Liquid nitrogen in a vacuum. Best explained here. Source here. Seen via IFLS.
Img. 1 is an overview in real time: as pressure drops in the vacuum, liquid nitrogen first boils then freezes.
Img. 2 & 3 show liquid nitrogen boiling.
Img. 4 shows liquid nitrogen freezing.
Img. 5 & 6 show the frozen nitrogen abruptly and quickly restructuring into a more compact configuration, similar to snowflakes.