In fact, according to the team from Rutgers, the State University of New Jersey, processors may even end up separating ingredients they thought were thoroughly mixed.
The findings could help manufacturers of products as diverse as breakfast cereals, bakery products and dairy products make better products with less manufacturing waste. This of course is an important consideration - processors are under increasing pressure from both governments and retailers to make efficiency savings.
"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 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," the team 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. The Rutgers engineers' observations are published in this week's issue of Nature, a UK scientific journal.