Originally Posted by Khan
I've been laid up with a fever, so I've been using bags of ice to keep my head cool. I just went to get some ice and found that two cubes in the tray had nearly vertical spires of ice poking up from the middle. The tray was sitting there undisturbed for hours, but these reverse-icicles formed somehow. Has anyone seen this before? What happened?
Yes, this has been the subject of many threads here, including some lively debate as to the cause; I believe it has now been conclusively resolved by means of time-lapse video recording; it's something like this :
-the surface of the ice freezes from the outer rim inwards, until there's just a little unfrozen hole in the middle
-because water expands a little as it freezes, liquid water wells up a little through the hole, the outer rim of this water droplet freezes, forming a little torus of ice
-More water wells up through the centre of this, freezing on the outside as it goes, building a needle-like tube of ice, until the whole thing freezes solid.