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An experimental study of bubbles and droplets rising in a nematic liquid crystal Khullar, Siddharth

Abstract

Liquid crystals are nature's beautiful examples of complex materials which are fundamentally fascinating. Their unusual properties have intrigued researchers from a wide variety fields including biologists, engineers, and even cosmologists. This thesis focuses on the dynamics of topological defects occurring near micro-droplets and micro-bubbles as they rise through an aligned nematic liquid crystal. The experiments were conducted in a fabricated flow-cell, and the observations were made using polarized light microscopy with the help of a motion control system. The results settle a controversy in the literature regarding the effect of hydrodynamic flow on the motion of defects by providing direct evidence of downstream convection of a Saturn ring defect and its transformation to a hyperbolic point defect. The point defect is convected further in the wake of the drop or bubble as the rising velocity increases. In equilibrium, both defect configurations may persist for long times. But the point defect sometimes spontaneously opens into a Saturn ring, indicating the latter as the globally stable configuration for the conditions used. A quantitative analysis of the rise velocities versus the location of defects yields graphs which are consistent with recent theoretical predictions. Besides these, we also observe interesting multiple drop and bubble interactions leading to the phenomenon of self-assembly and distorted defect structures.

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