The Leidenfrost effect is a phenomenon of levitation of liquids by its own vapours, observed when a liquid comes into contact with a surface which is a lot hotter than its boiling point. This levitation prevents the liquid from coming into direct contact with the hot surface, causing it to evaporate slower than expected. Each liquid has a corresponding minimum temperature of the surface, called the Leidenfrost point, above which it exhibits the Leidenfrost effect. Before we go ahead, watch this video:

Source: The University of Bath YouTube page

When a liquid comes into contact with a surface hotter than itself, it absorbs heat from the surface and gradually turns into vapour. But when the temperature is too high, the bottom surface of liquid is quickly converted to vapour, but this forms a layer of vapour between the hot surface and the liquid. Since the vapour is usually slower at conducting heat, the bottom surface of the liquid receives heat slower than usual. Therefore it gets converted to vapour slower and the drop of liquid evaporates slower than usual. For example, a drop of water that vaporized almost immediately at 168 °C lasted for 152 seconds at 202 °C.

By Vystrix Nexoth at the English language Wikipedia, CC BY-SA 3.0, Link

The time for which the drop lasts increases till the Leidenfrost point and as you increase the temperature further, the time again starts decreasing. The temperature at which the drop lasts for the longest is the Leidenfrost point. This temperature depends on both the liquid and the material of the surface. When placed on a surface with ratchets, the vapour layer below the drop moves mostly towards the downward incline of the ratchet, making the droplet over it move with it. The easiest way to think of this motion is like a hovercraft. The droplet is suspended over the vapour and moves with the vapour. This is however still under research to properly model this movement. Source: The New York Times Using a hydrophobic coating on the surface can make the Leidenfrost effect visible way below the Leidenfrost point, thus effectively making it more practical to use. There are many proposed uses for the Leidenfrost Effect. It is planned to be used for cooling microprocessors. Just the ratchet shaped ridges on the microprocessors would move the drop along and send the heat with it. This liquid drop and its vapours can be then sent back in after it cools down, thus forming an effective heat exhaust. It is also proposed to be used for controlling the motion of small droplets in biological and chemical research. If understood well, this can prove useful in numerous applications.

References

• Enhanced Droplet Control by Transition Boiling
• Leidenfrost Effect
• Propulsion on a superhydrophobic ratchet

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