Nearly Simultaneous Pieces in Nature Physics and Science

February 18, 2008

In a somewhat rare feat, McCormick Dean Julio M. Ottino and Senior Associate Dean Richard Lueptow are co-authors on two papers published this past week – one in Science and the other Nature Physics.

The Science piece, written by Ottino and Lueptow and published February 15, is a perspective on how to counter segregation in mixing granular materials. When trying to mix particles that are different sizes or densities, putting more energy into mixing may actually cause the particles to demix, or segregate into patterns. This is problem of practical importance – such mixing issues are found in processing pharmaceuticals, dry chemicals, ceramics, minerals, and powdered or granular foodstuffs – but it also one that has puzzled theoreticians for a long time.

But new studies are yielding insight. One promising avenue was recently discovered by one of Ottino’s former PhD students, Joseph J. McCarthy, now a professor at the University of Pittsburgh. As particles of different sizes flow, small particles fall through the spaces between larger particles and end up lower in the flowing layer. But this takes time. If the flow is interrupted and somehow flipped over before the particles have percolated to the bottom of the flowing layer, then the particles do not segregate completely and segregation can be prevented. The approach works in two-dimensions, which leads researchers to take on the next challenge – mixing such particles in three-dimensional tumblers.

“Recent theoretical work on mixing a single class of particles in three-dimensional tumblers – a far simpler case than mixing two classes of particles – suggests an explosive increase in the richness of problems that may be encountered when tackling mixing and demixing of granular materials,” Ottino and Lueptow write in the paper.

But for every answer researchers find, another question arises. The Nature Physics paper, authored by Steven Meier, chemical and biological engineering doctoral student now at ExxonMobil, with Ottino and Lueptow, shows that new findings in segregation patterns also raise new mixing issues.

The paper, published online February 12, describes how when particles are tumbled in certain systems where one would normally expect them to mix, the particles actually segregate into patterns of streaks that radiate out from the center of the mixture. But by extending the experiment by more tumbler revolutions, the pattern of several streaks eventually coarsens as one large streak – much like drops of oil and vinegar coalesce and separate – resulting in a new wedge-shaped pattern. This is a slow process that had not been observed before, and it raises new questions about the physics of granular segregation.

The Science paper is titled, “How to Counter Segregation.” The Nature Physics paper is titled, “Coarsening of granular segregation patterns in quasi-two-dimensional tumblers.” In addition to Ottino, Lueptow and Meier, Diego A. Melani Barreiro, an undergraduate from the Massachusetts Institute of Technology, who spent a summer doing research in the Ottino and Lueptow lab under a Research Experience for Undergraduates program, was also an author on the Nature Physics paper.

-Emily Ayshford