Krishnamurthy K S, Kumar Pramoda, Kumar M Vijay
Centre for Soft Matter Research, P.O. Box 1329, Jalahalli, Bangalore 560013, India.
Phys Rev E Stat Nonlin Soft Matter Phys. 2013 Feb;87(2):022504. doi: 10.1103/PhysRevE.87.022504. Epub 2013 Feb 15.
We report, for a rodlike nematic liquid crystal with small positive dielectric and conductivity anisotropies, and in the 90°-twisted configuration, low frequency (<2 Hz) square wave electric field generated Carr-Helfrich director modulation appearing transiently over a few seconds at each polarity reversal and vanishing almost completely under steady field conditions. Significantly, the instability is polarity sensitive, with the maximum distortion localized in the vicinity of the negative electrode, rather than in the midplane of the layer. This is revealed by the wave vector alternating in the two halves of the driving cycle between the alignment directions at the two substrates. Besides the Carr-Helfrich mechanism, quadrupolar flexoelectric polarization arising under electric field gradient is strongly indicated as being involved in the development of the transient periodic order. Similar transient instability is also observed in other nematic compounds with varying combinations of dielectric and conductivity anisotropies, showing its general nature. The study also deals with various characteristics of the electro-optic effect that emerge from the temporal variation of optical response for different driving voltages, frequencies, and temperatures.
我们报道，对于一种具有小的正介电各向异性和电导率各向异性的棒状向列型液晶，在90°扭曲配置下，低频（<2 Hz）方波电场产生的卡尔-赫尔弗里希指向矢调制在每次极性反转时会在几秒钟内短暂出现，并在稳定场条件下几乎完全消失。值得注意的是，这种不稳定性对极性敏感，最大畸变位于负极附近，而不是层的中平面。这通过驱动周期的两半中波矢在两个基板的取向方向之间交替得以揭示。除了卡尔-赫尔弗里希机制外，电场梯度下产生的四极挠曲电极化强烈表明参与了瞬态周期性有序的发展。在具有不同介电和电导率各向异性组合的其他向列型化合物中也观察到类似的瞬态不稳定性，表明其具有普遍性。该研究还探讨了不同驱动电压、频率和温度下光学响应随时间变化所产生的电光效应的各种特性。
