Chemistry and fluid dynamics often go hand-in-hand. Here chemical reactions produce visible precipitates as one chemical drops into the other. The shapes that form are distinctly fluid dynamical, with vortex rings, plumes, and instabilities all appearing.
In many applications, chemical reactions and fluid dynamics are tied inextricably to one another because the rate of chemical reaction depends on local concentrations driven by fluid dynamics, and the fluid motion is itself influenced by those concentration gradients. This is why reacting flows, like those found in combustion, are among the hardest topics in fluids. (Image and video credit: Beauty of Science)