Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment
Topological Microfluidics
by Anupam Sengupta
Date of Examination:2012-12-18
Date of issue:2013-02-22
Advisor:Dr. Christian Bahr
Referee:Prof. Jörg Enderlein
Referee:Prof. Dr. Stephan Herminghaus
Referee:Prof. Dr. Pawel Pieranski
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Description:Doctoral Thesis
Abstract
English
The doctoral thesis presented here is one of the first systematic attempts to unravel the wonderful world of liquid crystals within microfluidic confinements, typically channels with dimensions of tens of micrometers. The present work is based on experiments with a room-temperature nematic liquid crystal, 5CB, and its colloidal dispersions within microfluidic devices of rectangular cross-section, fabricated using standard techniques of soft lithography. To begin with, a combination of physical and chemical methods was employed to create well defined boundary conditions for investigating the flow experiments. The walls of the microchannels were functionalized to induce different kinds of surface anchoring of the 5CB molecules: degenerate planar, uniform planar, and homeotropic surface anchoring. Channels possessing composite anchoring conditions (hybrid) were additionally fabricated, e. g. homeotropic and uniform planar anchoring within the same channel. On filling the microchannels with 5CB in the isotropic phase, different equilibrium configurations of the nematic director resulted, as the sample cooled down to nematic phase. For a given surface anchoring, the equilibrium director configuration varied also with the channel aspect ratio. The static director field within the channel registered the initial conditions for the flow experiments. The static and dynamic experiments have been analyzed using a combination of polarization, and confocal fluorescence microscopy techniques, along with particle tracking method for measuring the flow speeds. Additionally, dual-focus fluorescence correlation spectroscopy is introduced as a generic velocimetry tool for liquid crystal flows. The flow of nematic liquid crystals is inherently complex due to the coupling between the flow and the nematic director. The presence of the four confining walls and the nature of surface anchoring on them complicate the flow-director interactions further. In microchannels possessing degenerate planar anchoring, four different flow-induced defect textures were identified with increasing Ericksen number: pi-walls, disclination lines pinned to the channel walls, disclination lines with one pinned and one freely suspended end, and disclination loops freely flowing in a chaotic manner. However, such textures and sequence of defects were not observed for flows within channels with homogeneous anchoring. Using experiments and numerical modeling the flow-director coupling was investigated within homeotropic microchannels. Complex non-Poiseuille multi-stream flow profiles emerged which provided a direct route to controlled shaping of the flow profile in a microfluidic channel. The dynamics have been characterized by the de Gennes characteristic shear-flow lengths e_1 and e_2 which, together with the channel's aspect ratio (width/depth), control the relative stability of the flow regimes. Additionally, by applying a local temperature gradient across the channel, the nematic flow could be steered in the transverse direction via mechanisms of viscosity anisotropy. The flow-director coupling was quantified through optical birefringence and in situ velocity measurements within a diverging microchannel. When a cylindrical obstacle was placed in the flow path, a reversible sequence of topological defects originated at the obstacle. The appearance of the topological structures has been analyzed on the basis of the flow-director interactions at different flow speeds. Using the dual-focus fluorescence correlation method, the velocity distribution within the defect structure was experimentally assessed. The flow of nematic 5CB within a microchannel with hybrid surface anchoring (combination of surfaces having uniform planar and homeotropic anchoring) generated and stabilized a topological defect line along the entire length of the microchannel. Colloid particles and small water droplets, the 'working horses' of common-style droplet-based microfluidics, were trapped at the disclination lines and consequently followed them through the microfluidic device. The topological defect line was utilized as a 'soft rail' whose position was controlled through easily accessible experimental parameters. Controlled threading of a defect line at a channel bifurcation and in situ switching of the defect guidance demonstrate the high potential of this technique, especially for the transport of a wide range of microfluidic cargo. The topological soft rail introduces a unique platform for targeted delivery of single particles, droplets, or clusters of such entities, paving the way to flexible micro-cargo concepts in microfluidic settings. Colloidal particles transported through the nematic matrix were further utilized to extract the information about the flow-induced local director field. The dependence of the particle orientation flowing through the ordered 5CB has been proposed as a route to stereo-selective transport of colloidal inclusions (with shape anisotropy) under appropriate boundary conditions. In addition, the interplay between the viscous and elastic interactions present in such systems has been utilized to derive the particle-disclination trapping force. A number of new questions evolved during the course of the research work. Suggestive experiments to address those questions, and a perspective view on the research of liquid crystal based microfluidics, are presented in the concluding parts of the dissertation.
Keywords: liquid crystal microfluidics; topological defects; colloids; surface functionalization; flow-shaping; guided transport; anisotropic fluids; non-Poiseuille behaviour; topological constraints