Just Add Water and Unmix

Drum roll, please. Add a little water, and grainy materials that ordinarily mix may suddenly separate.

It sounds as likely as toothpaste oozing back into the tube, but a mixture of two grainy materials can spontaneously separate. All you have to do is stir in a splash of water, a pair of chemical engineers has found. The effect might lead to better methods for controlling the mixing of drugs and other grainy industrial products.

The finding turns another curious phenomenon on its head. Researchers have known for decades that, when shaken or stirred, a mixture of two grainy materials will spontaneously separate if the grains of one material are much larger or denser than the grains of the other. Such sorting is known as the Brazil nut effect: It explains why Brazil nuts always end up on top of the smaller almonds and filberts in a can of mixed nuts. Water, however, can make the disparate grains stick together and keep them jumbled.

But water can also turn the opposite trick and cause two materials that normally mix to separate, say Hongming Li and Joe McCarthy of the University of Pittsburgh. If water clings more tightly to the grains of one material than to those of the other, the grains may separate even if they are otherwise identical and would ordinarily mix, the researchers report in a paper to be published in Physical Review Letters.

Li and McCarthy developed a simple theory to predict when grains of various sizes and affinities for water would intermingle and when they would keep to themselves. They then tested their theory by tumbling mixtures of glass beads in a drum. The researchers controlled the beads' penchant for water by coating them with a silicon-based substance, and they marked the beads with fluorescent dyes to track them. Adding a soupçon of water caused different combinations of beads to mix or separate just as predicted.

The work could open new avenues for controlling industrial processes, says Fernando Muzzio, a chemical engineer at Rutgers University in New Brunswick, New Jersey. “It's not that easy to make particles that mix and unmix depending on what you do to them,” Muzzio says. Currently, industrial engineers tailor the shapes of hoppers and mixing chambers to make grainy substances mix or separate, he says. The new work suggests that engineers might also manipulate the tiny forces between types of particles, he says.

Related sites
McCarthy's Web page
Muzzio's Web page

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