Cambridge, Mass. -- By studying the behavior of tiny particles at an interface between oil and water, researchers at Harvard have discovered that stabilized emulsions may take longer to reach equilibrium than previously thought.
Much longer, in fact.
"We were looking at what we thought would be a very simple phenomenon, and we found something very strange," says principal investigator Vinothan Manoharan, Associate Professor of Chemical Engineering and Physics at the Harvard School of Engineering and Applied Sciences (SEAS).
"We knew that the particle would stick to the interface, and other researchers had assumed this event happened instantaneously," he says. "We actually found that the timescale for this process was months to years."
The findings, published in Nature Materials (online) on December 4, have important implications for the manufacturing processes used in pharmaceuticals, cosmetics, and foods, among other chemical industries.
An emulsion is a mixture of two or more insoluble liquidsusually oil and water. A simple emulsion like vinaigrette takes energy to create (for example, by shaking it), and over time it will separate out, as the oil or water molecules cluster together again.
To give products like mayonnaise and sunscreen a reasonable shelf life, manufacturers typically add stabilizing particles to create Pickering emulsions. Ice cream, for example, is stabilized by tiny ice crystals that cling to the interfaces between the fat and water droplets, creating a rigid physical barrier between the two. In traditional mayonnaise, proteins from the egg yolk perform the same role.
When the oil and water in these types of emulsions are completely mixed and stable, the particles are said to be at equilibrium.
"There are certain rules for making different types of emulsions," explains Manoharan. "For example, do you get oil droplets in water, or water droplets in oil? The conven
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