Investigation of flexoelectricity and ionic contamination in nematic liquid crystals

Abstract

The work presented is a study of nematic liquid crystal physical properties. One of the phenomenon investigated is the bistability or in other words, the ability of the director of the material to rest in different orientations as function of the geometry of the sidewall. The main core of this research is the study of exoelectric effect and ionic contamination within the nematic layer. This can be split into three approaches. In the first part, the exoelectric coefficients are determined from the distortion of a nematic liquid crystal in response to non uniform electric fields created using an interdigitated electrode architecture. The distortion was deduced from the fringes movement in a Mach-Zhender interferometer built especially for this investigation. In the second part, the transient capacitance technique is used to measure the permittivity response of two homogeneous nematic materials subjected to different electric signal waveforms. The results remain to be fitted in order to give an alternative method to deduce the exoelectric coefficients values and a better understanding of the effect of ionic contamination. The last part of the work relates to the determination of the exoelectricity coefficients and ionic contamination and their effects on the Fréedericksz transition in homogeneous nematic liquid crystal using magnetic fields. The Mach-Zhender interferometer was also used to investigate two other phenomena, the first one is electrophoresis and the second is the resonance mode of insects