Three-dimensional liquid crystal hemisphere can be used as a micro-lens
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Because the petals of this “flower” are made of transparent liquid crystal and radiate out in a circle from a central point, the ensemble resembles a compound eye and can thus be used as a lens.
These results are an improvement over earlier studies when the material scientists, chemical engineers and physicists were able to produce patterns of “defects,” useful disruptions in the repeating patterns found in liquid crystals, in nanoscale grids and rings.
The researchers’ ongoing work with liquid crystals is an example of a growing field of nanotechnology known as “directed assembly,” in which scientists and engineers aim to manufacture structures on the smallest scales without having to individually manipulate each component.
The trick is to set out precisely defined starting conditions and let the physics and chemistry that govern those components do the rest.
The starting conditions in the researchers’ previous experiments were templates consisting of tiny posts. In one of their studies, they showed that changing the size, shape or spacing of these posts would result in corresponding changes in the patterns of defects on the surface of the liquid crystal resting on top of them. In another experiment, they showed they could make a “hula hoop” of defects around individual posts, which would then act as a second template for a ring of defects at the surface.
To yield the three-dimensional liquid crystal hemisphere, silica beads were used as the seed, planted at the top of a pool of liquid crystal. Flower-like patterns of defects then grow around each bead.
The key difference between the template in this experiment and ones in the research team’s earlier work was the shape of the interface between the template and the liquid crystal. In their experiment that generated grid patterns of defects, those patterns stemmed from cues generated by the templates’ microposts.
Domains of elastic energy originated on the flat tops and edges of these posts and travelled up the liquid crystal’s layers, culminating in defects. Using a bead instead of a post, makes it so that the interface is no longer flat but keeps changing. The liquid crystal re-arranges itself around the bead in petal-like shapes at smaller and smaller sizes, trying to match the angle of the bead until everything is flat.
Surface tension on the bead induce those petals to be arranged in a tiered, convex fashion. And because the liquid crystal can interact with light, the entire assembly can function as a lens, focusing light to a point underneath the bead.
It is anticipated that such directed assemblies could be used to make optical switches and in other applications.
Visit the University of Pennsylvania at www.upenn.edu
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