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Turning Bubbles into Snow Globes
Posted by Jacqueline Mifsud
Although several mesmerizing videos of freezing soap bubbles out in the snow are available on YouTube, this phenomenon lacked scientific explanation. The following video explores some of the physics involved in freezing bubbles.
Researchers from Virginia Tech in Blacksburg investigated this phenomenon by depositing bubbles on a silicon substrate having temperatures between -10 and -40 degrees Celsius, with the surrounding air at room temperature. It was observed that the freeze front moves very slowly up the bubble, and in some cases even comes to a complete stop after reaching a critical height. The slow propagation of the freezing front is a result of the poor thermal conductivity of the thin soap film. The speed of propagation of the freeze front can be more readily observed in larger bubbles or at higher surface temperatures.
This delayed freezing consequently allows enough time for the frozen portion of the bubble surface to cool the air within the bubble, while the top part is still liquid. As a result, a pressure imbalance is developed that either collapses the top or causes the top to pop. When the freeze front manages to reach the top of the bubble, a section of the top may melt and slowly refreeze. This melting and refreezing cycle can take place several times in a single bubble.
The last part of the video shows freezing bubbles in a freezer, where the surrounding air was maintained at -20 degrees Celsius. Under these conditions, the bubble freezes quickly and the ice grows radially from nucleation sites rather than perpendicular to the surface. This contrasts with the limited conductivity of bubbles deposited on a cold surface in room temperature.
Differences in surface tension (or Marangoni effects) create currents in soapy films that move ice crystals around. The result is a variety of ice crystals swirling across the surface of a soap bubble as it freezes, making it look somewhat similar to a snow globe.