Thursday, October 04, 2018

Tiny Water Droplets Self-Propel on Sizzling Surfaces

Many cooks have experienced this—sprinkle a few drops of water on a searing hot skillet, and watch them roll around like a couple of glass beads. Scientists had previously thought that this phenomenon is solely due to the cushions of steam that form underneath the droplets, but now researchers find these drops also propel themselves, with the churning fluid inside them acting like engines. This discovery could lead to new kinds of self-propelled devices, they add.

The successive states of a water droplet gently deposited on a hot plate (with constant temperature, about 300 degrees Celsius), and spontaneously propelled. Colors are used to better distinguish the successive images. About 50 milliseconds separate two images, and the drop radius is 1 millimeter.
Image Credit: Ambre Bouillant, Célia Boutilier and David Quéré

Drops of water that land on a surface heated well above the boiling point of water are highly mobile, and tend to wander around as soon as they land, as reported in 1756 by German physician and scientist Johann Gottlob Leidenfrost. This "Leidenfrost effect" results from the droplets levitating on layers of vapor, which are typically about 50 microns thick, or about half the average width of a human hair, and which cushion the droplets from the hot surface.

The mobility of the drops is routinely attributed to how levitation makes them move virtually without friction, rendering them sensitive to any force, such as surrounding airflows or the gravitational pull resulting from tiny slopes on the surfaces underneath them. But now scientists have discovered these droplets move not only due to outside forces, but to internal ones as well.

"Our initial and main surprise in this work was to observe, for the first time, more than 250 years after Leidenfrost's first observations, and after so many people from engineering and physics communities have studied the phenomenon of thermal levitation, that such drops cannot stay where they are placed but instead get self-propelled, as if they were containing a small motor," said study senior author David Quéré, a physicist at the City of Paris Industrial Physics and Chemistry Higher Educational Institution in France.

This research began when a student asked Quéré if the fluid flowed inside Leidenfrost drops. To learn more, the researchers used high-speed cameras and motion-tracking software to monitor the movements of tiny glass spheres injected in the droplets and then illuminated with red laser light as the droplets moved around on a flat, smooth and level surface.

When it came to large drops, Quéré said he expected and saw that the particles within the droplets flowed in a balanced manner—"the common case of boring research where you find what you expect."

However, unexpectedly, as these drops evaporated and became less pancake-shaped and more spherical, the flowing within them bcame imbalanced. Once these droplets reached about a millimeter in diameter, these imbalanced forces pushed the drops to spontaneously roll, "as wheels do," Quéré said.

In addition, the imbalanced forces also slightly tilted each hovering drop downward toward the same direction the fluid inside the droplet was flowing. Such tilting essentially acts like a ratchet, pushing these "Leidenfrost wheels" to keep rolling until they evaporate completely.

"Nice observations and great detective work to understand the dynamics," said fluid dynamicist Howard Stone at Princeton University in New Jersey, who did not take part in this research. "It is interesting to think about ways to try to harness this kind of motion."

Normally the direction that Leidenfrost wheels roll is random. However, the researchers also found that these drops roll toward cooler surfaces because lower temperatures result in thinner vapor layers, essentially resulting in an incline between hotter and cooler surfaces. This finding could lead to new kinds of self-propelling devices, where the rolling of Leidenfrost wheels is controlled with temperature gradients. Future work can explore how Leidenfrost wheels behave when they hit walls or other drops, the researchers noted. Scientists can also investigate why these drops occasionally both rotate and oscillate, they added.

"We are also looking at ways we can guide the drops with grooves on the solid surface to control their movement and possibly decide the direction of self-propulsion," Quéré said.

The scientists detailed their findings online Sept. 10 in the journal Nature Physics.

Charles Q. Choi, Inside Science News

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