Due to the forecasted frost last night, I placed yesterday evening, some 1.5l standard PET bottles filled up to 90% with warm tap water(+60°C) close to some vegetables that I wanted to protect in my garden. The temperature dropped to roughly -3 ~ -4°C last night.
This morning I went to see how it went. Of the 10 bottles, one was filled half with water, half with ice. In all the other bottles, the water was still in a liquid state.
So I decided to empty them. Here comes the interesting thing: I uncap the bottles, flip them upside down to empty the water, and give them a shake/twist so that it empties faster, and I noticed that some not well structured ice (it was more looking like melting snow actually) was forming almost instantaneously. Curious, I decided to give a strong shake on the next one while emptying it, and well, this mix of ice forming at that moment reminded me the texture of the icy fruit smoothies one can find during summertime.
How do you explain that the water, when still, was 100% liquid, and that when I shake the bottle, ice was forming in no time? I mean, for me, shaking = adding energy, so it should warm the water, not cool it to the point it will form ice? From this experiment I guess not, and that instead, it more or less 'helps' the remaining energy of the 0°C water to dissipate, forming ice super quickly. Am I right in my reasoning?
I'll redo the experiment the next night, trying to take a photo to add it here.
Edit 1: I've placed the same bottles, with the same warm tap water in them at the same position as yesterday. The upcoming night might even be colder... I'll try to take some photos tomorrow morning.
Edit 2: Okay, so this morning it wasn't as impressive as yesterday but it happened again:
Shaking a little, then
Fig.2 Ice particles have attached to the inside of the bottle as it is being emptied.
It's a totally "wild" and uncontrolled experiment so it's not as impressive as the videos linked by Philip hereunder.
I also made a tiny additional observation but this is probably entirely due to chance: because I filled the bottles with warm water yesterday, they were a little depressurized this morning, having kind of a global concave shape. I have shaken them all before opening; but the water stayed clear. It's only once I opened them, and emptied them, that ice was formed.