Liquid crystal materials are subject to contamination during the manufacturing process and generally degrade over time to generate ions. Nanoparticles act as effective ion capturing agents for purifying liquid crystal materials.
Liquid crystals are used in modern devices (laptops, tablets, smartphones) which are typically driven by an electric field. For ensuring the specific performance of these devices, liquid crystals should be extremely clean and free from any ionic impurity.
Presence of ions can compromise the performance in liquid crystal displays (LCDs). It is the mobile ions that are responsible for a slow response, image sticking, and image flickering. Therefore ions in liquid crystals are not desirable.
Purification of liquid crystals is of key significance in liquid crystal technology and nanotechnology. Nanomaterials dispersed in liquid crystals can trap and immobilize the ions thus providing a permanent cleansing of the liquid crystal from ions.
Numerous research groups around the world verified the concept of ion capturing effect in liquid crystals with nanoparticles. (Crystals, “Nano-Objects and Ions in Liquid Crystals: Ion Trapping Effect and Related Phenomena”).
“A fascinating and essential query is what happens to the ion trapping effect if temperature changes. Does the capability of nanomaterials continue to be same at low and high temperatures?”
Yuriy Garbovskiy, a researcher on the UCCS BioFrontiers Middle & Division of Physics, College of Colorado, tells Nanowerk, wonders,
“ For instance, on a sunny day, you determined to take your laptop, go to the park and work on your task there. Will ions in liquid crystals and their sensitivity to temperature spoil your day?”
Recently, a paper published in Liquid Crystals attempts to answer some these queries by modeling the temperature dependence of the concentration of mobile ions in liquid crystals doped with nanoparticles.
The keynote of this work is the use of Langmuir adsorption isotherm in conjugation with the Arrhenius equation and the measurement of the contamination of nanoparticles by the aid of dimensionless contamination factor.
Garbovskiy explains “Usually, ionic species in thermotropic liquid crystals are approximately fully ionized”.
“Therefore, the concentration of mobile ions practically does not depend on the temperature. In contrast, the same liquid crystal doped with nanoparticles shows totally different behavior. The concentration of the mobile ions exhibits a strong dependence on the temperature.”
Two Important Aspects
He revealed two important aspects with both 100% pure nanoparticles and nanoparticles contaminated with ions before their dispersion in the liquid crystal.
Systems with 100% pure nanoparticles dispersed in liquid crystals show a decrease in the concentration of the mobile ion. This ion capturing effect results in the permanent purification of liquid crystals from ions. However, this effect shows a dependence on temperature.
Contaminated nanoparticles dispersed in liquid crystals show the temperature-driven switching between the purification and contamination regimes enabling a thermal control of ions.
A purification regime is observed when the temperature of liquid crystals T is below Tc . The contamination regime holds true above this temperature (T > Tc).
“These outcomes add to our understanding of physics of liquid crystals doped with nanoparticles,” notes Garbovskiy.
The temperature dependence effects should be studied by R&D engineers using liquid crystal/nanoparticles colloids for their devices. For instance, the running temperature should be lower than the temperature Tc to assure that nanoparticles act as ion capturing agents.
Devices (optical switches) exist which make use of light scattering and thereby rely on the ions in liquid crystals. In such devices, ions in the liquid crystal are not harmful. This type of application takes advantage of the contaminated nanoparticles and temperatures (T > Tc).
Since it can be employed for various display and non-display applications of liquid crystals, nanoparticle-enabled thermal control of ions in liquid crystals establishes itself as an important phenomena.