Bubbles freeze in a swirl of ice crystals

An online search for “frozen bubbles” yields a bevy of mesmerizing photos and videos . Tiny bits of crystallizing ice dance around on a bubble’s surface until they grow large enough to impede each other’s motion and, eventually, occlude the surface entirely (see the images).

But what underlies that dynamic freezing behavior? Although researchers have extensively studied the freezing of droplets and surface-bound films, Jonathan Boreyko and coworkers at Virginia Tech discovered that the scientific literature on freezing bubbles was essentially nonexistent.

To explain the phenomenon that generated so much public interest, the researchers froze sessile glycerol–water soap bubbles in two different environments: isothermal and room temperature. In both cases, the surface temperature was cooled to below the bubble’s melting temperature, which caused the bubble to freeze. But the isothermal experiments were performed in a walk-in freezer, so the surrounding air was the same temperature as the surface, whereas at room temperature the air was warmer.

The above video, recorded by Farzad Ahmadi and Christian Kingett, both students in Boreyko’s lab, shows the isothermal freezing of a droplet. Watching the crystallites, it’s clear that a flow develops in the bubble film. Although the researchers considered different explanations, they ruled out all but one: The bubbles start freezing at their contact line with the surface, which releases latent heat and creates a temperature gradient along the bubble film. The resulting Marangoni flow shears off tiny bits of ice from the freezing front. The flow also carries the nanoscale crystals around the bubble’s surface and, as those crystals grow, creates a swirling effect like in a snow globe. The crystallization fronts eventually meet, which stops the dynamic behavior.

For room-temperature bubbles, the surface–air temperature gradient easily overpowers the much smaller gradient caused by freezing and suppresses the snow globe effect. The bubbles each froze with a single front that moved upward from the contact line. Although 5 μL bubbles on a –40 °C substrate would freeze completely, larger droplets and those at higher surface temperatures would only partially freeze and eventually collapse. (S. F. Ahmadi et al., Nat. Commun. 10, 2531, 2019 .)