MIT scientists reveal why water drops move faster on hot, oil-coated surface

There is a classic 2009 Down with the myths An episode in which the hosts show how someone can wet their hand and immerse their hand very briefly in molten lead without hurting themselves. The protection mechanism is known as the “Leidenfrost effect”, and it could someday prove useful for microfluidic devices, especially in microgravity environments, among other applications. We have taken a step closer to realizing these applications, thanks to new knowledge about the phenomenon discovered by scientists at MIT. They described their findings in a recent article published in the journal Physical Review Letters.

As we previously reported, the Leidenfrost effect dates back to 1756. It was around this time that German scientist Johann Gottlob Leidenfrost observed that as water splashed onto a very hot pot sizzles and sizzles. evaporates very quickly, something changes when the temperature of the pan is well above the boiling of water. indicate. When this happens, Leidenfrost discovered, “shiny quicksilver-like drops” will form and slide across the surface.

Over the next 250 years, physicists have found a viable explanation for why this happens. If the surface is at least 400 degrees Fahrenheit (well above the boiling point of water), cushions of water vapor or vapor form under the droplets, keeping them levitating. The droplet can slide across the surface with very little friction. The Leidenfrost effect also works with other liquids, including oils and alcohol, but the temperature at which it manifests (the “Leidenfrost point”) will be different.

Physicists are learning even more about this effect more than two centuries later. For example, French physicists discovered in 2018 that the drops don’t just roll on a cushion of vapor; as long as they’re not too big, they propel themselves too. An imbalance in the flow of fluid inside the Leidenfrost drops acts as a small internal motor. As the drops evaporated, becoming smaller and more spherical, they began to roll like a wheel (aka the “Leidenfrost wheel”).

And in 2019, an international team of scientists finally identified the source of the accompanying crunch reported by Leidenfrost. Scientists have found that it depends on the size of the droplet. The smaller drops will detach from the surface and evaporate, while the larger drops explode with this telltale crack. The culprit is particulate contaminants, found in almost all liquids. Larger drops will start with a higher concentration of contaminants, and this concentration increases as the droplets shrink. They end up with such a high concentration that the particles slowly form a kind of shell around the droplet. This shell interferes with the vapor cushion that keeps the drop in the air and it explodes when it hits the surface.

The co-authors of this most recent paper (MIT mechanical engineer Kripa Varanasi and his graduate student Victor Julio Leon) are interested in exploiting the Leidenfrost effect for practical applications. They carried out a series of experiments of dropping millimeter-sized water droplets onto hot solid surfaces – silicon wafers between 10 and 100 microns thick – covered with thin films of different types. oil and filming what happened with high speed cameras.

They expected the Leidenfrost droplets to move more slowly over an oil-covered surface, because the oil is about 100 times more viscous than air and therefore would produce more friction than a film of vapor. But when the researchers analyzed the images, they found the opposite had happened: the Leidenfrost droplets moved much faster in random directions on oil-coated surfaces than on simple metal surfaces.

“We were puzzled on many levels as to what was going on, because the effect was so unexpected,” Varanasi said. “It’s a pretty complex answer to what may sound simple, but it really does create that rapid propulsion.” Further study showed that under the right conditions a thin layer formed on the outside of each droplet, like a mantle. As the droplet got hotter, tiny bubbles of water vapor started to form between the droplet and the oil, then moved away. The following bubbles generally formed near the same places, forming a single vapor trail which served to push the droplet in a preferred direction.

Small bubbles

The formation of the tiny bubbles also occurs much faster than the transfer of heat through the oily film. The resulting asymmetry means less friction under the bubble, quickly propelling it off the hot oily surface. Varanasi drew an analogy between the oily film, or camouflage effect, and the rubber of a balloon. The force of the bursting vapor bubbles causes the “balloon” to fly up, “because the air comes out from one side, creating an impulse transfer,” he said.

Potential applications include microrobots, defrost systems, and self-cleaning surfaces that remove contaminants like salt to prevent damage from build-up. Until now, these modes of propelling water droplets were limited to a few millimeters per second. But the droplets in the Varanasi and Leon experiments moved much faster: around 10 cm / s.

Several different factors can affect the direction of self-propulsion, such as droplet size, oil film thickness and viscosity, as well as surface texture and thermal conductivity. The authors therefore admit that they cannot yet control precisely this direction. However, “further studies on the formation of the initial asymmetry and the occultation dynamics of rich oil droplets may shed light on how to control the Leidenfrost point on thin liquid films and the direction of self-propulsion on. a surface, ”they concluded. “Such a surface could quickly and controllably release corrosive and fouling droplets from heated surfaces.”

DOI: Physical Review Letters, 2021. 0.1103 / PhysRevLett.127.074502 (About DOIs).