Overlooked Ionic Phenomena Affecting the Electrical Conductivity of Liquid Crystals

Abstract

Liquid crystal devices, such as displays, various tunable optical components, and sensors, are becoming increasingly ubiquitous. Basic physical properties of liquid crystal materials can be controlled by external physical fields, thus making liquid crystal devices dynamically reconfigurable. The tunability of liquid crystals offers exciting opportunities for the development of new applications, including advanced electronic and photonic devices, by merging the concepts of flat optics, tunable metasurfaces, nanoplasmonics, and soft matter biophotonics. As a rule, the tunability of liquid crystals is achieved by applying an electric field. This field reorients liquid crystals and changes their physical properties. Ions, typically present in liquid crystals in minute quantities, can alter the reorientation of liquid crystals through the well-known screening effect. Because the electrical conductivity of thermotropic liquid crystals is normally caused by ions, an understanding of ion generation processes in liquid crystals is of utmost importance to existing and emerging technologies relying on such materials. That is why measuring of electrical conductivity of liquid crystals is a standard part of their material characterization. Measuring the electrical conductivity of liquid crystals is a very delicate process. In this paper, we discuss overlooked ionic phenomena caused by interactions of ions with substrates of the liquid crystal cells. These interactions affect the measured values of the DC electrical conductivity of liquid crystals and make them dependent on the cell thickness.

Steady-State and Transient Electrical Properties of Liquid Crystal Cells

“…These ionic processes will also continue until a steady state is reached. In our recent paper [33] we modeled DC conductivity of liquid crystals cells as a function of the cell thickness assuming a steady state. Because the present paper is a logical continuation of paper [33], we use similar physical parameters listed in Table 1.…”

“…where 𝜈𝜈 𝑆𝑆𝑆𝑆 is the contamination factor of substrates, and 𝑑𝑑 is the cell thickness [42,43]. Additional details can be found in papers [33,42,43]. It should be noted that a perfect alignment of liquid crystal samples is assumed (either planar or homeotropic).…”

“…As a rule, electrical measurements of thermotropic liquid crystals are carried out using sandwich-like cells of a fixed thickness. At the same time, interactions between ions and substrates of a liquid crystal cell can result in dependence of the measured electrical conductivity on the cell thickness [32,33]. Even though the importance to consider interactions between ions and substrates of a liquid crystal cell for a proper interpretation of electrical measurements was emphasized many times [28 and references therein], only a very limited number of experimental reports can be found [34,35,36].…”

“…Experimental results reported in works [34][35][36] unambiguously point to the necessity to consider the dependence of the measured electrical conductivity of thermotropic liquid crystals on the cell thickness. A general analysis of such dependence performed recently shows a very rich behavior with the possibility of both monotonous and non-monotonous dependence of DC electrical conductivity on the cell thickness [33]. Both experiments [34][35][36] and modeling [33] assume a steady state.…”

“…A general analysis of such dependence performed recently shows a very rich behavior with the possibility of both monotonous and non-monotonous dependence of DC electrical conductivity on the cell thickness [33]. Both experiments [34][35][36] and modeling [33] assume a steady state. The consideration of time dependence of DC electrical conductivity can reveal transient ionic processes in liquid crystal cells caused by interactions between ions and substrates of the cell.…”