(140d) Strain-Induced Topological Defects and Configurational Transitions in Liquid Crystals

In this work we explore the effect of geometrical confinement on the orientational ordering and transition of topological defects in nematic and smectic liquid crystals. The geometrical confinement is induced by uniaxial and biaxial stretching of a polymer dispersed liquid crystal (PDLC) film, with homeotopic anchoring. Our results show that upon uniaxial stretching, the nematic liquid crystal (LC) in the prolate cavity of a stretched polymer film retains a radial configuration, and the defect position at the droplet center remains unchanged. In the radial smectic LC droplet, however, the topological defect undergoes a transition from a point to a line, whose length is a function of the aspect ratio. The formation of this topological defect can be attributed to the misorientation and dilation of the smectic layers. Upon biaxial stretching, liquid crystal droplets become increasingly oblate. In the oblate smectic LC droplet, a layering configuration undergoes a transition from radial to parallel ordering, starting from the central part of the droplet. On the other hand, in the oblate nematic LC droplet, a topological distortion competes with homeotropic alignment and radial ordering, causing a configurational director twist. At a particular strain level, the LC molecules in the twisted oblate align normal to the respective stretch directions. Landau-de Gennes simulations are used to gain additional insights into the morphological changes of the nematic LC under biaxial stretching. Consistent with our experimental results, the continuum simulations capture the biaxial stretch-induced twist configuration, and demonstrate a uniform configuration when the mechanical strain exceeds 20%.