I saw in the YouTube video Don't Mix Coke with Liquid Nitrogen! that when you fill a bottle part way with water then pour liquid nitrogen into it then turn it upside down, it blasts off at high speed. Why does that happen? How is it possible?
I cannot figure out the answer for sure but here's my theory. According to the article Liquid Nitrogen Is Beautiful When It's Dancing Across Gasoline, liquid nitrogen is less dense than water. I'm guessing that means at first, the liquid nitrogen floats on the water. When it's turned upside down, the liquid nitrogen starts mixing with the water but surface tension limits its rate of mixing. However, the mixing still occurs to some extent which boils some of the liquid nitrogen creating thrust. Once it starts accelerating, the acceleration speeds up the mixing which in turn creates an even stronger thrust.
When the bottle is turned upside down, the liquid nitrogen floats to the top and absorbs heat from the water. It therefore boils, and creates pressure in the air space at what is now the top of the bottle. This pressure then blasts the water out of the narrow end at high speed, and the bottle is compelled to obey Newton's 3rd Law of Motion (i.e. for every action there is an equal but opposite reaction).