Turbulence is said to be one the unsolved problems in physics;
https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics#Condensed_matter_physics
Though it's actually the mathematical problem which is considered to be the "millenium problem";
https://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_existence_and_smoothness
My own thought is that there is surface created inside the fluid, even when the fluid is the same, and not only when there are two different fluids;
https://en.wikipedia.org/wiki/Capillary_surface