Accurate measurement of the twist elastic constant of liquid crystal by using capacitance method

ABSTRACT

In this study, the twist elastic constant (k₂₂) of liquid crystals (LCs) was accurately measured using capacitance method. The constant can be obtained on the basis of accurate measurement of other LC parameters, such as parallel and vertical dielectric constants (ε_// and ε_⊥), splay and bend elastic constants (k₁₁ and k₃₃), and rotational viscosity coefficient (γ₁). First, by using dual-cell capacitance method and an LC cell capacitance model to measure ε_// and ε_⊥, k₁₁ and k₃₃ can be obtained from the threshold voltage determined from the voltage–capacitance curve of the parallel-aligned nematic LC layer and the comparison between the experimental and theoretical results based on the Frank elastic theory, respectively. In addition, γ₁ can be obtained from the measurement of the dynamic response in the parallel-aligned nematic cell. Finally, k₂₂ can be accurately determined using the threshold voltage of the twisted nematic LC cell. By adopting the above method, the measured k₂₂ for LC E7 was 6.7 × 10⁻¹² N. The proposed method is more rigorous and yields a more accurate measurement result than the other methods reported in the literature.

Graphical Abstract

KEYWORDS:

• Twist elastic constant
• capacitance method
• threshold voltage
• twisted nematic

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [grant number 11374087], [grant number 11504080]; Natural Science Foundation of Hebei Province [grant number A2014202123], [grant number A2017202004]; Research Project of Hebei Education Department of China [grant number QN2014130], [grant number QN2015260]; Key Subject Construction Project of Hebei Province University. There is no fund number for Key Subject Construction Project of Hebei Province University.