Study Sheds Light on Oil-Water Mixtures

Emulsions—mixtures of oil droplets suspended in water—may take much longer to reach equilibrium than previously expected, according to a recent study by Harvard researchers.

In a class of mixtures called Pickering emulsions, solid particles bind to the interface between two liquids that usually do not mix, such as oil and water. The particles keep the two liquids from separating, allowing products like ice cream, mayonnaise, and many lotion products to have a reasonable shelf life.

But in a paper published in Nature Materials on December 4, a team led by Harvard Chemical Engineering and Physics Professor Vinothan N. Manoharan discovered that these particles may take months or even years to settle into place. The process was previously thought to be instantaneous, according to David M. Kaz, an author of the study who received his Ph.D. in Physics at Harvard this fall.

“We’ve introduced a lot of questions that are relevant to anyone preparing Pickering emulsions in a controlled fashion,” Kaz said. “We’ve provided a new insight into what might be happening after they’ve been prepared.”

In the study, the researchers observed a tiny particle, a few microns in diameter, as it approached a layer of oil floating on top of water. By using digital holography, they reconstructed three-dimensional images of the particle’s path, finding that the particle slowed down dramatically after reaching the oil-water boundary.

“Our experiments went up to about 100 seconds, but by extrapolating the data to the point where we think the equilibrium is, we estimated a time scale ... of months to years [for the particle to become stable],” said Manoharan.

According to Kaz, the oil-water interface snags on imperfections on the particle, impeding its movement. The process might be quicker if the particle was moving with more energy, he said. Thus, the amount of energy input should matter when preparing an emulsion.

The results have applications to understanding many common substances. In mayonnaise, for example, Kaz explained that egg yolk proteins play the role of solid particles which stabilize the emulsion. By binding to the surface of tiny oil droplets, they create “armored droplets” that are less likely to coalesce. This prevents the oil and water in mayonnaise from separating.

“When people consider how emulsions are aging, they consider temperature or chemical processes that would make the emulsion change over time,” Kaz said. “Now they can ask: how did we prepare it?”

Kaz added that he thinks the results are not limited to understanding emulsions, as a similar process is used in water purification and mineral recovery.

—Staff writer Brian C. Zhang can be reached at brianzhang@college.harvard.edu.