Leidenfrost effect induces bouncing droplets

Place a droplet on a surface whose temperature is well above the liquid’s boiling point, and curious things can happen. Hovering on a cushion of its own vapor, the droplet can skitter, roll , and even flow uphill . The study of that phenomenon, known as the Leidenfrost effect, has typically focused on the vapor layer between droplets and a surface. Felipe Pacheco-Vázquez of the Meritorious Autonomous University of Puebla in Mexico and his colleagues have now demonstrated the Leidenfrost effect acting between droplets of different liquids. Rather than coalesce, the droplets bounce off each other, because a barrier of gas forms between them.

In each of their trials, the researchers placed a droplet of one of 10 liquids at the center of a slightly concave hot plate and then deposited another droplet on the edge of the plate. Using a high-speed camera, they then monitored the outside droplet’s gravity-driven descent toward the inner liquid. Whereas droplets of the same liquid would combine upon colliding, droplets of different substances would sometimes carom off each other multiple times. The primary factor that determined the outcome of a collision, the researchers found, was the difference in boiling points between the two liquids.

Pacheco-Vázquez and his colleagues’ analysis of the recorded collisions led them to the conclusion that the same Leidenfrost effect that was keeping the droplets skittering across the hot plate was also preventing the droplets from merging. For two droplets heated on a surface much hotter than both their boiling points, the droplet with the higher boiling point acts as the superheated surface for the other droplet, just as the hot plate does for both droplets. The vapor that forms between the droplets prevents the liquids from contacting each other, and the droplets remain intact when they recoil. Merging finally occurs when one of the droplets becomes too small to retain its gaseous cushion.

To elaborate their proposed mechanism, which they call the triple Leidenfrost effect (since a film of vapor forms between the droplets as well as between each of the droplets and the hot plate), the researchers chose two liquids with a large difference in boiling temperature: ethylene glycol (190 °C at the 2200 m elevation of the university’s Ecocampus in Puebla) and chloroform (54 °C). The frames of the video, shown in the figure, reveal a distinct layer of vapor between droplets. After bouncing several times off the ethylene glycol, the chloroform droplet gets depleted, finally reaches the other droplet, and bursts violently. (F. Pacheco-Vázquez et al., Phys. Rev. Lett. 127, 204501, 2021 .)