Download or read book Nematic Liquid Crystal Sessile Droplets In Electric And Magnetic Fields written by Zoltán Karaszi and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nematic Liquid Crystal Sessile Droplets in Electric and Magnetic Fields Abstract of Dissertation Zoltán Karaszi Materials Science Graduate Program, Kent State University Sessile droplets with uniform director structure could be used as tunable optical lenses where the focal length could be controlled by light polarization, viewing angle, and magnetic or electric fields. To achieve that, one must understand the liquid crystal director structure in various external fields. In this dissertation, I presented detailed experimental studies. I summarized the theoretical description of the director structure of uniaxial nematic liquid crystals, such as the formation and dynamics of Néel wall type metastable inversion walls, either in magnetic or electric fields or magnetic and electric fields combined. Sessile nematic droplets allow for studying the combined effect of anchoring at solid and gas interfaces. The combination of various alignments at the two surfaces and external fields results in various director distribution schemes, ranging from a defect-free, almost homogeneous state to configurations with point-, line- and wall defects. We designed a polarizing optical microscope made of non-magnetic materials that could be placed between an electromagnet's poles. The design allowed us to study the effect of various combinations of electric and magnetic fields on nematic liquid crystal sessile droplets. Additionally, a long-range microscope was used to observe the side view of the LC drop. We also built another experimental setup that enabled us to measure the focal length in response to electric fields while rotating the sample between crossed polarizers. (1) Our main experimental findings can be summarized as follows. We showed that under low magnetic fields applied along the base plane of a sessile droplet with homeotropic alignment, the director structure becomes distorted and gradually leads to a defect wall that moves toward the periphery. We explained the director field's magnetic field dependence and the defect walls' formation and motion. We have shown that at a strong enough lateral magnetic field or even at a small field that makes more than 3° with respect to the base plane, the director can be uniformly aligned along the field without the presence of the defect wall. [1] (2) Replacing the magnetic field, an AC electric field was applied along the base plane of a nematic sessile droplet with positive dielectric anisotropy; we also found a rotation of the director toward the electric field and the formation of an inversion wall perpendicular to the applied field. While at low frequencies, the direction of the wall was stationary, just as observed in magnetic fields, above the Maxwell-Wagner frequency, it turned toward the external electric field. In both cases, the defect wall was also swept toward the periphery of the drop, where it eventually disappeared. The defect wall's rotation at high frequencies resulted from the antiparallel orientation of the effective moment vector and the electric field due to the lower dielectric constant and higher electric conductivity of the defect wall than of the rest of the liquid crystal droplet. An exponential time dependence could describe the time dependence of the displacement of the electric field-induced defect wall without any fitting parameter. That, combined with the threshold for director deformation, enabled us to determine both the bend elastic constant and the rotational viscosity using much less substance than existing techniques. Uniform electric field-induced generation, rotation, and linear movement of defect walls is a unique phenomenon in soft matters.[2] (3) Multidimensional solitons and their electric field-induced movement have recently been reported in achiral and chiral nematic liquid crystals. In the presence of competing magnetic AC electric fields on a sessile droplet with positive dielectric anisotropy, we found that the inversion wall induced by a horizontal magnetic field suffers buckling at sufficiently high electric voltage applied vertically. We characterized the time and field dependence of the buckled walls' shape and motion and proposed a physical mechanism to account for the behavior. We note that, due to their spontaneous propagation, the inversion wall can also be considered as a one-dimensional soliton, i.e., a spatially localized shape preserving traveling wave packet such as observed first by J.S. Russell in 1834 in the form of a one-dimensional water wave traveling along a canal near Edinburgh. [3] (4) We also studied the behavior of nematic liquid crystal sessile droplets with negative dielectric anisotropy. We observed several new director configurations depending on the specific combination of the magnetic and electric fields. For example, at high enough voltages applied across the droplet, the radial symmetry breaks, and a spiraling deformation of the Maltese cross appears near the central defect line. This can be attributed to the twist deformation in the vicinity of the central defect line that replaces the more costly bend deformation. Applying a magnetic field perpendicular to the vertical electric field, a twisted inversion wall formed together with a vertical central defect line. When the electric field was applied parallel to the base plane of the droplet, a homeotropic central region formed along the electric field. When this field was applied together with a magnetic field in the same direction, the homeotropic central region became perpendicular to the applied field. [4,5] (5) We measured the focal length of nematic sessile droplets with positive dielectric anisotropy as a function of electric fields applied along the base plane of the lenses. It was observed that the focal length decreases during increasing fields as the effective refractive index, which is inversely proportional to the focal length, increases from "no" to "ne". The focal length of NLC droplets could also be tuned by varying the polarization direction of a linear polarizer placed in front of the lenses. At the same time, a constant AC electric field was applied along the base plane. [6] References: [1] P. Salamon, Z. Karaszi, V. Kenderesi, Buka, and A. Jákli, Liquid Crystal Spherical Caps in Magnetic Fields, Phys. Rev. Res. 2, 023261 (2020). [2] Z. Karaszi, P. Salamon, Á. Buka, and A. Jákli, Lens Shape Liquid Crystals in Electric Fields, J. Mol. Liq. 334, 116085 (2021). [3] Z. Karaszi, M. Máthé, P. Salamon, Á. Buka, and A. Jákli, Electric Field Induced Buckling of Inversion Walls in Lens-Shape Liquid Crystal Droplets, J. Mol. Liq. 365, 120177-1-7 (2022). [4] Z. Karaszi, M. Máthé, P. Salamon, Á. Buka, and A. Jákli, Lens-Shaped Nematic Liquid Crystal Droplets with Negative Dielectric Anisotropy in Electric and Magnetic Fields, Liq. Cryst. DOI: 10.1080/02678292.2022.2134594, published online (October 24, 2022). [5] Ágnes Buka, Péter Salamon, Marcell Máthé, Zoltán Karaszi, and Antal Jákli, Liquid Crystals, 2023, Nematic liquid crystals in lens shape geometry. https://doi.org/10.1080/02678292.2023.2168307 [6] Z. Karaszi, K. Perera, and A. Jákli, Tuning the Focal Length of Sessile Nematic Droplets by Electric Fields, under preparation, (2023).