A YouTube video, a walk-in chest freezer at minus 20 degrees Celsius, and some very cold-tolerant engineering students needed to finally find out why frozen soap bubbles look like glitter in a snow globe. The trick itself is a popular winter science experiment when temperatures are below freezing: go outside, blow a bubble of soap, gently dip it on snow or ice, and watch as crystals dance around on the movie until the whole thing a delicate ice ball is. It's visually stunning – but until recently, people did not know exactly why bubbles freeze in this particular, intriguing way.
When a drop of water or a puddle freezes, it usually begins to solidify into ice at the coldest point where it comes in contact with other snow or ice. Fresh ice freezes the adjacent water and ensures a proper flow over the puddle, which is referred to as freezing front. But if you freeze a bubble in a cold room, all this order goes out the window quickly.
Boreyko, a mechanical engineer, runs a Virginia Tech lab that deals with the behavior of liquids – including the freezing of puddles and droplets – when some of his students wanted to find out why bubbles in popular YouTube videos in these different patterns he was excited. "I think this is the first time in my life No, that my work was inspired by YouTube, "says Boreyko. Doctoral student Farzad Ahmadi and student Christian Kingett periodically packed jackets and borrowed the walk-in chest freezer of a neighboring laboratory, which had cooled to minus 20 degrees Celsius, to carefully lay soap bubbles with pipettes on ice.
As a result of all this cold work, they discovered that the snowball effect was driven by something called the Marangoni Current. "This is basically just a fantasy, because liquids flow from hot to cold at an interface," says Boreyko. As the bubbles froze in the freezer, the still-liquid part of the bubble continued to move, tearing ice crystals from the growing freezer front and tossing them around. These ice crystals each formed their own freezing front, whereby the surface of the bubble solidified faster.
But how did a freezer, where all temperatures are equal, heat up enough of the bubble to create the flow? "It turns out that the answer lies in freezing itself," says Boreyko. "It's very intuitive for people who are not in the field, but when you freeze water, it actually heats it up." This tiny bit of heat (usually just a few degrees) is enough to move the soap towards the top of the bladder. where the freezer still keeps it cold.
After the students thawed a little, they tried the same experiment at room temperature and again blown bubbles on an ice block. The results were very different, as you can see in this video:
Instead of completely freezing, the freezing front stops halfway up the bladder …. The warmer air in the room keeps the bladder in a strange purgatory until slowly air oozes out of tiny holes in the frozen half of the bubble. The holes are so small that Boreyko says it took a few minutes for some of the half-frozen bubbles to completely collapse.
Both experiments have wild-looking results, and if you live in a cold climate, you may have the opportunity to try out the experiments this winter. All you need is a soap solution, a cold surface (like snow) and a day when the air is below freezing.
"It's pretty easy for the people and that's why I wanted it," says Boreyko. "Everyone can see the effects themselves, and this can shed light on the why behind the beauty he sees when he wants to know more."