Flow of a nematogen past a cylindrical micro-pillar

Anupam Sengupta,Stephan Herminghaus,Christoph Pieper,Jörg Enderlein,Christian Bahr

doi:10.1039/c2sm27337c

Abstract

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.

Highlights

The advent of micro uidics[1] has opened up possibilities to study ows within micron-sized con nements and past minute obstacles
Studies have shown that such a hydrodynamic regime may not be strict when a non-Newtonian uid e.g. polymer solution[3] is owed, or in the presence of appropriate morphological patterning of the microchannel.[4]
The boundary conditions existing on the surfaces that con ne the liquid crystal have a crucial contribution in determining the static equilibrium of the director.[9]

Summary

Introduction

The advent of micro uidics[1] has opened up possibilities to study ows within micron-sized con nements and past minute obstacles. Studies have shown that such a hydrodynamic regime may not be strict when a non-Newtonian uid e.g. polymer solution[3] is owed, or in the presence of appropriate morphological patterning of the microchannel.[4] Recent investigations by the authors have further revealed that the use of an anisotropic uid (liquid crystal) as the owing matrix can induce effects distinct from those observed for an isotropic uid.[5,6] The average orientation of the liquid crystal molecules, denoted by a vector ~n and referred to as the director,[7] plays a decisive role in determining the rheological properties of the anisotropic uid.[8] the boundary conditions existing on the surfaces that con ne the liquid crystal have a crucial contribution in determining the static equilibrium of the director.[9] The dynamics of liquid crystal ows, which are

Methods

Results

Conclusion