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Electrically driven dynamic effects in nematic liquid crystals

Li, Bingxiang
http://orcid.org/0000-0003-4727-1572
http://rave.ohiolink.edu/etdc/view?acc_num=kent156384124639024

Abstract Details

2019, PHD, Kent State University, College of Arts and Sciences / Chemical Physics.
Electric field causes a plethora of physical effects in liquid crystals (LCs) such as dielectric and flexoelectric realignment of molecular orientation, electrohydrodynamics and electroosmosis, that are rooted in fundamentally new mechanisms of response mediated by anisotropy of the LC properties, such as permittivity and conductivity. These effects are of prime practical importance in applications such as LC-based electrooptical devices. Usually, the electro-optic response is caused by reorientation of the director that specifies the average molecular orientation. This response explores dielectric anisotropy of the LC and is relatively slow, on the order of millisecond. The first part of the thesis explore electro-optical response that occurs at much shorter time scales of nanoseconds and microseconds. The main distinctive feature of the nanosecond response is that the electric field does not cause director realignment but modifies the scalar order parameter. We explore a new approach to enhance the amplitude of field-induced retardance by implementing multiple reflections of light in the cell with a nanosecond response. A special material with a giant dielectric anisotropy (+200) is used to reduce the operating voltage of nanosecond electric modification of the order parameter. The study also determines the temperature and electric dependencies of electro-optic responses in both isotropic and nematic phases of LCs. In the isotropic phase, the field induces a uniaxial order parameter, which is known as the Kerr effect. We predict and observe that at a high electric field, the Kerr effect features a nonlinear dependence of dielectric permittivity on the field-induced orientational order parameter. Finally, a microsecond electro-optical response that does not require the use of polarizers was demonstrated for a dual-frequency LC doped with a dichroic dye. Electric field is also known to cause periodic modulations of the director in various regimes of electroconvection. These director modulations form periodic patterns that occupy the entire electrode areas. The second part of the thesis demonstrates that the electric field can produce spatially confined particle-like propagating solitary waves of the oscillating molecular director. These waves are localized along the direction of propagation and along the two orthogonal directions, thus representing particle-like solitons that are called director bullets. The bullets are topologically unprotected solitons that preserve spatially confined shapes during their propagation and survive collisions. The bullets lack the fore-aft symmetry along the direction of their propagation. At high frequencies in the so-called dielectric regime, these director bullets move with very high speed perpendicularly to the electric field and to the initial alignment direction. At low frequencies in the conductivity regime, the bullets propagates either perpendicularly or parallel to the initial orientation of the director. At low frequency, the solitons of different symmetry can be transformed one into another by changing the amplitude and/or frequency of the driving electric field. The thesis establishes the dynamics of the director field within the solitons, the phase diagram of soliton stability and the dependence of soliton velocity on parameters such as the amplitude and frequency of the driving voltage.
Oleg Lavrentovich (Committee Chair)
Sergij Shiyanovskii (Committee Member)
Deng-Ke Yang (Committee Member)
Han-Bin Mao (Committee Member)
Michael Fisch (Committee Member)
175 p.
Condensed Matter Physics; Optics; Physics
Liquid crystals; electric field, order parameter; dielectric anisotropy; soliton; electro-optic switching; fast switching

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Citations

  • Li, B. (2019). Electrically driven dynamic effects in nematic liquid crystals [Doctoral dissertation, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent156384124639024

    APA Style (7th edition)

  • Li, Bingxiang. Electrically driven dynamic effects in nematic liquid crystals. 2019. Kent State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent156384124639024.

    MLA Style (8th edition)

  • Li, Bingxiang. "Electrically driven dynamic effects in nematic liquid crystals." Doctoral dissertation, Kent State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=kent156384124639024

    Chicago Manual of Style (17th edition)

kent156384124639024
872
© 2019, some rights reserved.Creative Commons License Electrically driven dynamic effects in nematic liquid crystals by Bingxiang Li is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by Kent State University and OhioLINK.