Hot bodies have less drag

If you’ve ever watched water droplets skitter about on the surface of a hot skillet, you’ve been entranced by the Leidenfrost effect in action. It can occur when you have a hot object in contact with a colder liquid. If the temperatures are right, the heat from the hot object will vaporize the liquid causing a think layer of gas between them. In the case of a skillet, the hot pan creates a thin layer of water vapor that the drop of water then floats on. The vapor layer is also usually a much better insulator of heat than the liquid so it causes the drop of water to last much longer than if it was just in contact with the pan.

Now researchers have shown the Leidenfrost effect works very well in reverse. They dropped metal balls heated to different temperatures into a liquid and watched how fast they fell. The chose a room temperature ball, a heated ball that wasn’t enough to make the Leidenfrost effect occur, and a ball heated above the Leidenfrost temperature.

If you click on this image, you’ll see the results.

Combined video showing the fall of a 20 mm steel sphere at 25 degrees C, 110 degrees C, and 180 degrees C. For 110 C there is an intensive bubble release and for 180 C there is a continuous vapor film. The frame rate used was 1000 fps and the video playback speed is 30 fps.

Combined video showing the fall of a 20 mm steel sphere at 25 degrees C, 110 degrees C, and 180 degrees C. For 110 C there is an intensive bubble release and for 180 C there is a continuous vapor film. The frame rate used was 1000 fps and the video playback speed is 30 fps.

The Leidenfrost effect not only creates a vapor layer but that then reduces the drag on the falling spheres. The researchers found that the drag was reduced by as much as 85% between a room temperature ball and a ball above the Leidenfrost temperature. Close-up photographs of the ball above the Leidenfrost temperature clearly show the vapor layer.

(a) Digital camera snapshot of a heated 15 mm steel sphere held stationary in fluorinated liquid with sphere temperature above the Leidenfrost temperature. A thin vapor layer streaming around the sphere can be observed by the ripples moving along the sphere surface. (b) Snapshot at the instant when the sphere has cooled to the Leidenfrost temperature that is marked by an explosive release of bubbles.

(a) Digital camera snapshot of a heated 15 mm steel sphere held stationary in fluorinated liquid with sphere temperature above the Leidenfrost temperature. A thin vapor layer streaming around the sphere can be observed by the ripples moving along the sphere surface. (b) Snapshot at the instant when the sphere has cooled to the Leidenfrost temperature that is marked by an explosive release of bubbles.

Ref: Phys. Rev. Lett. 106, 214501 (2011)

Posted May 31st, 2011 in Uncategorized.

2 comments:

  1. Evan:

    That’s really cool!

  2. dylan:

    Always wondered if there was a practical application of the Leidenfrost effect (apart from making it easier to firewalk.)

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