Washington D.C. [USA]: The Internet is loaded with video of bubble turning into ice crystals, which has made the engineers wonder over what caused the phenomenon.
Lead researcher Jonathan Boreyko and his student researchers were watching a YouTube video of a soap bubble freezing. The video made them wonder as to how soap films or bubbles freeze.
The results of the team’s query, which began as a simple ‘why,’ has been published in the journal of ‘Nature Communications’, explaining the physics behind what causes the ice crystals jump up into the bubble and swirl around, thus changing perceptions about the process of freezing.
“We started by freezing a bubble in the lab, using a frozen substrate. What we found was that the bubble would freeze from the bottom to a certain point and then stop. We didn’t get that lovely ‘snow globe effect’ that we saw on the video. But, Farzad made a nice model that can accurately predict where the freezing front will stop based on the size of the bubble and the air temperature,” Boreyko explained.
Because the shell of a bubble is microscopically thin, the warm air temperature in the lab prevented the cold stage from completely freezing the bubble. Moving to a walk-in freezer, the team tried the experiment again, believing they would discover how the floating ice crystals were formed.
“We didn’t see it in the freezer, either, at first. But we tried again depositing the bubble on ice instead of a dry substrate, and that is where we saw what we were looking for,” Boreyko said.
At minus 20 degree Celsius and using an ice substrate, the bubble quickly filled with floating crystals that hastened the complete freezing of the bubble and opened the researcher’s eyes.
“When you deposit the bubble on an icy substrate, the bubble begins to freeze, which releases heat. The bottom of the bubble, in this case, becomes warmer than the rest of the bubble,” said student researcher Farzad Ahmadi.
The molecular energy releases when the water molecules fuse together into a tight-packed solid lattice created a temperature difference of about 14 degrees minus 20 at the top of the bubble and minus six degrees at the frozen base.
“The temperature gradient from top to bottom changed the surface tension. The tension created a flow from the hot toward the cold,” Ahmadi added.
This flow is known as Marangoni Flow. When it occurs in the freezing bubbles, the flow rips ice crystals from the bottom of the bubble and swirls them around the liquid shell where they enlarge until the entire bubble is frozen.
“Previously we thought that how fast we could freeze something depended on how fast the freezing front could grow. This shows us that a freezing-induced Marangoni Flow will create hundreds of additional freeze fronts from the ice crystals removed from the bottom,” said Boreyko.
“So, we realised that it’s not just how fast one front grows, but in cases like our bubble, you can manipulate the system to have hundreds of freeze fronts working together to freeze something much faster,” Boreyko added.