If you slap a handful of droplets onto a very hot pan, you can watch them bounce and dance.
These drops, believe it or not, actually levitate. If the surface is hot enough, the heat vaporizes the side of the droplet closest to it, creating a cushion of gas on which the rest of the droplet floats.
This is known as the Leidenfrost effect, after the German physician Johann Gottlob Leidenfrost, who documented the phenomenon in the 18th century.
Now a team of scientists has worked out a way to lower the temperature at which this little water dance occurs. A microscopically textured surface transfers heat to droplets more efficiently, a finding that has implications for heat transfer applications – such as cooling industrial machinery and nuclear cooling towers.
“We thought that micropillars would change the behavior of this known phenomenon, but our results contradicted even our own ideas,” says mechanical engineer Jingtao Cheng of Virginia Polytechnic Institute and State University.
“The observed bubble-droplet interactions are a major breakthrough for boiling heat transfer.”
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We have known about the Leidenfrost effect for some time and its parameters are well known. For this to happen, it needs enough heat for the water to form steam immediately upon contact with the hob, but not so much heat that the entire drop of water evaporates immediately.
The reason water does not completely evaporate at Leidenfrost temperatures is that much of the energy from the hot surface is carried away as steam instead of entering the rest of the droplet.
The surface that Cheng and his colleagues devised is made up of hundreds of tiny posts about 0.08 millimeters high, about the width of a human hair. These are arranged in a grid, separated from each other by a distance of about 0.12 millimeters. When placed on a surface, a drop of water will cover about 100 columns.
As the water sits on the surface, the posts push into the drop of water, adding more heat to the interior and allowing the water to boil faster. This means that the Leidenfrost effect can be observed within milliseconds and at much lower temperatures than on a flat surface such as a hot plate or pan.
In fact, the team was able to induce Leidenfrost levitation at 130 degrees Celsius, much lower than the 230 degrees Celsius they estimated as typical for the effect under these conditions.
Now water is an excellent cooling medium. Water boils and evaporates at around 100 degrees Celsius (it varies a bit with altitude). Liquid water cannot be hotter than this boiling point because it turns into steam.
That’s why this person was able to cook soup in a plastic bag over a fire: the heat is transferred to the water, which cannot exceed the melting point of the plastic (note: don’t do this, there are chemicals in the plastic that you don’t want in your soup).
The micropillar surface therefore offers a more efficient heat transfer mechanism that could be much safer than water cooling technologies currently in use, the researchers say, helping to prevent dangerous accidents such as vapor explosions.
“Vapor explosions occur when bubbles of vapor in a liquid rapidly expand as a result [presence of an] intense heat source nearby. “One example where this risk is particularly relevant is nuclear power plants, where the surface structure of heat exchangers can affect the growth of steam bubbles and potentially trigger such explosions,” says Virginia Tech engineer Weng Huang.
“Through our theoretical exploration in the paper, we examine how surface structure affects the growth pattern of vapor bubbles, providing valuable insights into controlling and mitigating the risk of vapor explosion.”
The team’s research was published in Natural physics.