Study Of Negative Nematic Umbilic Defect

Ordered media are never perfect, but present deformations and even defects. In liquidcrystals, the order is defined by several parameters, mainly the local mean direction. Thereforemolecular director arrangement is a common method to describe liquid crystal defects.Predecessors have got a certain understanding and exposition on liquid crystal defects and theircauses. On the basis of these researches, a particular defect is studied in this paper as asupplement in this field.The research way of combining the theory and experiment is adoped in this paper. Thetheoretical derivation and calculation are based on the Fréedericksz transition, while for theexperiment part some characteristics of the umbilic defect are observed and analyzed by usingthe polarizing microscope.The study object is configured for the VAthin cell without friction under a constant voltage.Then the director on the substrate has an infinite degeneracy. For the static state, the componentof director (function of z) with different critical voltages is discussed firstly. After that, theparameters of this component increase to x, y, z. In order to deduce the definition of strength anddiscuss the distribution of director in umbilic defect with different elastic constants, theFréedericksz transition is analyzed again in the cylindrical coordinate system. For dynamic state,the Leslie-Ericksen dynamical equation is cited directly, and the flow case of umbilic defect isalso discussed when a shear force is applied.Polarizing microscope is one of the important tools to study liquid crystal texture, which isused to observe the VA cell under constant voltage, including the strength definition, the stripenumbers relationship, and the rotation characteristic of umbilic defect. Due to the limitation ofexperimental condition, instead of adding shear force, a higher voltage is applied to the liquid crystal cell to induce some wonderful phenomenons, which are needed to be explored andanalyzed furtherly.