Marie Freebody, email@example.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
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