that when fuel droplets clung to surfaces, they burned inefficiently, leaving behind unburned residue and wasting energy. But in an engine model with a liquid-repellent coating, the droplets beaded up and burned completely. These findings may lead to better fire-resistant materials and more efficient engines.
"Our study could help protect flammable materials like textiles from burning droplets," says Zhu. "Confining fires to a smaller area and slowing their spread could give firefighters more time to put them out."
