A few days ago, Alan Davidson and Dustin Boswell at Google Santa Monica posted about a large Fresnel lens they bought, and about the things they melted with it. That was impressive, and reminds me of a time when we froze soap bubbles at PriceGrabber.
Last year the people at some energy drink company dropped off a refrigerated cooler of their product. When it was emptied (I think that took about five minutes, the grabbers are a thirsty bunch) I thought it would be perfect for freezing soap bubbles. Several years earlier I saw a display of this effect at the wonderful Exploratorium in San Francisco. They had a cylinder about two feet in diameter and four feet high with a brick of dry ice at the bottom, and you could blow soap bubbles into the chamber.
I borrowed the cooler and bought a block of dry ice to put at the bottom of it. The cooler was smaller than the one at the Exploratorium — only about 18 inches across and maybe thirty inches deep — but it worked great. It’s important that the air in the cooler be as still as possible, so we cut a hole in some cardboard to lay across the top. Once it had a chance to cool down, we were ready to start.
Dry ice freezes at −109.3 °F, much colder than the freezing point of water. It cools the air around it, and fills the bottom of the cooler with carbon dioxide. The bubbles float in the cold air because carbon dioxide is heavier than the air you exhale, and (at least initially) your breath is warmer than the carbon dioxide. As the bubble cools and carbon dioxide seeps into it, the bubble sinks lower and lower toward its eventual icy fate at the bottom of the cooler. In the picture above you can see a new bubble floating above the dry ice, with several previous bubbles frozen on it.
The bubbles won’t pop the way they ordinarily do. In the air the bubbles might last for half a minute. In the cooler our longest lasting bubble went about ten minutes before finally settling onto the dry ice. As they sink to their doom, the bubbles do two fascinating things: they cloud up, and they get larger.
The cloudiness I assume is because they are slowly freezing. The growing larger is a puzzle. Our best theory was that ice expands as it freezes, but if anyone has a better explanation speak up.
Here’s a picture of an older bubble: