Scientists grow liquid crystal "flowers" to use as lenses
December 24, 2013
Scientists at the University of Pennsylvania have grown liquid crystal flowers, making it possible to create lenses as complex as the compound eye of a dragonfly. When perfected, the technology could allow the growth of lenses on curved surfaces, and structures to be assembled out of liquid crystals to build new materials, smart surfaces, microlens arrays and advanced sensors.
To create the lenses from the liquid crystals, the researchers planted tiny silica beads in a pool of transparent liquid crystal, to obtain a pattern of defects. Petal-like shapes formed in the liquid crystal around the defects in a tiered manner, resulting in a flower-like structure that resembles an insect's compound eye.
"It's a lot like how you make rock candy as a kid," Randall Kamien, Professor in Physics and Astronomy, Penn Arts & Sciences, tells Gizmag. "The sugar naturally makes crystals, but you need to put in a seed (usually a stick or a piece of string) to get it to grow where you want. We have just done this on a smaller scale, making smaller bits of ordered material cued by smaller elements, like our silica beads."
The research represents a significant step forward in "directed assembly" nanotechnology, where scientists try to build minute structures by specifying starting conditions and letting physical and chemical processes do the assembly work, instead of doing it themselves.
"We exploit a material's tendency to form ordered structures by adding cues to the system", explains Kamien. "It is how cultured pearls are made – a seed is deliberately planted in the oyster to get it to naturally produce nacre around the seed."
Each flower petal formed out of transparent liquid crystal can act as a lens since light can interact with its curved surface. Since individual lenses can be manipulated so that they are a few microns to tens of microns in diameter, it's possible to create a flower of collective lenses which are tens of microns to millimeters in size. The technique makes it possible to grow a liquid crystal eye that's as complex as a dragonfly's eye, for example, which has millions of spherical lenses that enable it to stack lots of images into a 3D image.
"Researchers try to fabricate such compound eyes, for example, for cameras or detectors," Shu Yang, Materials Science and Engineering Professor at Penn Engineering tells us. "But it’s typically by a top-down process. So it isn't possible to fabricate millions of eyes in one-step, nor will it be cheap. Self-assembly of flower pattern of lenses will make it possible to create compound eyes in one-step. And we can control the curvature of the template, i.e., how we plant the seeds in the first place to determine how the lenses will be put together, how many of them and in what shape."
Being able to grow lenses this way opens up many exciting possibilities. Lenses could change sizes from the center to the edge, in a single eye; a feature that's very useful for certain types of applications. Biosensors could collect information from many lenses at the same time. Since the lenses form spontaneously in liquid crystal, a medium that's easily reconfigured, it may be possible to create self healing re-configurable optical devices. And all that's just for starters.
"We could make inexpensive lenses that could cover entire surfaces," says Kathleen Stebe, Chemical and Biomolecular Engineering Professor at Penn Engineering. "They could transmit images from many focal points and be reconstructed by algorithms to give improved resolution. We could have a camera on flexible substrate or curved substrates with lens that form and re-form after being bent."
Creating new synthetic metamaterials, where the arrangement and shapes of individual components determines its physical properties, also becomes a possibility.
"We are also interested in dynamically changing the physical properties of the metamaterials, which can be achieved by tuning the shape and arrangement of the seeds," Yang reveals. "Such materials will be of interest for creating superlenses that can produce flawless images much smaller than the wavelength of light, drug delivery, shape conforming mechanical metamaterials and acoustically invisible cloaks."
The team anticipates these lenses working their ways into liquid crystal displays within the next 10 years.
A paper detailing the research, which is supported by by the National Science Foundation, Penn’s Materials Science Research and Engineering Center and the Simons Foundation, was recently published in Physical Review X.
Source: University of Pennsylvania
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