It took a YouTube video, a walk-in freezer kept at negative 20 degrees Celsius, and some very cold tolerant engineering students for scientists to finally find out why freezing soap bubbles resemble glitter in a snowflake. The trick is a popular winter science experiment when the temperature dips during freezing: head outside, blows a soap bubble, gently picks it up on some snow or ice and looks like crystals dance around the movie until the whole thing is a delicate ice ball. It is visually amazing – but until very recently, people didn't know exactly why bubbles freeze in this particular, fascinating way.
When a drop of water or a sausage freezes, it normally begins to solidify in ice at the coldest point where it contacts other snow or ice. Fresh ice freezes the nearby water, creating a beautifully ordered advance over the pillow called a freezing front. But when you freeze a bubble in a cool room, the order goes out of the window quickly.
Boreyko, a mechanical engineer, heads a laboratory at Virginia Tech that focuses on how liquids behave – including how frogs and drops freeze. As some of his graduate students wanted to know if they could look at why bubbles in popular YouTube videos froze in the different patterns, he was thrilled. "I think this is the first time in my life I can say that my paper was inspired by YouTube," says Boreyko. For a long time, graduate students Farzad Ahmadi and bachelor Christian Kingett will regularly tie in jackets and borrow a nearby lab's walk-in freezer – cooled to negative 20 degrees Celsius – to gently deposit soap bubbles onto ice using pipettes.
As a result of all the cold work, they found that the snowdrop effect was driven by something called a Marangoni flow. "It's just fancy talk for basically fluids flowing from hot to cold on an interface," Boreyko says. As the bubbles froze in the freezer, the ever-floating part of the bubble continued to move, tearing ice crystals away from the growing freezing front and throwing them around. These ice crystals each created their own freezing front, which made the bubble's surface solidify faster.
But in a freezer where everything is the same temperature, how were parts of the bubble warming enough to create the current? "It turns out that the answer is in the freezing," Boreyko says. "It is very contradictory for people who are not in the field, but when you freeze water, it is actually heated." The little bit of heat (usually only a few degrees) is enough to start soap that moves up towards the top of the bubble, where the freezer still keeps it cold.
After the graduates had thawed a little, they tried the same experiment at room temperature and blown bubbles again on an ice box. The results were wildly different as you can see in this video:
Instead of being completely frozen, half of the bubble stops the freezing front just … The warmer air in the room holds the bubble in a strange scrubber until the air begins slowly looking out of small holes in the frozen half of the bubble. The holes are so small that Boreyko says it took several minutes for some of the semi-frozen bubbles to collapse completely.
Both experiments have great results, and if you live in a cold climate, you might get the chance to try the experiments for yourself this winter. All you need is some soap solutions, a cold surface (like snow), and a day when the air is freezing.
"It's pretty easy for people to do, and that's part of why I would do this," Boreyko says. "Someone can see the effects themselves, and it can explain why, behind the beauty they see, if they are interested in learning more about it."