In this article, we shall study in detail about Leidenfrost effect. We have provided a PDF link for the same.
Sometimes you notice the activity like when we sprinkle droplets of water on a hot pan to check its temperature the water forms into really small balls and starts skittering around the pan and take longer to evaporate also the ability of liquid nitrogen to skitter on the floor are all examples of leidenfrost effect.
What is Leidenfrost effect
The Leidenfrost effect is a phenomenon that occurs when a liquid is introduced on a very hot surface, creating a vapor layer between itself and the surface of contact. This is because the liquid suddenly gets so hot that it starts to evaporate, forming a vapor barrier that prevents direct contact of the liquid with the hot surface.
Working of Leidenfrost effect
The Leidenfrost effect can be seen when water droplets are poured onto a hot pan. Initially, at lower temperatures, the water evaporates slowly or remains liquid. As the pan temperature surpasses 100 °C (212 °F), the water droplets start hissing and evaporate quickly upon contact. However, when the temperature exceeds a certain point known as the Leidenfrost point. The water droplets form into small balls and skitter around the pan, taking longer time than before to evaporate. This effect continues until a much higher temperature causes rapid evaporation of any additional water droplets.
The Leidenfrost effect arises because, at temperatures equal to or above the Leidenfrost point, the bottom part of the water droplet instantly vaporizes upon touching the hot pan. The resulting steam suspends the remaining water droplet just above the surface, preventing direct contact. As we know steam has poor thermal conductivity compared to the metal pan, and heat transfer between the pan and the droplet slows down significantly. This allows the droplet to glide across the pan on the layer of steam beneath it.
Determining the exact Leidenfrost point is challenging, as it depends on various factors such as the impurities in the liquid and its surface properties. Using superhydrophobic surfaces we can stabilize the leidenfrost vapor layer of water, allowing for sustained droplet levitation and preventing nucleate boiling.
Application of leidenfrost effect are as follows
- Mass Spectrometry for enhanced sensitivity.
- Heat Engines providing benefits like reduced friction.
- Effect is applicable when surface are at room temperature and subsequent liquid is cryogenic in nature which means allowing nitrogen droplets to roll of the human skin without any damages.
- The inverse of this effect is the levitating of warm liquid droplets on a liquid nitrogen bathtub.
Equation for leidenfrost Point
The Leidenfrost point is the temperature at which the Leidenfrost effect occurs. It is the threshold temperature above which a liquid droplet on a hot surface forms a vapor layer, levitates, and exhibits the characteristic behavior associated with the effect.
The Leidenfrost point can be approximated using the following equation
Tl = Tb + (2/3) * (Ts – Tb)
- Tl is the Leidenfrost point
- Tb is the boiling point of the liquid,
- Ts is the temperature of the hot surface.
This equation is based on the assumption that the vapor layer is thin and neglects any effect caused by surface texture, liquid properties, and other factors that can influence the Leidenfrost point.
Characteristics of leidenfrost effect
- Droplet Shape: When a liquid droplet experiences this effect, it forms a spheroidal shape due to surface tension forces. To facilitate the vapor cushion formation This shape minimises the contact area with the surface
- Mobility: The droplet demonstrates mobility on the vapor cushion, allowing for different movements across the surface. These movements are influenced by the surface gradient and are in that direction or can be influenced by other external factors
- Extended Lifespan: The Leidenfrost effect increases the lifespan of the droplet compared to when it is in direct contact with the surface. The vapor cushion acts as an insulator, reducing heat transfer and slowing down the vaporization process.
- Hissing or Sizzling Sound: When a droplet experiencing the Leidenfrost effect a characteristic hissing or sizzling sound is generated upon contact of water droplets with hot surface. This sound is a due to the rapid vaporization occurring at the periphery of the droplet.
- Temperature Dependency: This effect is dependent on the temperature of the surface for its occurrence. Only when the surface temperature exceeds a certain threshold known as the Leidenfrost point this effect is exhibited .
- Insulating Vapor Layer: The presence of a vapor layer between the liquid droplet and the hot surface is another characteristics of this effect. This vapor layer acts as an insulator, preventing direct contact and reducing heat transfer .
Real-world examples of Leidenfrost effect
- Water Droplets on a Hot Pan: When we sprinkle water droplets on a heated pan. The droplets form into small balls and start skidding around the surface, taking longer to evaporate than they would at lower temperatures of the heated pan.
- Liquid Nitrogen and Exposed Skin: Extremely cold Liquid nitrogen demonstrates the Leidenfrost effect when it comes into contact with exposed skin. Instead of causing harm, the liquid nitrogen droplets roll off gently due to the vapor layer formed between the skin and the liquid.
- Hot Objects and Water Splashes: If a hot object like a glowing metal sphere, is submerged in water, the water droplets that touch the surface of the object form into small bouncing droplets rather than instantly evaporating.
- Droplets on a Hotplate: When water droplets are contacted on a hotplate, they start hovering and gliding above the surface,again taking longer to evaporate as it would with direct contact
- Cryogenic Liquid Droplets: Cryogenic liquids include liquid nitrogen or liquid helium, when it is dropped onto a surface warmer than their respective boiling point, It creates a vapor layer and exhibits the Leidenfrost effect, levitating and sliding on the vapor cushion.
- Hot Oil in Cooking: Whenever in hot oil during cooking any water droplets fall mistakenly, they can display the Leidenfrost effect and take longer to evaporate
Our Experiment to Understand the Leidenfrost Effect
What is the inverse leidenfrost effect
Let us understand by watching a video where we introduce hot water droplets on the liquid nitrogen surface. We observe that the droplets bounce of the surface and exhibit other characteristics.
In conclusion, the Leidenfrost effect is a mind-boggling phenomenon where a liquid forms a vapor layer when it comes in contact with a much hotter surface. Exhibiting unique characteristics like extended lifespan, droplet levitation, and mobility on the vapor cushion. A deep understanding of and harnessing of this effect can lead to burn prevention, improved heat transfer, and energy efficiency in different fields. Further exploration of the Leidenfrost effect promises advancements in thermal management and liquid interactions, with potential applications across various industries.