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.
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.