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Title: Structure and dynamics of filled liquid crystals
Author: Katyan, Navneeta
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2020
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In the formulation world, rheological properties like flow behavior and viscoelastic response determine the quality of the product. For the first time, we explore and explain the dynamic response of filled nematic thermotropic liquid crystal phases. We then discuss the behavior in lyotropic filled nematic liquid crystalline (NLCs) media and compare it with the more commonly understood filled lamellar phases. Conventional rheometry, coupled with polarizing microscopy, was used to formulate an understanding of the microstructure of the colloids and their effect on the flow behavior of the colloids and LCs composite. A class of soft solids exhibiting exceptional stability is made from dispersing PMMA microspheres in thermotropic nematic liquid crystal (NLCs). When a microsphere induces weak homeotropic anchoring in NLCs, the director around the colloid elastically distorts to accommodate the particle giving rise to disclinations or defect lines. The type of defect present depends on the anchoring strength, (W), between colloid and NLCs, the elasticity of the NLCs, (K), and the size of the dispersed particle, (r). For Wr K 1, the colloid induces a Saturn-ring defect in NLCs. These Saturn-ring defects remain isolated without interacting with each other in the dilute composite. As the concentration of the colloids in NLCs increases, the encircling loops of these Saturn-rings no longer remain isolated but entangle to form a more stable topological structure which holds the colloid in the defect matrix — thus forming a stable gel composite. Dynamic moduli of these composites increase with volume fraction with G0 and G00 ∝ φ2, possibly because each colloid supports a two-dimensional Saturn-ring. These ring defects can connect at different points around the circumference of the disclinations and therefore the number of percolating paths increases quadratically with the volume fraction. For the first time, we show that G00 ∝ ω1/2 on yielding. We derive a theory that describes this yielding behaviour is governed by the Ericksen number, Er, associated with confined nematic region within the composites. We find that the frequency dependence of the composites is independent of the volume fraction, φ, indicating that it is neither an active or passive filled system and that the behavior of composite is determined by the intrinsic properties of the nematic phase. The colloids merely serve to create and support Saturn-ring defects. The structure and dynamics of filled lyotropic NLCs were studied for the first time. Uncharged PMMA particles were dispersed in surfactant and water-based lyotropic mesophase to form a class of composites similar to the thermotropic system. Filled lyotropics exhibit similar rheological behavior to their thermotropic counterpart. However, the surface charge of colloids disrupts the composite properties in the charged micellar nematic liquid crystal system. A comparison of micrographs showed clustered networks for the uncharged composite but a disconnected array-like structure for anionic composites. Nematic emulsions made from dispersing PDMS droplets in lyotropic nematics show similar rheological behavior like the solid-sphere dispersion up to φ ≤ 0.54 but deviate near the glass transition volume fraction. The flow behavior of these unique NLCs composites was also examined from steady-state measurements. The flow behavior of filled nematic is complex, owing to the coupling between the flow field and the director field. Both thermotropic and lyotropic composites showed remarkable shear-thinning behavior with the viscosity curve following power-law behavior. The breaking of the network structure into smaller clusters further explains this exceptional shear-thinning behavior on the application of shear. These clusters then align along the direction of flow, thus providing less resistance to flow, reducing the viscosity, and some evidence of shear-banding is evident. Relative viscosities (ηr = ηφ ηLCs) at high shear follow Krieger-Dougherty relation for the lyotropic composites. However, the deformable colloids (PDMS) in nematic emulsion diverts from Krieger-Dougherty relation beyond φ ≥ φg = 0.58. Through extensive rheological experiments and microscopy, we describe the physical properties of a new type of gel with exceptional stability and shearthinning performance that could find wide application in the formulation industries.
Supervisor: Wood, Tiffany ; Poon, Wilson Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: microstructure ; soft solids ; flow behaviour ; nematic liquid crystal ; colloids ; Rheology ; nematic LCs emulsion underflow ; viscosity