Heat Race Across a Maze with the Leidenfrost Effect

A photograph showing the Leidenfrost effect of liquid nitrogen in action. The experimenter's warm hand is seen plunged into a vat of liquid nitrogen, which spills over. Don't Try This At Home!

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… 

A diagram explaining the physics behind the Leidenfrost Maze hovering droplet. The title reads: "Drop of liquid held up by layer of vapour." The droplet is held above the hot surface by as much as 0.2 mm at its centre, and 0.1 mm at the edge.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.

 

(Another state of matter with which we are less familiar with on Earth, is plasma.  This 4th state of matter is by far the most common in the Universe, and believed to account for 99% of all visible matter, according to the National Science Foundation.  Plasma, the fourth state of matter, is an incredibly hot gas-like  state, made up of free ions and electrons that can carry an electrical current.  Stars are made of plasma.  Sometimes we use plasma on Earth.  In neon lights, and plasma TV.  It also occurs in lightning.)

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…

 

Liquids and HeatA photograph showing a perfectly round water droplet on a hot plate, illustrating 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.

A graph and diagrams explaining the heat transfer process that is leading to the Leidenfrost effect for water at a pressure of one atmosphere. The graph is S-shaped when heat flux q is compared to temperature.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.

Watch this cool Introduction To The Wonderful Leidenfrost Effect by The Hot Buds, 2010 Winner of the IoP Best SciCast Physics Film.

An animation showing an actual meandering water droplet under the influence of the Leidenfrost effect in the maze.So, how could it effectively be used to solve one of life’s most frustrating problems of Physics?  😉

 

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”.