It’s nice to know that in this ever-changing world, there are some things we can rely on… like the laws of thermodynamics! Solids, liquids and gases change phases at predictable temperatures and pressures, right? Well, not quite… there are a few phenomena that can test this seemingly infallible field of science and its fundamental laws—the Leidenfrost effect is one of them.
The Leidenfrost effect is what you see when you pour a liquid over a superheated object/surface that is much hotter than the liquid’s boiling point. When this happens, the liquid doesn’t simply boil away, as you might expect. Instead, it hovers over the superheated object/surface, suspended on a tiny cloud of gas. This phenomenon is named after the German physicist Johan Gottlob Leidenfrost, who explained this strange liquid behavior in his scientific paper, A Tract About Some Qualities of Common Water in 1751.
Now, let’s try to understand the Leidenfrost effect in more detail by examining some of the popular examples where it is witnessed.
Skittering of Water Droplets on a Frying Pan
You can witness the Leidenfrost effect yourself at home without the need for a complex or expensive scientific lab setup. Take a frying pan, put it on a stove and start heating it. Now, start sprinkling water on the frying pan. Initially, when the temperature of the pan is below the boiling point of water, i.e., 100oC, the water flattens out on the pan’s surface and then starts to boil and evaporate. With the passage of time, as the temperature of the pan exceeds 100 °C, the water droplets hiss when you sprinkle them over the pan and then quickly evaporate. A little later, as the temperature exceeds what’s called a ‘Leidenfrost point’, you will start to see some unexpected behavior.
At this juncture, upon sprinkling water on the pan, you’ll see that the water droplets don’t flatten and hiss out easily on their way to evaporation. Instead, they bunch up into small balls of water and skitter around—lasting much longer on the pan than when the pan was at the lower temperatures. It must be noted that this effect works until a pan is heated further to much higher temperatures, causing any further sprinkles of water droplets to evaporate too quickly to witness this effect.
As the pan crosses the Leidenfrost point, the bottom part of the water droplet touching the pan vaporizes immediately upon contact. The resulting vapor gas that is formed suspends the remaining part of the water droplet just above it and prevents any further direct contact of the water droplet with the hot pan. Also, steam is known to have poor thermal conductivity. This implies that further heat transfer from the hot pan to the droplet is slowed down significantly. This is why the water droplet skids around the pan, as there is a layer of vapor gas acting as a barrier between the hot pan and the water droplet.
Calculating Leidenfrost Temperature is Not Easy
The temperature at which the Leidenfrost effect starts to occur is not easy to predict. Upon experimentation, it has been observed that, despite the volume of a drop of liquid staying the same, the Leidenfrost point may be quite different, due to its complicated dependence on the properties of the surface, as well as the impurities in the liquid. Although a few researchers have attempted to create a theoretical model to aptly summarize the effect, those models are still very complicated. As a very rough estimate, the Leidenfrost point for a drop of water on a frying pan might occur at around 200 °C.
Dipping Fingers in Molten Lead
Have you ever a seen an experiment wherein a brave experimenter dips his finger inside hot molten lead? Well, dipping one’s fingers in molten lead without getting burnt is another classic example of the Leidenfrost effect. However, it is not the usual case of water or a liquid rising up and remaining insulated from the hot pan due to a layer of vapor beneath it.
To successfully perform this experiment, an experimenter first dips his finger in a bowl of water. The drops are then shaken off and the hand is quickly dipped in and out of the molten lead container. Roughly 7-8 cms of finger length are pushed inside. Heat from the lead begins evaporating the surface water layer on the wet hand and building a layer of steam, which prevents the skin from coming directly in contact with the lead and thus allows the hand to escape unburnt!
Pouring Liquid Nitrogen on a Bare Hand
Liquid nitrogen is nitrogen held at an extremely low temperature. It is at around -190oC when poured from containers at room temperature for experimental purposes. Many experimenters have even demonstrated pouring this liquid nitrogen on their bare hands. They are able to pull off this shocking sight because of the Leidenfrost effect.
Now, room temperature is much higher than the boiling point of nitrogen (-196oC), so when it touches the surface of a hand that is above the Leindenfrost point, a layer of steam isolates most of the liquid from touching the skin. In this way, the experimenter does not suffer from any frostbite.