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Physicists have looked deeper into the Leidenfrost effect, which you probably noticed when you poured water into a pot to test its temperature.
Sprinkle water over a hot pan and the droplets will run around the pan, speeding up like a little crazy hovercraft on steam cushions.
This is the Leidenfrost effect that you have probably experienced while cooking. Johann Gottlob Leidenfrost, a German physician and theologian, described the phenomenon in 1756 in a book on the properties of water.
But French scientists have now found something new about these sparkling drops. When they are small enough – about a millimeter in diameter – the heat in the liquid causes the tilt and rotation of the drop. This, in turn, propels the droplet to roll.
Scientists – and home cooks – had never noticed this before, as no one had tried to fix a drop of water on a perfectly flat surface. Moreover, as the water is clear, you can not usually see in which
It was already known that the droplets, levitating over a layer of vapor, moved easily, but it was assumed that they were slipping along a slope or were being pushed by the currents. ;air. New research shows that they can move on their own.
"It's of an embarrassing simplicity," said David Quéré, a scientist at the French National Center for Scientific Research and at École Polytechnique.
"The drop runs away," he said. "There is a small engine inside, which is surprising. From this point of view, it is incredibly different from the usual drops, which of course stay where you place them.
Dr. Quéré and his colleagues describes the research this week in the journal Nature Physics.
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In the experiments, water droplets were placed on a very flat surface, very hot and non-absorbent, held in place by a needle. (For the Leidenfrost effect to occur, the surface is at least equal to about 400 degrees Fahrenheit, well above the boiling temperature of the water. Some recipes use it as a test to see if a saucepan is hot enough.)
The largest drops, more than 1.5 millimeters in diameter, have a more flattened shape (Dr. Quéré describes them as puddles). In the droplet, the liquid divides into two convective cells that rotate in opposite directions. Think of two wheels spinning in opposite directions, one canceling out the other widely. (The tracers mixed with water allowed the scientists to observe the flow.)
As the water evaporates, the droplet contracts and becomes almost spherical, leaving room for only one convective cell. When the needle is raised, the smallest droplet rises in the direction of the convective rotation.
Dr. Quere calls it a Leidenfrost wheel.
The next step in their experiments is to guide the droplets into grooves or temperature differences, Dr. Quere said.
There are other fundamental phenomena to study, such as the temperature of the surface which must be well above the boiling point before the drops begin to levitate.
"Very remarkably, there is no answer to this question today," said Dr. Quéré.
Three other interesting things about the water
Kenneth Chang has been at the Times since 2000 to write about physics, geology, chemistry and planets. Before becoming a science writer, he was a graduate student whose research involved controlling chaos. @kchangnyt
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