When water droplets hover over a hot surface, they are held in balance by a physical phenomenon known as the Leidenfrost effect. Researchers from ETH Zurich, Switzerland, have discovered that not only do the droplets float on a cushion of hot vapor, they also bounce up and down in a self-initiated move. This “trampolining” may make it necessary to revise our understanding of the well-known mechanism behind this effect.

 

 

The Leidenfrost Dance of the Droplets

Imagine some water droplets hovering over a hot pan, or drops of liquid nitrogen chasing each other across a lab bench. Both cases illustrate the Leidenfrost effect, which describes why a liquid does not simply boil away when placed on a hot surface. Instead, it breaks into droplets that float on a cushion of hot vapor and lose their masses more slowly, through evaporation.

Two-and-a-half centuries ago, the German doctor Johann Gottlob Leidenfrost first described this phenomenon, which was later named after him. Apart from being an entertaining physical observation, the Leidenfrost effect has far-reaching implications for manufacturing and industrial processes. As the vapor cushion reduces the heat transfer between surface and liquid, engineers must account for this effect when they design apparatuses for applications where heat transfer is crucial, such as nuclear reactors and spray cooling machines.

The cushioning effect comes from the vapor draining from the droplet’s lower surface. The forces of the vapor pressure lift the droplet upwards and are counteracted by gravity pulling it down. Scientists have long assumed that Leidenfrost equilibrium is quasi-static, and only forces coming from the environment, such as surface irregularities, uneven heat transfer, and the kinetics from depositing the droplet, can trigger motions such as gaining speed or jumping off the pan.

However, at ETH Zurich, material scientists working with Thomas Schultzius and Dimos Poulikakos from the Department of Mechanical and Process Engineering have discovered that some motions of Leidenfrost droplets, from vibrations to visible bouncing have nothing to do with surface texture or how the droplets were handled.

 

 

A New Move: Trampolining

Using a high-speed camera, the researchers observed that water droplets carefully placed on hot surfaces rested there for a few milliseconds and then started to bounce as if they were jumping on a trampoline. Their trampolining only ceased when the droplets had lost most of their size through evaporation.

Looking more closely at the details of this phenomenon, the researchers found that it all began with small oscillations around the center of gravity. “After a few oscillations, the droplet performs its first jump, so a visible gap appears,” they noted. Although other scientists have reported bouncing effects before, the researchers remark they were the first to describe this self-initiated, regular trampolining process.

 

 

Leidenfrost Equilibrium Out of Balance

To find out what caused this behavior, the researchers scrutinized the droplets from below. Using interference imaging, they discovered that the surface facing the vapor was rippled with interference waves, like those which appear when we throw a stone into a lake. In this case, the waves evolve from the vapor radially draining away, the authors explain.

These ripples disturb the balancing forces between the vapor pressure lifting the droplet upwards and gravity pulling them downwards and lead to an uneven heat transfer. The team argues that at the peaks of the ripples, the liquid is closer to the hot surface and takes up heat faster than in the valleys. This enhances evaporation and leads to vapor overpressure, which pushes the droplet upwards. When gravity takes over, the droplet falls back to the surface, only to take up more heat. Over time, the oscillations grow in height until the droplet can be seen to visibly trampoline.

There are size limitations, though. “Large droplets just wobble, while intermediate-sized droplets trampoline,” the researchers noted. However, once the larger droplets lost mass, bouncing started at radii under 1.4 mm.

The scientists also found that Leidenfrost trampolining was not limited to water: They tested other liquids such as acetone and ethanol and found similar behavior. Only adding glycerol ended the show. “For very viscous liquids, the ripples and the oscillations of the droplet are no longer underdamped and cannot persist. This renders Leidenfrost trampolining impossible,” the researchers concluded.

They suggest a thorough reexamination of the Leidenfrost mechanisms. Self-initiating droplet trampolining could have implications in several applications where heat transfer is critical.

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• Leidenfrost Droplet Trampolining,
  G. Graeber, K. Regulagadda, P. Hodel, C. Küttel, D. Landolf, T. M. Schultzius, D. Poulikakos,
  Nat. Commun. 2021, 12,1727.
  https://doi.org/10.1038/s41467-021-21981-z