Understanding how particles mix in fluids has a number of important applications--among them modeling chemical reactions and cloud dynamics. Normally the particles--like creamer in your morning coffee--will follow the fluid's flow; in your mug, they trail the eddy created by your spoon. Similarly, in a cylindrical lab container--in which spinning disks mix the contents--added particles tend to form a doughnut-shaped swirl. Some particles, however, orbit around the doughnut's surface, passing the same spots again and again. Large tracer particles, for instance, will concentrate along these specific trajectories.

Now Fernando Muzzio and his colleagues at Rutgers University have discovered by accident another way in which particles are forced to follow specific trajectories--namely, if their buoyancies or densities gradually change. They report their findings in the February 12th issue of Physical Review Letters.

The researchers injected two different fluorescent dyes into a cylindrical mixer filled with glycerine. The green dye, which was neutrally bouyant, soon formed the familiar doughnut. But the red dye, which was slightly less dense, took its own course (see image). After additional experimentation and computer simulation, the team figured out what was going on: as the red dye floats up in the glycerine, it spreads and becomes less buoyant. As a result, the dye particles become trapped in horizontal swirls of glycerine, unable to rise any farther.