Prof. Deng-Ke Yang from Department of Physics and the Liquid Crystal Institute at Kent State University was invited to give an online lecture, titled “Flexoelectric Effect of Liquid Crystals and Its Applications” On May 20th.
Nematic liquid crystals, consisting of elongated molecules, in uniformly oriented state do not exhibit spontaneous electric polarization and are dielectric media. They can be reoriented by externally applied voltage under dielectric interaction and are not sensitive to the polarity of the voltage. Nematic liquid crystals of specially shaped molecules in special confinements or configurations exhibit, however, spontaneous polarization, a phenomenon known as flexoelectricity. They can also be reoriented by applied voltage under flexoelectric interaction and is sensitive to the polarity of the voltage. The flexoelectric effect can be made use to develop many devices, which are impossible under dielectric interaction.
Professor Yang introduced the application scenarios of the liquid crystal flexoelectric effect from three aspects. The first is the construction of liquid crystal gratings based on the liquid crystal flexoelectric effect. The liquid crystal grating based on the flexoelectric effect has high transmittance (~90%), with the grating period adjustable by the electric field. The grating is almost independent of the angle of the incident light, and does not require polarizers and additional electrode strips. They can be applied to diffraction optics, laser display, beam steering, switchable holograms and so on. The second aspect is to use the flexoelectric effect for waveguide display fabrication. Under the action of low frequency and low amplitude AC voltage, the liquid crystal molecules will follow the electric field movement while maintaining a uniform periodic structure and are transparent; if high frequency or high amplitude voltage is applied, the liquid crystal molecules can't follow the electric field, and has a dielectric effect, so a multidomain structure will be formed and light scattering will occur, which is non-transparent. Finally, Professor Yang explained how to design a dual-mode smart window based on the flexoelectric effect. The smart window designed using liquid crystals with flexoelectric effect and dichroic dye molecules. In this case, they can not only control radiant energy flow but also control privacy.
Prof. Yang joined Kent State University in 1989. Professor Deng-Ke Yang's main research interests are cholesteric liquid crystal, liquid crystal/polymer composites and electro-optic devices. He is a world-famous scientist in liquid crystal physics and devices, and one of the founders of polymer stabilized liquid crystal materials and bistable liquid crystal display materials and devices. Professor Yang has been engaged in the research of liquid crystal science and application for nearly 30 years, and is recognized as an influential scholar in this field. He received the Society for Information Display’s (SID) Special Recognition Award in 2005. He was elected a fellow of the SID in 2007. He received SID’s Slottow-Owaki Prize in 2017 for his outstanding contributions to the education and training of students and professionals in the field of liquid-crystal displays.