In the cold of Canada, a man blows a soap bubble, which immediately freezes. Since that’s awesome, the guy makes a nice video about it.

Who knows, maybe he even knew about all the physics that was going on in front of his camera.

Let’s start from the easiest thing: why does the bottom of the bubble freeze last? The answer is gravity: the outermost layers of the bubble slide down, so the base just has more water to freeze. And it takes longer.

But why does it freeze in spots, instead of just from the top down?
Because cold is not the whole story. Water needs a point to start building its crystals, if it can’t find one, it stays liquid well below zero degrees (Celsius), but is very unstable and freezes at the slightest disturbance.

In a stunning turn of events, a soap bubble has plenty of soap molecules floating around, and they are excellent starting points for the ice crystals we see growing on the surface.

Finally, why does the bubble pop instead of just staying there? In the comments to the video, the author says he popped it, but I do believe he could have just waited a little.

The air he blew in the bubble came from his lungs, so it was over 30 degrees. Pressure inside the bubble, initially, is the same as the atmospheric one, but it drops as the air cools down. This puts a lot of stress on that thin and stiff ice surface. Sooner rather than later it was bound to collapse anyway.

All this magnificent physics in less than 30 seconds. What a wonderful world…

 

Foto: Frozen, CC-BY-NC-ND Benjamin Lehman, via Flickr. Some rights reserved.

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