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Evaluating the concentration of ions in liquid crystal cells: hidden factors and useful techniques
1  Department of Physics and Engineering Physics, Central Connecticut State University, New Britain, CT, USA

Published: 06 November 2020 by MDPI in The 2nd International Online Conference on Crystals session Liquid Crystals

Many of the liquid crystal devices are driven by electric fields. Ions, typically present in molecular liquid crystal materials in minute quantities, can compromise the performance of mesogenic materials (in the simplest case, through a well-known screening effect). Even highly purified liquid crystals can be contaminated with ions during their production and handling. Therefore, measurements of the concentration of ions became an important part of the material characterization of liquid crystals. Interestingly, even a brief analysis of existing publications can reveal a quite broad variability of the values of the concentration of ions measured by different research groups for the same liquid crystals. It reflects the complexity of ion generation mechanisms in liquid crystal materials and their dependence on numerous factors. In this paper, an overview of ion generation mechanisms in liquid crystals and modern ion measurement techniques is followed by the discussion of frequently overlooked factors affecting the measured values of the ion concentration. Ion-generating and ion-capturing properties of the alignment layers (or substrates) of liquid crystal cells are considered and used to evaluate a true concentration of ions in liquid crystals. In addition, practical recommendations aimed at improving the measurements of the ion density in liquid crystals are also discussed.

Keywords: liquid crystals; ions; electrical properties
Comments on this paper
Charles Rosenblatt
Alignment layer effects
This is a very nice work. But I am curious about the effects of patterning of the alignment layers on the ion capture and release process. Does introducing a surface topography have a significant effect on the ion concentration? I would suppose it would not, since the overall surface roughness is still rather small. But perhaps the author h8as some sense of this. Also, does a substrate patterned with topological defects have an effect, as the defects may serve as traps or sources for ions.
Yuriy Garbovskiy
Thank you so much for your thoughtful comments. In general, both surface roughness and topological defects can affect the ion capture and ion release process. These effects can be measurable if relatively thin cells are used to evaluate the concentration of ions. Assuming we are dealing with such “thin” cells, the total surface area of substrates with zero and non-zero roughness can differ by a factor of pi/2, a measurable difference. For a given substrate, the measured outcome will also depend on the type of liquid crystal materials used in experiments (low resistivity or high resistivity). A series of electrical measurements is required to observe the proposed effects.

Charles Rosenblatt
Thank you
Dear Dr. Garbovskiy,

Thank you for the clarification. I had expected an effect, but it's perhaps larger than I would have guessed.

Best wishes,
Yuriy Garbovskiy
Dear Dr. Rosenblatt:
Thank you for your forward-looking questions. I hope in the nearest future my research group will collect enough experimental results to give you a more specific answer.