University of Pittsburgh
June 9, 2003

Pitt Researchers Find Moisture Helps and Hinders in Particle Mixing

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Pitt Researchers Find Moisture Helps and Hinders in

Particle Mixing

PITTSBURGH—If you want to mix batches of two granular particles, just add a

little moisture.

If you want to separate a mixture of two granular particles, just add a little moisture.

Mixed up?

A University of Pittsburgh engineering professor and his graduate assistant have found that adding moisture to a batch of two different granular particles will either cause them to mix or separate, depending on the composition of the particles.

Joseph McCarthy, assistant professor in the Department of Chemical and Petroleum Engineering, and graduate student Hongming Li developed and tested a theory regarding particle mixing and segregating. Their findings were published last month in the journal Physical Review Letters.

Granular materials in many instances "flow" like liquids, but there are important differences that scientists don't fully comprehend.

"It's well known that granular materials may segregate-or unmix-due to differences in size, density, shape, etc.," the researchers write. What is less well-known, they write, is the role of cohesion, in this case due to moisture.

Conventional wisdom says that adding moisture to the materials to make them stick to one another more easily allows one to mix systems that would typically unmix.

In their experiments, McCarthy and Li tumbled particles-red and green glass and plastic beads-of different sizes and densities in a drum. The researchers also varied the particles' wetting properties by covering some of the beads with water repellent and others with water attracting coatings.

What they found was that for some combinations of size, density, and coatings, the dry beads mixed, while adding moisture caused them to segregate. For other combinations, however, the opposite occurred and the dry beads unmixed, while the wet beads segregated.

Their findings are of "considerable industrial importance" to business sectors like pharmaceuticals, metals, and ceramics, in which mixing or separating granular materials such as powders needs to be controlled.

"In many instances the wetting properties and the density of a material may be fixed, but our theory lets you know exactly what size ratios to use to get a cohesive system to mix, or not mix," says McCarthy. "And it's not always a combination your would expect."

In general, McCarthy says, "If like particles stick to each other when dry, then adding moisture will cause them to segregate. If dissimilar particles stick to each other, then adding moisture will cause them to mix."

The research is being sponsored by the Chemical and Transport Systems Division of the National Science Foundation and the American Chemical Society's Petroleum Research Fund.