Fluids to a certain particle size (about 500 nm) are able to mix.
Over this size "there is not enough space between the particles". There is oil and coloured water in the tube. The size of their particles is too big:
So they are non-mixing fluids!
Accidents with oil tankers and oil-rigs cause serious environmental catastrophies, because does not mix with water, and it floats on the top of it, this way shutting off oxygen from the sea's flora and fauna.
