Penn team making waves with liquid crystals

Liquid Crystals

Felice Macera, Daniel Beller, Apiradee Honglawan, Simon Čopar

By changing the geometry at the bottom layer of a stack of liquid crystals, the researchers were able to get much finer control of the features in the top layer than has previously been possible.

Liquid crystals have become a workhorse of technology; they are the standard for television and computer displays. In fact, there is a good chance you are looking at some liquid crystals right now.

An interdisciplinary team at Penn is working with liquid crystals like no researchers have before, opening the door for new applications in displays, lenses, sensors, and even nano-manufacturing.    

The team includes Shu Yang, an associate professor in the Department of Materials Science and Engineering; Kathleen Stebe, the Richer and Elizabeth Goodwin Professor of Engineering and Applied Science and deputy dean for research in the School of Engineering and Applied Science; and Randall Kamien, the Vicki and William Abrams Professor in the Natural Sciences in the School of Arts & Sciences.

They came together through Penn’s Materials Research Science and Engineering Center, which recently received a $21.7 million National Science Foundation grant to support this kind of interdisciplinary research. Stebe and Kamien are leaders of the Center’s sub-group focused on elasticity in soft materials, and knew they had the expertise on hand to do groundbreaking work with liquid crystals.   

Crystals are materials that have molecules arrayed in regular three-dimensional patterns; liquid crystals contain these patterns, but their molecules can flow around one another and change the direction they face. This behavior produces defects, places on the surface where the molecular orientation of the liquid crystals is disrupted.

Despite their name, such defects are highly desirable. If the location of the defects can be controlled, the change in pattern or orientation can be put to use. In the case of a liquid crystal display, for example, the crystals’ orientation in different regions determines which parts of the screen are illuminated.   

Electrical fields are often used to change the crystals’ orientation, but the Penn research team was interested in manipulating defects by using a physical template. Employing a type of liquid crystal that forms stacks of layers, they set out to show that by altering the geometry of the molecules on the bottommost layer, they could produce changes in the patterns of defects on the topmost.

Their templates were sheets with microscopic posts, like a bed of nails. By altering the size, shape, symmetry, and spacing of these posts, as well as the thickness of the liquid crystal film, the researchers discovered they could make subtle changes in the patterns of the defects.  

“You could even use the sheet as a template to assemble other molecules,” Yang says. “You could put nanoparticles, quantum dots, or carbon nanotubes in the liquid crystal and they would be expelled into to the defects.”

A template consisting of circular posts could even be dynamically altered with heat, for example, making the posts in a certain region elliptical. This microscopic geometric cue would travel up the layers of liquid crystal and produce nanometer-scale changes on the surface.     

“We are providing a very crude cue and getting exquisite molecular level organization,” Stebe says. “It’s quite beautiful.”

Originally published on December 20, 2012