RESEARCH NEWS UPDATES (LIQUID CRYSTAL)

Researchers Grow Liquid Crystal 'Flowers' That Can Be Used as Lenses

Dec. 20, 2013 — A team of material scientists, chemical engineers and physicists from the University of Pennsylvania has made another advance in their effort to use liquid crystals as a medium for assembling structures.

 

 
  A liquid crystal "flower" under magnification. The black dot at center is the silica bead that generates the flower's pattern. (Credit: Image courtesy of University of Pennsylvania)




In their earlier studies, the team produced patterns of "defects," useful disruptions in the repeating patterns found in liquid crystals, in nanoscale grids and rings. The new study adds a more complex pattern out of an even simpler template: a three-dimensional array in the shape of a flower.
And 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.
The team consists of Randall Kamien, professor in the School of Arts and Sciences' Department of Physics and Astronomy; Kathleen Stebe, the School of Engineering and Applied Science's deputy dean for research and professor in Chemical and Biomolecular Engineering and Shu Yang, professor in Engineering's departments of Materials Science and Engineering and Chemical and Biomolecular Engineering. Members of their labs also contributed to the new study, including lead author Daniel Beller, Mohamed Gharbi and Apiradee Honglawan.
Their work was published in Physical Review X.
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. Rather, they 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.
In their latest work, the researchers used a much simpler cue.
"Before we were growing these liquid crystals on something like a trellis, a template with precisely ordered features," Kamien said. "Here, we're just planting a seed."
The seed, in this case, were silica beads -- essentially, polished grains of sand. Planted at the top of a pool of liquid crystal flower-like patterns of defects 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, as the researchers did in their latest experiment, makes it so that the interface is no longer flat.
"Not only is the interface at an angle, it's an angle that keeps changing," Kamien said. "The way the liquid crystal responds to that is that it makes these 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 also makes it so these petals are 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's like an insect's compound eye, or the mirrors on the biggest telescopes," said Kamien. "As we learn more about these systems, we're going to be able to make these kinds of lenses to order and use them to direct light."
This type of directed assembly could be useful in making optical switches and in other applications.


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Controlling Liquid Crystals: Another Tool in the Directed Assembly Toolkit

 

Nov. 12, 2013 — An interdisciplinary team of University of Pennsylvania researchers has already developed a technique for controlling liquid crystals by means of physical templates and elastic energy, rather than the electromagnetic fields that manipulate them in televisions and computer monitors. They envision using this technique to direct the assembly of other materials, such as nanoparticles.

 

A submerged micropost causes the surrounding liquid crystal to form a ring pattern, directing nanoparticles on the surface. (Credit: Image courtesy of University of Pennsylvania)

 

Now, the Penn team has added another tool to its directed assembly toolkit, developing a new kind of template for rearranging particles and a new set of patterns that can be formed with them.
The team consists of Kathleen Stebe, the School of Engineering and Applied Science's deputy dean for research and professor in Chemical and Biomolecular Engineering; Randall Kamien, professor in the School of Arts and Sciences' Department of Physics and Astronomy; and Shu Yang, professor in Engineering's departments of Materials Science and Engineering and Chemical and Biomolecular Engineering. Members of their labs also contributed to the new study, including lead author Marcello Cavallaro Jr., Mohamed Gharbi, Daniel Beller, Simon Čopar, Zheng Shi and Tobias Baumgart.
Their work was published in the Proceedings of the National Academy of Sciences.
Crystals are materials that have molecules arrayed in regular three-dimensional patterns; liquid crystals contain some but not all of these patterns, and their molecules can flow around one another and change the direction they face. This behavior allows 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 a liquid crystal display, for example, the crystals' orientation in different regions determines which parts of the screen are illuminated.
In an earlier study, the team had used a template consisting of microscopic posts to arrange the defects on the surface of smectic liquid crystals, a class of the material that forms layers. The position, shape and symmetry of the posts allowed the researchers to manipulate the bottom layer of these liquid crystals which in turn generated patterns of defects on the top layer that were orders of magnitude smaller than the original template.
In their new study, the researchers use nematic liquid crystal, which have less long-range order in their patterns but are the kind found in liquid crystal displays.
"These nematic structures are very reconfigurable. That's the basis of why they're good for displays," Stebe said. "Everyone knew that materials can be moved and positioned with electric and magnetic fields, but we're doing it with fields of elastic energy and showing that this technology can be used in assembling materials."
As in their previous experiments, the team started with a template consisting of microscopic posts that was then topped with the liquid crystal. In this experiment, however, instead of a pattern of defects forming only on the surface of the liquid crystal, a ring-shaped defect encircled each of the posts at their midpoints. This ring then acts like another template, directing the arrangement of patterns on the liquid crystal surface, more than 50 microns away.
"With the smectic liquid crystals," Kamien said, "the patterns of defects we could make were closely associated with the corners of the posts. With nematic liquid crystals, we can form these rings, which is a new way to tell what to go where."
"And we're showing," Yang said, "that, whether we use smectic or nematic crystals, we can use a template that directs these surface defect arrays, which can then hold things like colloids, nanoparticles or quantum dots."
Being able to control the spacing and arrangement of these secondary materials could allow for making new types of antennas, sensors or displays. The team's latest study provides a new set of shapes and patterns to work with in the directed assembly approach to making such devices.
"We're building up the toolkit of the different structures we can make via different confinements of these materials," Stebe said. "Once we have our toolkit filled out, it's going to become more readily apparent how we can make these structures dance -- how we can make them rearrange themselves however we want -- the next wave of applications will come."
The research was supported by the National Science Foundation through Penn's Materials Research Science and Engineering Center, the Laboratory for Research on the Structure of Matter, as well as The Mark Howard Shapiro and Anita Rae Shapiro Charitable Fund, the Kavli Institute for Theoretical Physics and the Simons Foundation.



Molecular Switch Changes Liquid Crystal Colors

 

Aug. 26, 2013 — Dartmouth researchers have developed a molecular switch that changes a liquid crystal's readout color based on a chemical input. This new development may open the way for using liquid crystals in detecting harmful gases, pathogens, explosives and other chemical substances.

 

One of the challenges in the field of molecular switches and machines is the translation of molecular level motion into macroscopic level events by harnessing light or chemical energy -- think of a molecular-sized light switch that can be turned on and off. With an actual light switch, this can be easily done by hard wiring the switch to a light source, but doing this at the nanoscale is challenging.
In their study, the Dartmouth researchers used liquid crystals such as the ones in LCD (liquid crystal display) monitors and TV screens to address this challenge. They synthesized a pH activated molecular switch that can control the long range assembly of a commercially available liquid crystal called NP5. This manipulation changed the readout color of NP5 from purple to green depending on the applied pH, confirming the molecular level motion is responsible for the change in the photophysical properties of the liquid crystal.
The findings open the way for researchers to design molecular switches that produce different liquid crystal readout colors when harmful chemical substances are detected. If these liquid crystals are used as pixels -- similar to the ones in LCD screens -- researchers may be able to bunch them together and develop groups of sensors that can quickly analyze and detect hazardous materials.



Researchers Show New Level of Control Over Liquid Crystals

 

Jan. 7, 2013 — Directed assembly is a growing field of research in nanotechnology in which scientists and engineers aim to manufacture structures on the smallest scales without having to individually manipulate each component. Rather, they set out precisely defined starting conditions and let the physics and chemistry that govern those components do the rest.

 

 

An interdisciplinary team of researchers from the University of Pennsylvania has shown a new way to direct the assembly of liquid crystals, generating small features that spontaneously arrange in arrays based on much larger templates.
The study was led by Shu Yang, associate professor in the School of Engineering and Applied Science's departments of Materials Science and Engineering and Chemical and Biomolecular Engineering; Kathleen Stebe, Engineering's vice dean for research and professor in Chemical and Biomolecular Engineering; and Randall Kamien, professor in the School of Arts and Sciences' Department of Physics and Astronomy. Apiradee Honglawan of Yang's lab, Daniel Beller of Kamien's group and Marcello Cavallaro Jr. of Stebe's lab also contributed to the research.
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.
Their work was published in the Proceedings of the National Academies of Science.
Crystals are materials that have molecules arrayed in regular three-dimensional patterns; liquid crystals contain some, but not all, of these patterns, and their molecules can flow around one another and change the direction they face. This behavior allows 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 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 researchers were able to generate more complex relationships, such as getting four defects to sit atop each circular post or defects that formed over the points of a triangular post. They were also able to deduce the rules that govern these relationships and predict what defect patterns a given set of post parameters would produce.
"The first layer's molecules tend to be pinned to the edges of the posts," Kamien said, "so changing a post's size and shape will change how many defects can sit on its edges at the same time."
The size discrepancy between the posts and individual molecules of the liquid crystal is also a key feature for using this class of liquid crystal in directed assembly. The posts are each a few microns wide and tall, still microscopically small, but large enough to be easily and economically made to specification. This is much more attractive than trying to directly control the size and arrangement of the liquid crystals' defects.
"The liquid crystal layers are very thin, so the defects are on the order of several nanometers across" Kamien said. "Those defects would normally be very hard to control, especially compared to the posts, which are more like a few thousand nanometers across."
Beyond sensors and displays, these defects can be used in nanomanufacturing.
"If you make defects like dimples, you could put ink in them and use them like a stamp," Kamien said. "Or you could make the inverse of the dimples and make points, which could be used as localized surface plasmon resonance hot spots for chemical and biological sensing or as a topographic protrusion for creating a superhydrophobic surface."
And because the layers of liquid crystals transmit elastic energy, they can also be used to do mechanical work. This means that the top layer could be used as a template to assemble even larger molecules.
"You could put nanoparticles, quantum dots or carbon nanotubes in the liquid crystal layers and they would be expelled into to the defects," Yang said.
A template consisting of circular posts could even be dynamically altered with heat and an electric field, for example, making the posts in a certain region elliptical. This microscopic geometric cue would travel up the layers of liquid crystal and produce micrometer-scale changes on the surface.
By establishing the mathematical relationships between the post parameters and the surface-layer defects, the researchers are laying the foundation of a directed assembly technique that can be used with any smectic liquid crystal.
"We are providing a very crude cue and getting exquisite molecular level organization," Stebe said. "Any liquid crystal that makes layers can now be used to make rather beautiful control over textures on the surface.
The research was supported by the National Science Foundation.

 

Liquid Crystal Research May Lead to Creation of New Materials That Can Be Actively Controlled

 

Dec. 27, 2012 — Contributing geometric and topological analyses of micro-materials, University of Massachusetts Amherst mathematician Robert Kusner aided experimental physicists at the University of Colorado (UC) by successfully explaining the observed "beautiful and complex patterns revealed" in three-dimensional liquid crystal experiments. The work is expected to lead to creation of new materials that can be actively controlled.

 

 

Kusner is a geometer, an expert in the analysis of variational problems in low-dimensional geometry and topology, which concerns properties preserved under continuous deformation such as stretching and bending. His work over 3 decades has focused on the geometry and topology of curves, surfaces and other spaces that arise in nature, such as soap films, knots and the shapes of fluid droplets. Kusner agrees with physicist and lead author Ivan Smalyukh of UC Boulder that their collaboration is the first to show in experiments that some of the most fundamental topological theorems hold up in real materials. Their findings appear in the current early online issue of Nature.
UMass Amherst's Kusner explains, "There are two important aspects of this work. First, the experimental work by the Colorado team, who fabricated topologically complex micro-materials allowing controlled experiments of three-dimensional liquid crystals. Second, the theoretical work performed by us mathematicians and theoretical physicists while visiting the University of California Santa Barbara's Kavli Institute for Theoretical Physics (KITP). We provided the geometric and topological analysis of these experiments, to explain the observed patterns and predict what patterns should be seen when experimental conditions are changed."
Kusner was the lone mathematician among four organizers of last summer's workshop on "Knotted Fields" at KITP, which led to this work. The workshop engaged about a dozen other mathematicians and about twice as many theoretical and experimental physicists in a month-long investigation of the interplay between low-dimensional topology and what physicists call "soft matter."
In their experiments, the physicists at UC Boulder showed that tiny topological particles injected into a liquid crystal medium behave in a manner consistent with established theorems in geometry and topology, Kusner says. The researchers say they have thus identified approaches for building new materials using topology.
UC Boulder's Smalyukh and colleagues set up the experiment by first creating colloids, solutions in which tiny particles are dispersed but not dissolved in a host medium, such as milk, paint and shaving cream. Specifically, they injected tiny, different-shaped particles into a liquid crystal, which behaves something like a liquid and a solid. Once injected into a liquid crystal, the particles behaved as predicted by topology.
Smalyukh says, "Our study shows that interaction between particles and molecular alignment in liquid crystals follows the predictions of topological theorems, making it possible to use these theorems in designing new composite materials with unique properties that cannot be encountered in nature or synthesized by chemists. These findings lay the groundwork for new applications in experimental studies of low-dimensional topology, with important potential ramifications for many branches of science and technology."
For example, he adds, these topological liquid crystal colloids could be used to upgrade current liquid crystal displays like those used in laptops and television screens, to allow them to interact with light in new, more energy efficient ways.

 

Liquid Crystals Light Way to Better Data Storage

 

June 24, 2010 — As cell phones and computers continue to shrink, many companies are seeking better ways to store hundreds of gigabytes of data in small, low-power devices.

 

A special type of liquid crystal, similar to those used in computer displays and televisions, offers a solution. Unlike CDs and DVDs, which store information only on their surface, lasers can encode data throughout a liquid crystal. Known as holographic storage, the technique makes it possible to pack much more information in a tiny space.
But attempts to use liquid crystals for data storage have had limited success. In order to reliably record and rewrite data, researchers must figure out a way to uniformly control the orientation of liquid crystal molecules. Currently, most liquid crystal technologies rely on physical or chemical manipulation, such as rubbing in one direction, to align molecules in a preferred direction.
In an important advance, scientists at the Tokyo Institute of Technology have created a stable, rewritable memory device that exploits a liquid crystal property called the "anchoring transition." The work is described in the latest issue of the Journal of Applied Physics, which is published by the American Institute of Physics (AIP).
Using either a laser beam or an electric field, the researchers can align rod-like liquid crystal molecules in a polymer. Their tests show that the liquid crystal created by the team can store data, be erased and used again.
"This is the first rewritable memory device utilizing anchoring transition," said Hideo Takezoe, who led the research. And because the device is bi-stable -- the liquid crystals retain their orientation in one of two directions -- it needs no power to keep images, adds Takezoe.

 

 

 

 

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