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

Abstract

ABSTRACTIn 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.