Flow of a nematogen past a cylindrical micro-pillar

We study the flow of a nematic liquid crystal past a micron-sized cylindrical pillar within a microfluidic confinement of a rectangular cross-section. The liquid crystal molecules are anchored perpendicularly (homeotropic anchoring) to the surface of the pillar and the channel walls. Flow past the cylindrical obstacle generated topological defect structures whose nature, dimensions and morphology varied with the flow velocity and channel dimensions. On increasing the flow speed, we observed sequential evolution of a semi-integer loop, which transformed into an integer hedgehog defect, and finally equilibrated to an extended defect wall. On stopping the flow, the topological defect states reversed its sequence of appearance. Additionally, we introduce dual-focus fluorescence correlation spectroscopy as a general velocimetry technique for microfluidics of liquid crystal systems - with or without topological defect structures.