NEW BRUNSWICK/PISCATAWAY, N.J. – Rutgers engineers have demonstrated that stirring a mix of granular ingredients faster or longer in food or pharmaceutical manufacturing won't always give you a smooth blend. In fact, you may even end up separating ingredients you thought were thoroughly mixed.
While these findings won't change the way cooks at home blend flour, salt and sugar to bake a cake, they could help manufacturers of products as diverse as breakfast cereal, medicine, cosmetics or even fertilizers and road paving materials make better products with less manufacturing waste.
The Rutgers engineers' observations are published in this week's issue of Nature, a prestigious British scientific journal.
"While mixing dry ingredients would seem to be a simple undertaking, getting uniform batches on a large scale can in fact be a challenge for industries," said Ben Glasser, professor of chemical and biochemical engineering at Rutgers, The State University of New Jersey. "The consequences of uneven blending could range from a box of raisin bran without enough raisins to pills that don't have the safe or effective amount of active pharmaceutical."
Glasser said that for years researchers have studied how liquids and gases act when stirred and discovered how "uniform flow degrades into turbulence." This concept has been applied to manufacturing, aviation, pollution control and weather prediction. But, he added, "we still don't know that much about powder or granular mixing dynamics."
The most striking finding reported by Glasser and his colleagues, Biomedical Engineering Professor Troy Shinbrot and Stephen Conway, a doctoral student, was the tendency of fine particles to separate into distinct layers under conditions that would otherwise seem to cause thorough mixing. In their paper titled "A Taylor vortex analogy in granular flows," they noted that at lower mixing speeds, fine glass beads of different sizes started to mix uniformly. But as the speed increased, the beads started forming distinct layers, the number and thickness varying with rotation speed. The researchers were able to identify patterns of granular motion that promoted layer formation and interfered with achieving uniform mixing.
While the researchers did their work under controlled laboratory conditions, they believe they have uncovered principles about mixing behavior in solid particles that could be useful not only for manufacturing, but also in understanding and manipulating "particle" flow on a larger scale, such as rock slides and avalanches.
"Just as the earlier studies of fluid mixing have helped us understand how river water mixes when it reaches the ocean, we expect our understanding of granular mixing could help scientists better understand how soil, sand, rock or snow flow," said Glasser. "Such knowledge could help engineers design barriers that effectively divert slides from developed and populated areas."
The engineers' research was partially supported by grants from the National Science Foundation, American Chemical Society, Merck & Co., Inc., and NASA.
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