It took a YouTube video, a 20-degree refrigerator, and some cold-tolerant researcher engineers to understand why freezing soap bubbles resemble a glow in the snow globe. Stunt itself is a popular winter science experiment when temperatures fall under frost: go out, blow the soap bubble, gently rub it on snow or ice and watch the crystals dance on the film while the whole thing is a lightweight ball. This is visually stunning – but until recently people did not know exactly why the bubbles froze in this particular, hypnotic way. Typically, when a drop of water or pudding freezes, it begins to solidify on ice in the coldest place where it comes into contact with other snow or ice. The fresh ice freezes the neighboring water, creating a nice ordered progression through the puddle called the freezing front. But when you freeze a balloon in a cold room, all of this order quickly goes out the window. It starts to grow normally, freezes from the bottom where it touches the ice, up to the top, but then, suddenly, hundreds of frozen fronts appear on the surface of the bubble. "It looks like the rotating crystals you'll see in the snow cage." That's why we call it the effect of the snow world, "says Jonathan Boreiko, a co-author of a new article on the effect of the snow world, published in the journal Nature Nature Communications .
Boreiko, a mechanical engineer, leads a Virginia lab that focuses on fluid behavior – including how to freeze puddles and droplets. When some of his students wanted to know if they could understand why the bubbles in popular YouTube videos were frozen in these different models, he was excited. "I think for the first time in my life I can say that my book was inspired by YouTube," says Boreiko. For years, graduate Faddah Ahmadi and student Christian Kingt periodically gather in jackets and take a freezer in a neighboring laboratory – cooled to negative 20 degrees Celsius (negative four degrees Fahrenheit) – to carefully deposit soap bubbles on ice with the help of pipettes .
As a result of all this cold labor, they found that the effect of the snow world was driven by something called the Marangoni stream. "It's just a fantasy, because liquids run from hot to cold on an interface," says Boreiko. As the bubbles froze in the freezer, the liquid bladder still kept moving, tearing the icy crystals out of the growing front and throwing them around. Each of these ice crystals creates its own freezing front, making the surface of the balloon faster. But in a freezer where everything is the same temperature, how parts of the bubble heats up enough to create the flow? "It turns out that the answer is in the freezing itself," says Boreiko. "It is very contradictory to people who are not in the field, but when you freeze the water, it actually heats it. where the freezer still holds it cold.
After the students melted slightly from the cold, they tried the same experiment at room temperature, again releasing bubbles in an ice block. The results were wildly different as you can see in this video:
Instead of freezing completely in the middle of the bubble, the freezing front just … stops. The warmer air in the room holds the balloon in a strange purgatory as the air starts to slowly come out of small holes in the frozen half of the balloon. The holes are so small that Boreiko says it took a few minutes until some of the half-frozen bubbles break down completely.
Both experiments have wild results, and if you live in a cold climate, you may get the chance to try experiments for yourself this winter. All you need is a soap solution, a cold surface (like snow) and a day when the air is below zero.
"It's pretty easy for people, and that's part of the reason why I wanted to do that," says Boreiko. "Everyone can see the effects themselves, and that can tell why behind the beauty they see if they're interested in learning more about it."