Scientists study light scattering particles

A year of adding polymers to nanoparticles and testing their opacity left chemistry Ph.D. student Lindsay Bombalski frustrated. Although she was using a standard procedure to test the particles and characterize them by how much light they scatter, two samples were not giving her any signal at all, as though they had no opacity.

“At no point did we think we were looking at something interesting,” said Michael Bockstaller, assistant professor of materials science and engineering who worked with Bombalski and chemistry professor Krzysztof Matyjaszewski on the project.

As it turns out, they discovered that that “something” could be used to improve product packaging, windshields, and sunblock.

“Lindsay wanted to understand why the technology wouldn’t work out,” Bockstaller said. “This led to redefining her thesis, to understand this phenomenon,” he added.

Particle additives such as polymers are often added to particles in other materials to increase heat resistance or mechanical strength of materials. However, the problem with this technique is that the reinforced particles scatter, altering the particles’ original appearance and making them virtually unusable for any matter that requires light to pass through.

This scattering is caused by the change in the particles’ refractive index — an assessment of how fast light travels within a material — when the inorganic particles are added to an organic solution. The researchers found that by planning a target refractive index, they could eliminate the scattering of particles.

Bombalski likened the phenomenon to how a straw appears bent as it is immersed in water, which is the result of the straw going from the refractive index of air to the refractive index of water.
“In our case, we made the air and the water match,” Bombalski said.

After two and a half years of research, the team’s findings were published in the most recent edition of Advanced Materials magazine. In addition to Bockstaller, Matyjaszewski, and Bombalski, who graduated last December, other researchers included two current graduate students: Hongchen Dong, a Ph.D. student in chemistry, and Jessica Listak, a Ph.D. student in materials science and engineering.

“We gained a better understanding of the optical properties of particles and of their compatibility, and the development of new synthetic techniques that will become bound to the particle surface,” Bockstaller said.

The significance of the project to the students lies in the various potential applications of their discovery.

Dong agreed. “I like things that have applications that are close to real life,” she said. “I’m happy this stuff has a real application and will have an impact in academia and an impact in industry.”
Dong was responsible for synthesizing the particles, while Listak worked on the characterization of the particles via an electron microscope, according to Bockstaller.

Bockstaller has his own ideas about how the group’s research will affect the development of new materials, which he says is a multi-billion dollar industry.

“Rather than use this for new applications, I would rather see this as a big step in enhancing existing applications,” he said. The particles could be used in product packaging where transparency is important to consumers, Bockstaller said.

Additionally, they could aid in the development of transparent suntan lotion. Sunscreen contains nanoparticles that are responsible for UV protection and give the lotion its white color.
The particles could also be added to windshields to make them absorb more light — something that is not currently possible because nanoparticles that scatter light would block the driver’s view.

“The Airforce has also expressed interest in this technology, so eventually we might be thinking about applications in this direction,” said Bockstaller. But the results of the research are more than purely scientific.

“This is a textbook case on how research on new discoveries is being made,” Bockstaller said.

“This is proof that the discovery of an invention comes from the work of a student on a problem because of his or her persistence, discipline, and curiosity,” he added. “Students could learn from this story.”