The Leidenfrost Effect is a phenomenon the majority of us will come across on a regular basis but probably never give it a second thought. It is easy to dismiss it as “just one of those things” but what is it, when does it occur and why does it happen?
What is the Leidenfrost Effect?
The best way to demonstrate the Leidenfrost Effect involves something we do every day, cooking. When you use a hot cooking surface, such as a frying pan, have you ever wondered why water droplets don’t always evaporate when touching the surface? Sometimes they seem to dance around the surface making random movements until they eventually evaporate. But how can this be?
If the cooking surface is hotter than the boiling point of water, 100°C, then surely the water would immediately evaporate on contact? When you pour water into a cold pan, heat up the pan, as the surface gets hotter the water begins to evaporate and disappear. There is no dancing about the surface, no random movements and no extended life. So, why do water droplets have a longer lifespan on a surface with a temperature well above their boiling point?
Who discovered the Leidenfrost Effect?
The Leidenfrost Effect is named after Johann Gottlob Leidenfrost. He covered this phenomenon in great detail in “A Tract About Some Qualities of Common Water” which was published in 1796. In layman’s terms, the Leidenfrost Effect occurs when the surface in the proximity of liquid is hotter than the liquid’s boiling point. In the case of water this occurs when the surface is hotter than 200°C (twice the boiling point of water).
How does the Leidenfrost Effect work?
At 200°C part of each water droplet will vaporise. This effectively creates a type of thermal jacket between the droplet and the hot surface. As vapour is extremely poor at conducting heat this protects the rest of the water droplet from the extreme temperature. As a consequence this will slow down the vaporising process. Eventually the water droplet will evaporate but this will occur more slowly the higher the temperature, above and beyond the Leidenfrost point of water.
If the temperature of the surface is below the boiling point of water the liquid will just flatten out across the surface. When the temperature reaches boiling point the water will begin to evaporate but it will still be extended over the flat surface. In order to demonstrate the Leidenfrost point of water, you will need to preheat the surface to 200°C and sprinkle the droplets from above.
Erratic movements at and beyond the Leidenfrost point
One thing you will notice, whatever liquid you use, is that when the surface area reaches the liquid’s Leidenfrost point each droplet will begin to move in an erratic manner. It will dart from side to side at extremely quick speed until it eventually evaporates. Why does this occur?
There are many theories as to why droplets move in such an erratic manner when at or above their Leidenfrost point. Some experts believe that it is gravity which is pulling the droplets, while others highlight differences in air pressure, similar to how an ice puck travels over the surface. If the surface is grained and shaped in some way it is actually possible to control the direction of the droplets which to the naked eye seem to be self-propelled.
How to make water flow up hill
As scientists continue to delve deeper into the Leidenfrost Effect we have seen the release of some fascinating footage. One particular experiment shows how water droplets experiencing the Leidenfrost Effect can become self-propelled and effectively flow up hill. In this experiment a bowl-shaped surface was heated up to the Leidenfrost point of water. The droplets were released into the centre where they appear to hover due to the layer of vapour insulation.
In order to show the self-propulsion the surface was structured with round ridges moving up from the central point. There is clear video footage which shows the droplets travelling up the raised sides. Magnified footage shows that the droplets are able to lock onto the ridges. In a manner similar to a tidal wave they effectively push themselves forward towards the next ridge – in effect we have created Leidenfrost wheels. It is worth noting that some liquids can be used to create droplets many times the size of water droplets. What kind of power could self-propulsion achieve with a heavier weight behind it and the effectively frictionless ride up hill?
There is also a fascinating video which uses the self-propulsion concept to create a Leidenfrost maze. As you will see in the video link below, the droplets appear to be moving randomly around the maze. However, it is the position, gradient and ridge pattern on certain tiles which propels the droplets around maze. Random to look at, but controlled by the structure of the maze.
Where does the power come from to propel these droplets? If only there was a way to harness that power?
Creating a Leidenfrost engine
The theory behind the Leidenfrost Effect is well covered in the scientific and engineering press. However, new ideas are emerging on a regular basis. One such idea involves using the Leidenfrost Effect to create a liquid vapour engine very different to a steam engine. Experiments have been carried out which show that utilising the surface of hot turbines and blocks of dry ice can create extremely useful motion. Using a model specifically built to encourage movement of the dry ice, which is levitating above the hot turbine’s surface, scientists were able to rotate the blocks of dry ice.
The motion of the dry ice was then converted into electric power when coupled to a magnetic coil system. While researchers are nowhere near creating the finished engine, there are high hopes that the Leidenfrost Effect could revolutionise many inefficient systems we use today.
Leidenfrost Effect FAQ
The Leidenfrost Effect is a phenomenal of nature which is difficult to explain but extremely easy to demonstrate. As a consequence, there are many frequently asked questions which include:
Insulation from excessive heat
One experiment which makes full use of the Leidenfrost Effect involves dipping a wet finger into molten lead. To explain the Leidenfrost Effect lava flow experiment we need to look at the temperatures and substances involved. If you dip a dry finger into molten lead your skin tissue would simply burn instantly. However, molten lead has a temperature in excess of 300°C which is well above the Leidenfrost point of water. As a consequence, the water around a wet finger will partially vaporise, briefly acting as insulation against the molten lead.
This is perhaps best compared to using oven gloves to remove an extremely hot container from your oven. It will insulate your hands for a small period of time before the heat is felt.
Harnessing power
We know scientists are working on an engine which is based upon a turbine and rotating dry ice. There has also been another very useful experiment demonstrating how to transfer power from the hot surface to a liquid protected by the vaporised layer. In this experiment scientists used hydrogel balls (98% water) which have no shell and are extremely flexible.
When dropping the hydrogel balls onto a hot surface, at the liquid’s Leidenfrost point, part of the water vaporised. As this water vaporised, the interaction with the liquid sphere effectively transferred energy from the hot surface to the liquid. This was done via rapid oscillations in pressure and deformation of the balls. Moving so fast much of it was not visible to the naked eye. Scientists describe this as a modern day engine. The sphere absorbed the fuel, had the piston mechanism to transfer into movement and mechanical output.
Summary
The majority of natural phenomenon are easy to explain in theory but often difficult to present in practice. It is the opposite way round for the Leidenfrost Effect. Possibly without realising, many of us will come across the Leidenfrost Effect every day without giving it a second thought. If scientists could harness the energy created during this process we could see some revolutionary changes in mechanical engineering.
