What happens when you freeze a water droplet from the outside? It might not sound that interesting, but it’s actually rather cool – pun most definitely intended.

In a paper published in Physical Review Letters, researchers led by Sander Wildeman at the University of Twente in The Netherlands placed spherical droplets a few millimeters across on a hydrophobic surface, one that repels water, inside a vacuum chamber.

They then supercooled the droplets to -8°C (17.6°F) using evaporative cooling, taking the temperature well below zero but keeping the droplet liquid. Touching the droplet with a silver iodide tip started the freezing process with ice crystals forming on its exterior, which then created a skin of ice on the outside that moved inwards.

Cavities of vapor began to form under the skin, due to the rapid cooling effect. After shedding flakes of ice, the ice droplet then suddenly explodes, propelling out icy debris at about 1.5 meters (5 feet) per second.

^{The explosion takes place in a split second after the freezing process is started. Wildeman et al}

“We have shown that millimetric water droplets that freeze radially inwards undergo a sequence of intermittent fracturing and healing events, culminating in a final explosion,” the authors write.

“By modeling the elastic stresses in the ice shell and the fast dynamics following its failure, we have unveiled the important competing energies involved in this phenomenon.”

Interestingly, the researchers found that below a size of 50 microns, the ice droplets did not explode, most likely due to surface tension keeping the droplets together. This may be particularly useful in studying the formation of rain, when droplets can explode in the cool tops of clouds. Freezing rain and cloud droplets can often exist in this size range.

Check out a video showing the droplets exploding below.