Prachi Thareja

Shear-induced phase separation (SIPS) with shear banding in solutions of cationic surfactant and salt

The rheological behavior and microstructure of branched, cationic wormlike micellar (WLM) solutions of 40 mM erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) are studied as a function of added salt (sodium salicylate) concentration, temperature, and shear rate via Rheosmall-angle light scattering (Rheo-SALS). These WLM solutions exhibit shear-enhanced concentration fluctuations leading to shear-induced phase separation (SIPS), manifested as visual turbidity under shear and the appearance of a characteristic “butterfly” scattering pattern in Rheo-SALS experiments. Flow kinematics measurements in a Couette geometry are used to determine the relationship between SIPS and shear banding, i.e., the splitting of the flow into shear bands with different local shear rates. Modeling using the Giesekus constitutive equation aids in discrimination between banding and nonbanding solutions. The combination of Rheo-SALS, dynamic rheology, velocimetry, and constitutive equation modeling allows detailed exploration of the relationship between SIPS, shear banding, fluid microstructure, and the equilibrium phase behavior.The rheological behavior and microstructure of branched, cationic wormlike micellar (WLM) solutions of 40 mM erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) are studied as a function of added salt (sodium salicylate) concentration, temperature, and shear rate via Rheosmall-angle light scattering (Rheo-SALS). These WLM solutions exhibit shear-enhanced concentration fluctuations leading to shear-induced phase separation (SIPS), manifested as visual turbidity under shear and the appearance of a characteristic “butterfly” scattering pattern in Rheo-SALS experiments. Flow kinematics measurements in a Couette geometry are used to determine the relationship between SIPS and shear banding, i.e., the splitting of the flow into shear bands with different local shear rates. Modeling using the Giesekus constitutive equation aids in discrimination between banding and nonbanding solutions. The combination of Rheo-SALS, dynamic rheology, velocimetry, and constitutive equation modeling allows detailed exploration ...

Rheology of colloidal particles in lyotropic hexagonal liquid crystals: the role of particle loading, shape, and phase transition kinetics

The rheology of self-assembled elongated iron oxyhydroxide (FeOOH) and spherical silica (SiO2) particles in hexagonal (H1) liquid crystal (LC) phase of water and non-ionic surfactant C12E9 is investigated by varying particle concentration and cooling rate. The rheology data shows that both SiO2/H1 and FeOOH/ H1 LC composites exhibit a higher G′ when compared to the particle-free H1 phase, with increasing particle loading and cooling rate. FeOOH particles improve G′ of the H1 phase more significantly than SiO2 particles due to the formation of an interconnected network at H1 domain boundaries at cooling rates of 1 and 2 ∘C/min. We hypothesize that self-assembly of particles at domain boundaries leads to a decreased mobility of defects causing an increase in elasticity of particle-laden H1 phase. Dynamic strain sweep and creep experiments show a non-linear stress–strain relationship attributed to the alignment of micellar cylindrical rods under shear.

Rheology and microstructure of pastes with crystal network

This article reviews scientific studies which have been focused on the structure–property relationships of pastes with crystal network by considering fat crystal network as the key example. The review illustrates how rheology can be a valuable tool to characterize the mechanical behavior in these pastes and also provide information about the underlying microstructure in conjunction with various complimentary techniques. Rheological tests which can be adopted to determine the yielding behavior of the crystal network containing pastes are also presented. We have used these methods and concepts to characterize fatty acid crystal network to study a system emulating a skin cream formulation. It is hoped that the rheological fundamentals and the microstructural characterization techniques discussed in context of fat crystal network can be translated to other experimental systems where crystal-forming components are added to impart pasty material characteristics.

Partitioning and self assembly of silica and hematite particles at grain boundaries of hexagonal liquid crystals: Implications on rheology

We investigate the rheological implications of partitioning and self-assembly of colloidal particles at the grain boundaries (GBs) of hexagonal (H1) liquid crystal (LC) phase as a function of particle loading, shape and phase transition kinetics. The rheology of spherical silica particles (SiO2, diameter = 140 nm)/H1 and irregular hematite particles (Fe2O3, size = 110 nm)/H1 composites is measured as the samples are cooled from an isotropic to H1 phase at 2 and 0.2 °C/min. At 2 °C/min, SiO2/H1 composites show a consistent increase in G′ as the particle loading increases from 0.5 to 7.5 wt. % while Fe2O3/H1 composites exhibit a small drop in G′ above 2.5 wt. % particle loading. On the other hand, SiO2/H1 and Fe2O3/H1 composites show a monotonic increase in G′ with particle loading at a cooling rate of 0.2 °C/min. Microscopy observations reveal that at 0.2 °C/min, both SiO2 and Fe2O3 particles aggregate at the H1 GBs. The different rheological responses of SiO2/H1 and Fe2O3/H1 composites at 2 °C/min are due...

Experimental study of surfactant driven nematic liquid crystal (NLC) anchoring transitions at solid surfaces: role of solid surface energy and anisotropic NLC – solid interfacial energy

Abstract The fundamental understanding of nematic liquid crystal (NLC) alignment on solid surfaces is significant for the operation of liquid crystal displays and devices. We investigate the effect of structure and concentration of surfactants on the anchoring of NLC 4-cyano-4′-pentylbiphenyl (5CB) at 5CB–solid interface. 5CB molecules undergo an ordering transition from parallel to normal anchoring near the surfactant critical micelle concentration depending on the chemical structure of the surfactant tails. In contrast to the previous studies which point to the solubilization of 5CB in surfactant micelles, our experiments indicate that NLC anchoring transition occurs due to the ability of 5CB molecules to penetrate into the surface surfactant layer. The surfactant-driven 5CB anchoring is further related to the surface tension of 5CB (γ5CB), surfactant adsorbed substrate surface energy (γsa), anisotropic 5CB–surface interfacial energy (Δγsl), and spreading coefficient (S). Our experimental results agree with the Creagh–Kahn criterion of predicting surface anchoring from relative values of γsa and γ5CB. A change in the sign of Δγsl and S is also observed along with the anchoring transition. Using a semi-empirical approach, we show that dipolar interactions dominate over dispersion interactions when 5CB molecules exhibit perpendicular anchoring at the solid–5CB interface.

Flow-directed assembly of non-spherical titania nanoparticles into superhydrophilic thin films

Superhydrophilic thin films of 21 nm sized non-spherical titania nanoparticles are fabricated from a colloidal suspension by fixed blade flow coating without UV illumination. At a blade angle of a = 36° and a gap of d = 300 µm, hierarchically structured films with increasing surface roughness along with microscopic voids are formed depending on the substrate velocity and the titania volume fraction. Increasing the roughness is shown to be concomitant to an increase in the hydrophilicity, eventually leading to superhydrophilicity or water contact angle less than 5°.

Surfactant Induced Interfacial Anchoring Transitions In Nematic Liquid Crystal Droplets On Glass Surfaces

The interfacial adsorption of surfactants at planar nematic liquid crystal (NLC)–water interface induces an ordering transition from a tilted to perpendicular anchoring with the increase in surfactant concentration at CN=C∕Ccmc≪1, where Ccmc is the Critical Micelle Concentration of surfactants in water. In this study, we show that depending upon the surfactant structure a tilted to perpendicular NLC anchoring transition is observed at C∕Ccmc≥1 in 5CB droplets of size 50–70μm. Micrometer sized 5CB droplets are deposited on glass surfaces using flow coating of 5CB-in-ethanol solutions. When placed on 5CB drop decorated glass surfaces, the aqueous surfactant solutions of aliphatic chain surfactants (SDS, CTAB and CPBr) at CN<1, result in an optical transition to a bright-cross texture attributed to the tilted anchoring of 5CB molecules at 5CB–water interface. At C∕Ccmc≥1, perpendicular anchoring of 5CB molecules at 5CB–water interface results in a droplet texture with a hedgehog defect. In contrast, aqueous solutions of SDBS lead to 5CB droplets with a bright-cross texture regardless of the surfactant concentration in the aqueous phase. These results indicate that the orientation of 5CB molecules is independent of the nature of the surfactant headgroup. In addition, 5CB droplet decorated OTS treated glass substrates show a hedgehog texture which disappears completely on exposure to organic vapors with the response time-dependent on the polarity of the vapor molecules.