Generation Mechanism of Shear Yield Stress for Regular Defect Arrays in Water-in-Cholesteric Liquid Crystal Emulsions

Abstract

We have investigated the rheological behavior of the quadrilateral defect array formed in cholesteric emulsions, which are composed of water, surfactants, and cholesteric liquid crystals. The defect array is spontaneously formed after stopping the shear to the cholesteric emulsions, between glass plates with a homeotropic anchoring surface. In the formation process of the defect array, Newtonian flow behavior was detected immediately after termination of the preshear; however, the shear yield stress appeared and was increasing with rest time, corresponding to the texture change. The maximum yield stress, which is detected in the well-developed defect array, tends to be proportional to the number of point defects, among the samples with different thicknesses. The stress-strain relation shows the elastic response in the low shear strain region, which obeys Hooke's law; therefore we have discussed the pitch dependence of the elastic constant per one point defect in the defect array. We proposed a simple model to estimate the yield stress of the defect array quantitatively, by considering that the shear deformation induces the compression and elongation of the cholesteric helix. The estimated yield values were in approximate agreement with the experimental results, which means that the shear stress propagates in the defect array through the liquidcrystalline order of the cholesteric helix between neighboring point defects separately anchored on upper or lower glass surfaces.

Colloids in Cholesterics: Size-Dependent Defects and Non-Stokesian Microrheology

“…Our results can be illuminated by considering the bulk shear rheology of cholesterics undergoing permeation flow along the pitch direction. In this case, in any finite sample there is a regime of linear rheology, but the shear viscosity increases linearly with system size, diverging in the thermodynamic limit [18,19]. Crudely treating the colloid radius R as an effective sample size then gives α = 2, not far from the observed value (α ≈ 1.7).…”

Direct Observation of Anisotropic Interparticle Forces in Nematic Colloids with Optical Tweezers

“…Thus induced distortion of the liquid crystalline order assumes a dipolar property, which leads to the chain formation owing to the balance of long-range dipolar attractive and short-range defectmediated repulsive interactions. In other systems of LC emulsions, there are some attractive reports about spontaneously formed three-dimensional defect structures, which are composed of water droplets and nematic [2] or cholesteric fluids [4,10]. Liquid-crystal dispersions can be thus expected to show further new liquid-crystal structures and unique phenomena [11], owing to the interparticle and/or particle-LC interactions.…”

Assessing flow alignment of nematic liquid crystals through linear viscoelasticity

“…In most of these theoretical studies it was found that monodomain nematic liquid crystals are viscoelastic in a frequency region surrounding the director relaxation time, and that the response in the small frequency terminal region corresponds to pure viscous material. Obviously, introduction of stable defect lattices would predict elastic response in the terminal zone, as observed by Ramos et al [15] and by Yada et al [16] for cholesteric liquid crystals. In this paper we show that the relatively simple SAOF measurements are also a useful tool to determine flow alignment in liquid crystals.…”