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Pitt scientists find mixing materials isn't as simple as adding water

Monday, May 26, 2003

By Byron Spice, Post-Gazette Science Editor

Since pestle was first put to mortar, people have been grinding and mixing things. On those occasions when materials prove difficult to mix, people have often done exactly what children who build sandcastles at the beach do -- add a little water to make things sticky.

Top: Glass beads of similar sizes and densities mix evenly when tumbled in a narrow drum. Bottom: In the same drum, after the addition of water, the beads that repel water, appearing as white, clump together while beads that are attracted to water become unmixed and shift to the edge of the drum. (Photos courtesy of Physical Review Letters)

And so it has been for thousands and thousands of years of shaking and stirring, as people have mixed grains and other foods, ceramics, pharmaceuticals, metal powders and a whole array of granular material.

But experiments by chemical engineers at the University of Pittsburgh have shown that surprises still remain in the science of mixing.

Or, you might call it the science of unmixing.

Joseph McCarthy, an assistant professor, and Hongming Li, a graduate student, have found that, in some cases, adding a little water can actually cause mixed granular materials to unmix, or segregate, as scientists would say.

"It's somewhat counterintuitive," said McCarthy, who reported their findings in the May 9 issue of the journal Physical Review Letters.

Likewise, a new series of experiments is showing some surprising effects of shear forces -- stirring. It seems that, in some cases, the faster a mixture is stirred, the more it unmixes.

The experiments, which involve tumbling specially coated glass beads in 6-inch high drums, were designed to test a particle-mixing theory that McCarthy and Li are developing. They predict the ability of two materials to mix or unmix based on their relative size, density and the degree to which they are attracted to or repelled by water.

Regardless of the theory's merits, "they've found new behaviors that people hadn't expected," said Peter Schiffer, a Penn State physicist whose research includes the properties of granular materials. Observing grains that get separated by the addition of moisture to the mix, he added, "is a big step forward."

The tumbling experiments are fairly standard laboratory procedures, Schiffer said, but what McCarthy and Li did with them was quite clever. They coated red beads with a material that is hydrophilic -- that is, water-loving -- and green beads with a hydrophobic, or water-repelling, material. The colors allowed the researchers to track the behavior of the beads.

The experiments involved a number of combinations of beads, which could vary in size, density and in affinity to water.

When other factors are equal, a set of hydrophilic particles can mix perfectly well with hydrophobic particles. But add a little water and the two types will readily unmix, the experiments showed.

Researchers have long appreciated that particles of different sizes are difficult to keep mixed. In a can of mixed nuts, for instance, the Brazil nuts always work their way to the top; in a box of cereal, the crumbs fall to the bottom and the largest flakes move to the top. Everything else being equal, adding liquid, such as milk to a bowl of cereal, increases the stickiness and helps keep everything mixed.

Density also matters. Light tends to stick to light and heavy tends to stick to heavy.

Affinity to water is something that has been less studied, McCarthy said. Many researchers tend to simplify these factors, regarding particles as either hydrophilic or not, when in actuality materials can be hydrophilic or hydrophobic to varying degrees.

Mixing is an important industrial process. In the pharmaceutical industry, for instance, pills typically are made with only a small amount of medicine that must be evenly mixed with a large amount of binder.

The improved understanding of how materials behave when mixed won't necessarily allow manufacturers to do anything they couldn't do before, but should remove some of the guesswork as they develop processes for new products, McCarthy said.

Already, for instance, McCarthy is working with the National Energy Technology Laboratory in South Park on how to combine coal with switchgrass, a hardy grass that can be grown in poor soil and that has been promoted as an alternative energy source. McCarthy is studying ways that the switchgrass and coal can be mixed uniformly so that furnaces can burn the combination efficiently.

Byron Spice can be reached at bspice@post-gazette.com or 412-263-1578.

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