The freeze front creates an unusual liquid flow on the surface of the soap bubbles, new research suggests.
You may have seen the viral videos of photographers freezing soap bubbles during a recent Snowmageddon. They’re magical, turning into ethereal globes filled with ice crystals when the surrounding temperature is just right.
Unlike water droplets, puddles or other liquid surfaces, the thin, rounded shape of bubbles makes them poor heat conductors. So when soap bubbles freeze, lacy crystals break off from cooler points on the surface and swirl around on currents of warmer liquid. Recently, scientists set out to explain the transfer of heat that affects how bubbles freeze.
“We’ve seen the unique freezing dynamics of bubbles in nature, but we’ve never understood the physics behind it,” said Jonathan Boreyko, who studies condensation and frost phenomena at Virginia Tech.
Dr. Boreyko and colleagues’ results, published Tuesday in Nature Communications, make for fascinating viewing. The study could also have applications for flash freezing food, creating tastier ice cream or even developing antifreeze materials.
How bubbles behave on ice pose many questions, Dr. Boreyko said, but “You can only study that by looking at bubbles in carefully controlled situations in the lab.”
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The team started by pipetting tiny soap bubbles onto an ice block that they chilled to minus 4 degrees Fahrenheit and kept in a walk-in freezer. Using high-speed cameras, the team then filmed the soap bubbles as they froze from the bottom up.
They noticed that as soon as the bubble came into contact with the icy surface, water that was sandwiched inside the soap began flowing upward from the base. As the bottom of the bubble cooled and solidified, ice crystals also formed a boundary between the frozen and unfrozen part of the bubble — a freeze front — that inched up the surface. But within milliseconds, chunks of ice crystals started breaking off from the freeze front. These were swept up by tiny water streams, known as Marangoni currents, until hundreds of ice crystals danced across the bubble.
After about 10 seconds, the Marangoni flow dissipated and died out as the floating ice grew and crystals became interlocked. At that point, the rest of the bubble froze over.
The phenomenon of the Marangoni current also explains why food dyes disperse in a bowl of milk and create a tie-dye effect when you add soap, or why wine appears to leave “tears” behind in your glass as you drink: Liquids move from areas with low surface tension to areas with high surface tension.
In a bubble that’s slowly freezing, the changes in temperature change the surface tension, too. That results in Marangoni currents that produce swirls of liquid and ice crystals, resulting in a dramatic snow globe effect.
The researchers wondered how this phenomenon affected bubble behavior at different temperatures. Could they learn to tweak Marangoni currents to speed up or prevent liquids from freezing?
Dr. Boreyko’s team placed soap bubbles on an icy stage outside their walk-in freezer. They found that the Marangoni currents were weaker when the surroundings were at room temperature. The soap bubbles already had an externally imposed temperature difference. They were cold at the point of contact with the stage and warm on top. So the researchers didn’t see any ice crystals break off from the freeze front or whirl around the bubble.
Poor conduction stopped the freeze front from climbing more than halfway up the bubble. “Bubbles stayed in this partially frozen state for a while until the liquid dome collapsed 20 to 30 minutes later,” said Christian Kingett, an undergraduate student who helped conduct the study.
Anyone can see this physics in action, Mr. Kingett said. You may not have a walk-in freezer at home, but as long as you’ve got soap, water and a surface that’s cold enough, you should be able to watch soap bubbles freeze, form snow globes and fall apart, too.
“We hope this gets people thinking about fun home experiments they can do to see how beautiful nature can be when you have just the right conditions,” Dr. Boreyko said.
Additional reporting on bubbles, droplets and physics