Nematic Liquid Crystals and Nematic Colloids in Microfluidic Environment

Abstract

Where the mind is without fear and the head is held high;Where knowledge is free;Where the world has not been broken up into fragments by narrow domestic walls;Where words come out from the depth of truth;Where tireless striving stretches its arms towards perfection;Where the clear stream of reason has not lost its way into the dreary desert sand of dead habit;Where the mind is led forward by thee into ever-widening thought and actionInto that heaven of freedom, my Father, let my country awake.Rabindranath Tagore (1861Tagore ( -1941 To my parents, my brother, and all the sacrifices they made for me AbstractThe 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 roomtemperature 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 i ii 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: π-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...

Solution landscapes in nematic microfluidics

“…with k ∈ Z arbitrary. However, the boundary conditions (15b.c) are not satisfied by (16) and hence we expect to find boundary layers near z = −1, 0 and 1, in order to match the boundary conditions. The solution in the two outer regions −1 < z < 0 and 0 < z < 1 are given by (16) for any two particular integer values of k, say k 1 and k 2 .…”

“…We numerically solve the three boundary layer problems (17), (18) and (19), using Chebfun, matching to the constant values in (16). For our particular choice of dimensionless nematic viscosities α 2 and α 3 , all values of σ ± k (defined in (16)) are close to some odd multiple of π 2 , and thus the inner director field is largely flow-aligned and is rotated kπ times with respect to the flow direction. There are multiple choices for the outer solutions, σ ± k 1 and σ ± k 2 , for −1 < z < 0 and 0 < z < 1 respectively, yielding different asymptotic approximations.…”

“…There are multiple choices for the outer solutions, σ ± k 1 and σ ± k 2 , for −1 < z < 0 and 0 < z < 1 respectively, yielding different asymptotic approximations. In Figures 6(a) and 7(a) we compare the asymptotic approximations (16), (17), (18) and (19) with numerical solutions of the full system (4) for large values of G. The two cases are labeled as θ * a 0 ,G and θ * a 1 ,G respectively, depending on the initial condition used to generate them. The values of σ ± k 1 and σ ± k 2 are extracted from the numerical solution and used in the asymptotic approximation (16)-(19) (these values are different for solutions θ * a 0 ,G and θ * a 1 ,G ).…”

Thermal characterisation of thermotropic nematic liquid-crystalline elastomers

“…The order parameter vs. temperature curves in Figure 9 also lack the characteristic discontinuity or sharp change in slope as the sample enters the isotropic phase. [1,[39][40][41] The lack of a sharp discontinuity at the nematic-to-isotropic transition as observed in conventional liquid crystals has received attention in recent work relying on nuclear magnetic resonance (NMR) measurements. [42][43][44][45][46] This work, on thermotropic liquid crystal elastomers, used NMR [42][43][44][45][46] to study residual orientational ordering that was present following departure from the nematic mesophase upon heating.…”

“…For a lowmolecular-weight nematic liquid crystal, the nematicto-isotropic transition would be sharper. [1,37,[39][40][41] Transition breadth reflects the local variation in bonding environment. If most of the mesogens are embedded in a similar local environment, the transition will be sharper.…”