Mixing two fluids is a tougher task than you might think. One of my favorite asides from a fluids lecture concerned how to mix fruit into yogurt in an industrial setting. Mix too quickly, and you’ll obliterate the yogurt’s consistency, but mix too little and you may as well sell it as fruit-on-the-bottom. Apparently that particular problem got solved by sending the fruit and yogurt flowing through a series of specially-shaped ducts to slowly and carefully mix them together.
In this study, researchers tackle a similar problem — mixing two fluids in a circular cross-section — through optimization. As you can see above, circular stirrers on their own don’t do a great job of mixing. So the researchers searched for the right combination of stirrer shape, mixing speed, and mixing trajectory to give the best mixing for a set mixing time and energy input. Their final stirrer shapes are anything but circular and often move in jerks and fits to help shed vortices that do the actual job of mixing. (Image and research credit: M. Eggl and P. Schmid; via APS Physics)