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Physicists at New York University have developed a new method for packing microscopic spheres by juggling the properties of emulsions, a mixture of two or more immiscible liquids.

Led by Jasna Brujić, assistant professor of physics, researchers studied droplets of oil in water, which form the basis of a range of consumer products, including butter, ice cream, and milk.

In earlier research, Brujić determined that the relative size of a sphere will determine how they pack.

In the new study, the researchers wanted to create a method to manipulate further how particles pack, and relied on a physical property known as “depletion attraction,” a force that causes big particles to stick together by the pressure from the surrounding small ones.

The research is reported in the Proceedings of the National Academy of Sciences.

Previous research has employed this process of attraction to create particulate gels, but these studies have tended to rely on thermally activated particles—below one micron in size—that result in complex structures known as fractals that look similar on all length scales.

In the new study, larger particles, which are not sensitive to room temperature and therefore pack under gravity alone, were used.

To bring about depletion attraction, Brujić added tiny polymers to the larger particles suspended in water, using the smaller polymers to force together the larger spheres.

In order to regulate the nature of this packing—how tightly or loosely the larger particles fit together—the researchers developed a statistical model that determines the fluctuations in the local properties of the packing.

“What we discovered is that you can control the connectivity of the particles—how they stick together and their properties—by manipulating the extent of the attraction,” Brujić says.

As a result of the discovery, the researchers have developed a method for potentially creating a range of materials—from loose to dense—based on the packing of their component parts.

Researchers from Rensselaer Polytechnic Institute contributed to the study.

More news from NYU: www.nyu.edu/public.affairs/