[howaboutnope]

When I made it with WebGL (with no clue what I was embarking on, I just wanted to try it), I ended up with 8 substeps. Looking at it now it confuses the hell out of me, but IIRC it had to do with with checking each cardinal direction both ways ("both ways" meaning both "do I want to swap positions with this other pixel", and "does this other pixel want to swap position with me").

edit: I don't have a good video of this atm, but just to demonstrate it: https://www.youtube.com/watch?v=rItnlDmGALU

it's not visualized in the video, but the pixels also have temperature, and materials a sort of stickyness (basically sand that takes longer to form into a pyramid), that's about it.