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Liquid crystals: Size and shape matter

Photonics Spectra
May 2010
Marie Freebody, marie.freebody@photonics.com

COLLEGE STATION, Texas – When it comes to liquid crystals, scientists at Texas A&M University have discovered that size and shape really matter. Dr. Zhengdong Cheng, an assistant professor of chemical engineering, and his colleagues oriented disk-shaped molecules of liquid crystals into distinct and separate layers for the first time. The achievement could result in more effective industrial sealants, improved food packaging and even enhanced electronic displays.


Dr. Zhengdong Cheng, left, discusses the self-assembling of microdisks with graduate student Andres F. Mejia. Images courtesy of Zhengdong Cheng.


The molecules used in today’s electronic displays are rod-shaped one-dimensional objects, whereas their disk-shaped cousins are two-dimensional and behave very differently. Until now, disk-shaped molecules had never been observed in the form of layers – known as the smectic phase.

It is the discotic smectic phase – several layers of disks stacked together – that Cheng believes will make a better sealant. To illustrate his point, he said that covering a roof with large-size tiles instead of rod-shaped ones provides better coverage.

“If the gaps between tiles in different layers are located in different positions, then it will take longer for water droplets to find ways to penetrate through the layers,” he said. “The structure of the discotic smectic phase is similar to the structure of several layers of roof tiles. It offers good barrier properties for paints and food packaging.”

The discovery also could be expanded into the field of fuel-cell technology, preventing the problematic methanol crossover through a polymer electrolyte membrane in fuel cells. These areas – as well as many more that employ liquid crystal technology – stand to benefit from the new finding. For example, liquid crystals are found in soaps and detergents as well as in the proteins and cell membranes within the human body.


Graduate student Andres F. Mejia, at left, and Shengmei Ye use the UV-VIS spectrometer to investigate the disk suspensions.


Cheng further speculates that future LCD TVs, cell phones and portable gaming devices might boast improved visual properties while being more energy-efficient. This is thanks to the disk-shaped liquid crystal molecules’ being easier to manipulate and more sensitive to electric fields than are the rod-shaped liquid crystal molecules currently used in displays.

The trick to manipulating disk-shaped liquid crystal molecules into layers seems to come down to controlling three main molecular properties: thickness, aspect ratio and size. The Texas team found that the molecules had to have identical thickness, a large aspect ratio and polydispersity in size; i.e., a broad size range because uniform-size disks tend to form columnar structures.

In Cheng’s experiment, which appeared in the October 2009 issue of Physical Review E, each layer is composed of many inorganic crystals with an identical thickness of 2.68 nm and a diameter of approximately 2000 nm. The disks (also referred to as platelets because of their large size-to-thickness ratio) are created by exfoliating crystals of zirconium phosphate into single layers using a positively charged chemical.


Graduate student Andres F. Mejia, left, and Shengmei Ye discuss nanoparticle synthesis.


The platelet suspensions are left to self-assemble into various phases depending upon their concentrations. “In other words, each individual platelet moves around due to collisions with water molecules and interacts with neighboring platelets,” Cheng said. “Given enough time, the sample will find its thermodynamic ground state.”

Ground state normally finds the disks aligning into columnlike structures; however, in Cheng’s experiment, the disks behaved in an atypical manner, assembling themselves into separate layers.

The next step for Cheng and his collaborators, Dr. Hung-Jue Sue from the department of mechanical engineering at Texas A&M, and Dr. Yuri Martinez-Raton from Universidad Carlos III de Madrid and Enrique Velasco from Universidad Autónoma de Madrid, both in Spain, is to mass-produce the disks.

“We seek industry collaboration to test ideas for practical applications, particularly for photonic applications,” Cheng said. “We are testing the response of these disks to electric and magnetic fields, and we hope to develop the next generation of displays using this new discotic liquid crystal phase.”


GLOSSARY
smectic phase
A form of liquid crystal in which flow does not take place in the usual manner. When examined with polarized light, the substance often forms drops that display a series of fine lines. X-ray diffraction patterns can be obtained only in a single direction.  
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