Ions and size effects in nanoparticle/liquid crystal colloids sandwiched between two substrates

The concentration of mobile ions in liquid crystals have been identified as a physical quantity that is very critical when it comes to several electro-optical applications. For the case of display applications, mobile ions in liquid crystals are undesirable since they result in effects such as image flickering, slow response, image sticking, and reduced voltage-holding ratio. Unfortunately, even highly purified liquid crystals are susceptible to uncontrolled ion contamination. Therefore, display applications call for high resistivity liquid crystals. It is therefore important to develop new methods of mitigating the uncontrolled ion contamination of liquid crystals in electro-optical gadgets.

Mobile ions in liquid crystals are not always undesirable. There are a number of research works reporting applications of liquid crystals dependent on ions. Such applications include, liquid crystal wave front correctors, optical switches implementing dynamic light scattering, liquid crystal generators of low frequency oscillations, and electrically tuned ionic modification of surface anchoring. These applications will benefit greatly from development of new methods of enriching liquid crystals with ions.

Recent advances in liquid crystal nanoscience have shown that nanodopants such as carbon nanotubes, metal, dielectric, magnetic and ferroelectric nanoparticles dispersed in liquid crystals can alter mobile ions concentration through desorption/adsorption processes. There has also been an attempt to describe an array of existing experimental results in relation to the Langmuir adsorption formalism. However, some of these studies have not focused on interactions of ions with alignment layers of the liquid crystal cell.

Based on the research on the design and attributes of the alignment layers, it has been found that the adsorption of ions on the alignment layers of the liquid crystal cell could lead to the dependence of the electro-physical attributes of liquid crystals on the thickness of the cell.

Yuriy Garbovskiy at University of Colorado Colorado Springs explored the combined effects of alignment layers and nanoparticles on the concentration of ions in liquid crystal nano-colloids. His main objective was to consider the combined effects of alignment layers of the liquid crystal cell and nanoparticles on the concentration of ions in liquid crystal nanocolloids assuming two forms of fully ionized species. A very important aspect of his work was the consideration of possible effects associated with the ionic contamination of both nanoparticles and alignment layers of the liquid crystal cell. This ionic contamination was quantified by the dimensionless contamination factor thus enabling a quantitative analysis of ion capturing / ion releasing process in the liquid crystal cell utilizing contaminated nanomaterials and alignment layers. His research work is published in journal, Chemical Physics Letters.

The consideration of the adsorption/desorption process on the alignment layers of the liquid crystal cell led to a series of consequences. The author found that the concentration of mobile ions in liquid crystals was dictated by the thickness of the cell. The effect of size was more pronounced when thin cells were applied and became negligible when thick cells were applied. Liquid crystals characterized by single type fully ionized species contaminants exhibited monotonous dependence of the concentration of mobile ions on the cell thickness.

The author observed that the dispersion of nanoparticles in liquid crystals changed the form of behavior of the total concentration of ions as a function of the cell thickness (Figure 1). Liquid crystals nanocolloids characterized by two forms of fully ionized species also exhibited non-monotonous and monotonous dependence of the total concentration of ions on the nanoparticle concentration

Figure 1. The dependence of the total concentration of mobile ions in liquid crystals doped with nanoparticles on the cell thickness calculated at several values of the weight concentration of nanoparticles. The ionic contamination of the cell substrates  is characterized by the dimensionless contamination factor, . (Image: Dr. Yuriy Garbovskiy, University of Colorado, Colorado Springs)

The adsorption of ions on the alignment layers altered the type of dependence. The use of thin cell could change the non-monotonous behavior to the monotonous one (Figure 2).

Figure 2. The dependence of the total concentration of mobile ions in liquid crystals doped with contaminated nanoparticles on the weight concentration of these nanoparticles calculated at several values of the cell thickness. The ionic contamination of nanoparticles is characterized by the dimensionless contamination factor, . (Image: Dr. Yuriy Garbovskiy, University of Colorado, Colorado Springs)

Yuriy Garbovskiy results demonstrate that the uncontrolled contamination of alignment layers and nanoparticles lead to predictable behavior that can be modelled and analyzed.