Skip to main content
Surfaces and interfaces

Surfaces and interfaces

Fizzy water droplets levitate at room temperature

23 Jul 2021 Isabelle Dumé
Leidenfrost droplet
A moving droplet. Courtesy: Philippe Bourrianne (Princeton)

If you’ve ever thrown water into a red-hot frying pan and watched the droplets roll around the pan like tiny glass marbles, you’ve seen the Leidenfrost effect in action. The effect occurs because an insulating cushion of water vapour forms between the droplets and the pan, which allows the liquid to behave in ways it normally wouldn’t – including some that might have practical uses, were it not for the high temperatures required.

Researchers at the Massachusetts Institute of Technology (MIT) have now put forward an alternative, room-temperature way to levitate liquid droplets. While their initial experiment focused on carbonated water placed on a strongly water-repellent surface, the technique could be extended to other liquids such as oils, clearing the way for applications in industrial processing.

Lowering the Leidenfrost temperature

In 1756, the German scientist Johann Leidenfrost described how water droplets levitate on their own vapour when placed on a hot solid surface. The vapour acts as a repulsive layer, preventing the droplets from evaporating rapidly and allowing them to hover above the surface. Since there is virtually no friction between the droplet and surface, the droplet glides quickly and effortlessly across it. The droplet can also be made to accelerate in different directions by changing the texture of the solid surface – something that could be useful for making self-propelling devices.

The high temperatures required for the Leidenfrost effect somewhat limits its applications, however, and in recent years researchers have been trying to reduce the temperature required from 200 °C to 100 °C or below. A team led by Kripa Varanasi and Gareth McKinley has now succeeded in doing so, reporting liquid levitation at ambient temperatures.

Entering the levitating regime

In their experiments, the researchers prepared their “fizzy” water by pumping gaseous carbon dioxide (CO2) into deionized water. They then deposited millimetre-sized water droplets supersaturated with the dissolved gas on a superhydrophobic solid – that is, one that strongly repels water. They imaged the interface underneath the drop using an optical microscope and also deposited the droplets on a curved solid surface to study their high mobility.

McKinley and colleagues found that the fizzy droplets generate a carbon dioxide gas cushion that keeps them levitated for up to a minute. During this time, droplets that are initially just sitting atop rough, air-trapping surface structures on the solid – the Cassie-Baxter state – enter a levitating regime once the COconcentration reaches a certain value. This critical concentration is analogous to the Leidenfrost temperature, but instead of being driven by external energy sources such as heat or mechanical forces, the researchers explain that it reflects the excess chemical energy stored inside the droplet in the form of dissolved gas.

Application areas

According to the team, applications for this new, frostier Leidenfrost effect include frictionless transport of drops; sorting droplets by varying the level of carbon dioxide within them; developing heat transfer technologies that benefit from air insulation; and being able to “de-pin” droplets on demand. “Our result is of practical interest as it extends the benefits of levitating liquids to room temperature and avoids the energetic cost of heating a surface,” team member Philippe Bourrianne explains. “It is exciting to see that levitation can be achieved under ambient conditions with a common liquid such as fizzy water.”

Team member Divya Panchanathan adds that the effect could be extended to oil and other organic liquids by using appropriate textured surfaces and soluble gases. Extending the effect to such non-volatile liquids will bring out the full potential of this fizzy levitation effect, she tells Physics World.

The research is detailed in Science Advances.

Copyright © 2024 by IOP Publishing Ltd and individual contributors