Lauren Zarzar

Lauren Zarzar thinks liquids deserve some materials attention.

“Liquids are the most dynamic and responsive materials we have,” the Pennsylvania State University researcher says. Unlike solids, she explains, liquids change shape easily, and two liquids in contact can essentially communicate, passing molecules between each other.

Zarzar has developed ways to control the shapes and compositions of tiny droplets of complex emulsions. She thinks these liquid materials could act as microscopic lenses, simple sensors, and colorful coatings.

The droplets are also part of a research theme that Zarzar has pursued since she was a graduate student at Harvard University: materials that can respond to their surroundings. Most materials, Zarzar says, are static and designed for a specific purpose. But materials aren’t always used under one set of circumstances, she says. “If they could adapt and change, we could make materials that are more functional and useful.”

As a postdoc at the Massachusetts Institute of Technology, Zarzar started working with complex emulsions. These droplets are made of liquids that don’t mix, such as a droplet containing a fluorocarbon oil inside a hydrocarbon oil floating in water.

She developed an easy way to build these droplets and then change their structure. For example, she could pick which oil was on the inside of the droplet and which was on the outside. She started with oils that didn’t mix at room temperature but could when heated. By cooling these heated mixtures with surfactants, she could control the structure of the emulsions as the oils stopped mixing. By adding different surfactants to a solution containing the emulsions, she could change the droplets’ structure again—flipping which oil was inside the droplet and which was outside, for example.

Surfactants influence the droplets’ structure by controlling the surface tension of the different liquids in the emulsions. That surface tension also dictates the curvature of the droplets, which can influence how light interacts with the materials, affecting their optical properties. Researchers could use the droplets as lenses and tune them as they would a microscope.

Zarzar also recently reported a colorful example of the droplets’ optical properties.

Light bounces off the curved surfaces of these droplets and causes interference, leading to iridescence, as you might see in soap bubbles. Zarzar and her lab found that they could control the curvature of their droplets by adding different surfactants, allowing them to change this iridescence. They describe this effect as a tunable version of structural color, which is color that arises not from dyes but from how light interacts with the structure of a material, like the color on some butterfly wings.

“You can tune that curvature in the droplets dynamically, and that changes the color, which you can’t do easily in a solid,” Zarzar says. She thinks this phenomenon could be exploited in colorful coatings and reflective displays.

The work, published earlier this year, “showed Lauren’s creativity and ability to bring together orthogonal fields, thinking how chemistry can be combined with materials science and with optics to come up with ideas that don’t fit in just one field,” says Joanna Aizenberg, who was Zarzar’s graduate adviser at Harvard.

Zarzar plans to keep pursuing the theme of responsive materials and think about ways to create what she calls artificial living materials—materials that can adapt and evolve over time.