A Familiar Sight in The Kitchen
The Leidenfrost effect is a phenomenon in which a liquid, brought in near contact with a mass significantly hotter than the liquid’s own boiling point, produces a thin vapour layer. This insulating vapour layer keeps liquid from boiling rapidly.
The Leidenfrost effect is most commonly seen in a kitchen environment when cooking. If you sprinkle drops of water in a pan to gauge its temperature, you will find that, provided the pan’s temperature is at or above the Leidenfrost point, the water skitters across the hot metal plate and takes longer to evaporate than it would in a pan that is above boiling temperature, but below the temperature of the Leidenfrost point.
The effect is also responsible for the ability of liquid nitrogen to skitter across floors at room temperature…
The Leidenfrost effect was named after German scientist Johann Gottlob Leidenfrost (1715-1794), who first described the phenomenon in 1756, in his work De Aquae Communis Nonnullis Qualitatibus Tractatus (“A Tract About Some Qualities of Common Water”).
States of Matter
Matter is most often described in terms of being a solid, a liquid, or a gas.
These three forms of matter are the most commonly known states of matter:
- A solid has a definite shape and a definite volume.
- A liquid has a distinct volume, yet no definite shape. Rather, a liquid retains the shape of its container.
- A gas (or vapour) has neither distinct volume nor shape. It conforms to the volume and shape of its container.
Each of the three common states contains varying observable physical properties.
Amongst other things, the properties of liquids under different physical conditions lead to the strange behaviour of matter, known as the Leidenfrost effect…
The heat capacity is a measurable quantity that is used to specify the amount of heat required to change the temperature of an object by a given amount. The SI unit of heat capacity is in Joule per Kelvin, J/K or J K-1.
Heat capacity is proportional to the size of a system. When expressing the same phenomenon as an intensive property, the heat capacity is divided by the amount of substance, mass or volume, so that the quantity is independent of the size or extent of the sample.
The molar heat capacity is the heat capacity per mole of a pure substance and the specific heat capacity, or specific heat, is the heat capacity per unit mass of a material.
Critical heat flux is the thermal limit of a phenomenon where a phase change occurs during heating (such as bubbles forming on a metal surface used to heat water), which suddenly decreases the efficiency of heat transfer, thus causing a localised overheating of the heating surface. When a liquid undergoes a change in phase due to the absorption of heat from a heated solid surface, a higher transfer rate occurs.
The more efficient the heat transfer from the hot surface (in the form of heat of vaporisation and sensible heat) and the motions of bubbles (bubble-driven convection, turbulence).
The Leidenfrost Effect in Action
The Leidenfrost effect occurs when a drop of liquid comes into contact with a hot surface that produces an insulating layer of vapour, keeping the drop from evaporating rapidly. This vapour layer cushions the drop and allows it to glide effortlessly over the plate surface, which is when the fun really begins.
Elegant Solution to the ‘Ball-in-a-Maze’ Puzzle
Physics students from the University of Bath, U.K., have been having fun with this natural phenomenon and have come up with a whole new use for it. Know those challenging ball-in-maze games you get in toy shops and Christmas crackers? You need no longer get frustrated over one of those old-fashioned games.
It turns out that if you replace a smooth surface with the sort of asymmetrical teeth found in a ratchet, the drop will move rapidly in one direction. By using ratchet surfaces to accelerate liquid drops, the team has made the drops move uphill and even follow a predetermined path through a maze.
And if you wonder what would happen if you combined the Leidenfrost effect with the paramagnetic response of a liquid, check out the beautiful image in the article “Levitating drops controlled by fridge magnets”.