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 a liquid crystal display, for example, the crystals' orientation in different regions determines which parts of the screen are illuminated.
"Liquid crystals naturally produce a pattern of close-packed defects on their surfaces," Yang said, "but it turns out that this pattern is often not that interesting for device applications. We want to arbitrarily manipulate that pattern on demand."
Electrical fields are often used to change the crystals' orientation, as in the case with liquid crystal displays, but the Penn research team was interested in manipulating defects by using a physical template. Employing a class of liquid crystals that forms stacks of layers spaced in nanometers known as "smectic" liquid crystals the researchers 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.
"The molecules can feel the geometry of the template, which creates a sort of elastic cue," Stebe said. "That cue is transmitted layer by layer, and the whole system responds."
The researchers' template was a series of microscopic posts arrayed 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.
For example, a smectic liquid crystal that would naturally form a hexagonal array of dimple-like defects on its surface could be templated to form a square pattern or to have dimples that were more closely or loosely packed.
Critically, these induced defect patterns weren't one-to-one reproductions of the pattern of posts on the template layer. The
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University of Pennsylvania