Leidenfrost effect

With the Leidenfrost effect, also called Leidenfrost phenomenon, the effect of jumping on hot ground ( "dancing " ) is drop meant the Johann Gottlob Leidenfrost described.

Overview

This phenomenon was first explored by Johann Gottlob Leidenfrost in Duisburg and is described in the De Aquae Communis Nonnullis Qualitatibus Tractatus in 1756. The writing itself is based on the ancient, Empedocles (or Aristotle ) attributed four- element theory (fire, water, air and earth ) and intended to demonstrate the preparation of earth from fire and water. Presumably " succeeded " Leidenfrost this experiment because he ausdampfte the dissolved salts of hard water Duisburg.

Even today, when scientifically explained, described by the Leidenfrost effect a physical effect, the delayed implementation of substance, ie the time- stretched change of state implies. This effect can be observed at a hot stove, on which you can drop a drop of water. If the temperature of the interface is high enough to achieve a rapid primary evaporation, as floats or slides of water droplets on a vapor cushion which isolates it from direct heat transmission. The steam is trapped under the water drops and escapes slowly. At the same time created new water vapor and the droplet to slide something like a hovercraft over the hot material.

This gaseous protective layer is formed even at liquid nitrogen when it encounters objects with room temperature. Due to the Leidenfrost effect, therefore, it is possible to let liquid nitrogen run (-196 ° C) over the hand or immerse the hand for a moment, without getting hurt. However, it must be ensured that the nitrogen can take place anywhere and does not accumulate in depressions. Risk of the liquid nitrogen if materials with a high thermal conductivity contacts (such as a ring on a finger ). Here there is a considerable danger of frostbite.

An unwanted effect causes the Leidenfrost phenomenon during hardening of metals in a liquid medium such as oil or water. The desired short time for cooling down of the workpiece is extended through the steam layer between the workpiece and the cooling medium. By means of additives to the cooling medium, the effect can be affected due.

Another process in which the Leidenfrost effect supposedly protects against burns, but actually does not matter, is running on hot coals.

In the series Mythbusters dipping the finger moistened with water has been successfully tested in liquid lead, without being damaged. This is also due to the Leidenfrost effect.