Evaluation of applied methods for the determination of viscoelastic effects in chiral liquid crystals

The material characteristics of liquid crystals play a crucial role in shaping the structure, order, and properties of a particular mesophase and its adjacent phases. This is vital for the development of high-performance optoelectronic devices such as liquid crystal displays, light modulators, apertures, and filters. While the elasticity and viscosity constants are seldom measured, they are essential for evaluating a material's applicability. Leveraging the ferroelectric and antiferroelectric properties of smectics enhances various parameters of information visualization devices, including grayscale, switching speed, contrast, and information content [1, 2].

The newly established method for calculating the absolute values of the coefficients related to the linear and quadratic electro-optical effects in low electric fields enables the assessment of elasticity constants (derived from the depth of light modulation) and rotational viscosity (based on frequency dependence), all while maintaining laminar flow and low strain conditions. Additionally, the type and characteristics of the phase transformations that occur in the material being examined can significantly impact both the magnitude of the determined coefficients and their dependence on temperature [3].

This study focuses on reviewing the methods employed to assess viscoelastic effects in ferroelectric and antiferroelectric chiral liquid crystals, their mixtures, composite materials, and even dielectric systems. The goal is to establish a universal method that permits the application of relatively low electric fields. For chiral liquid crystals with ferroelectric and antiferroelectric phases, as well as their subphases, the following principle holds: adherence to Hooke’s law (for elastic coefficients) and maintenance of laminar flow (for viscosity coefficients) [4].

References

[1] D. Dardas, Phase Transitions, 89 (4), 368-375 (2016)

[2] D. Dardas, Rheological Acta, 58, 193-201 (2019)

[3] D. Dardas, S. Lalik , Z. Nowacka, T. Yevchenko, M. Marzec, Crystals, 13, 164 (2023)

[4] D. Dardas, Materials, 17(16), 3993 (2024)

Acknowledgments: The work was supported by the Polish National Science Center, Project No. 2019/03/X/ST3/01516.