Splash water on a hot skillet, and it will evaporate very quickly. But if you heat it up a little bit more, you'll start to see some new physics: the water droplets will skitter across the surface without evaporating. Upon contact, a protective vapor forms between the water droplet and the skillet, allowing it to levitate across the surface with very little friction.
Called the Leidenfrost effect, this phenomenon occurs at temperatures much higher than a liquid's individual boiling point. Now, researchers have transferred the Leidenfrost effect from the kitchen to the lab. By controlling liquid oxygen droplets with magnets, scientists have uncovered some of the physics behind these dancing droplets.
Video courtesy of Keyvan Piroird, Baptiste Darbois Texier, Christophe Clanet and David Quéré.
Because oxygen's boiling point is so low (-183 degrees celcius), the team of French researchers were able to observe this effect at room temperature. Also, liquid oxygen is paramagnetic, so it will respond to magnetic fields. When the droplets passed close to magnets, they could slow down, speed up or change their shape depending upon the direction of the magnetic field relative to the droplet's velocity. All of this can be seen in the video above.
For instance, the team was able to slingshot the drops around moving magnets, similar to gravity assists used to shoot spacecraft toward the outer planets of the solar system. When a spacecraft approaches a planet with the right velocity, it can whip around them and accelerate in a new direction. Although the spacecraft's velocity relative to the planet remains the same, its velocity relative to the Sun can change. This allows spacecraft to save fuel for longer journeys.
Similarly, the water droplets could be accelerated around a moving magnet. The moving magnetic field would shoot the liquid oxygen in a new direction and increase its speed. In the future, the team hopes to investigate how magnets could affect orbiting droplets, possibly causing them to rip apart.