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The use of nematics in microfluidic devices offers methods of flow control that are inaccessible to isotropic fluids. While nematics have been used in channel flows found in lab on chip devices, their flow and microstructure formation still needs to be fully understood in geometries with a cylindrical cross-section. This paper investigates liquid crystal flows through capillaries via 1) the Beris-Edwards (BE) model and 2) the Leslie-Ericksen (LE) model, which emerges as a limiting case (uniaxial, constant order parameter) of the BE model. We show that the microstructure distribution is controlled by the Ericksen (Er) and Deborah (De) numbers in the Beris-Edwards model. Defects at low Ericksen numbers arise when the elastic energy dominates the thermotropic energy. Defect destruction provides a new shear-thinning mechanism, which cannot be observed in the Leslie- Ericksen theory. [ABSTRACT FROM AUTHOR]

Thin films consisting of polymer solutions are typically produced through a combination of extrusion and shearing processes, where the anisotropic, non-Newtonian solution is deformed and subjected to thermal treatment. This paper investigates the shearing of polymeric thin films by studying the channel flow rheology of polymer solutions that experience yield stress. The material rheology is described by the transversely isotropic fluid (TIF) model, which contains a yield behavior term related to microstructure distortion. Our results show that this distortional stress is able to resist the pressure gradient, and non-trivial stress distributions can exist in the absence of a flow. This represents a significant improvement over existing viscosity-based yield stress models (e.g., the Heschel–Bulkley model). The unyielded state is achieved as the end result of a transient process, where a pressure gradient produces a short-lived flow that ceases when opposing stresses from microstructure distortion are produced. Predictions of the TIF model are compared with the phenomenological Saramito model. Both models are found to predict yielding when a threshold stress is exceeded. In both cases, the velocity profile is Newtonian near the wall, while plug flows are encountered close to the centerline. [ABSTRACT FROM AUTHOR]

We explore the use of an Elastic Perfectly Plastic (EPP) constitutive equation for the modelling of yield stress fluids. Contrary to many other models, stresses in the EPP model arise from elastic deformation rather than as a viscous effect. In this paper, the EPP model is coupled to a standard viscous treatment of the post-yield flow stresses to produce Bingham-like behaviour, and the timescale associated with the yielding mechanism is linked to material parameters. We also show that when the yield stress is much smaller than the elastic modulus, EPP and Bingham models can produce very similar flow fields in channel and contraction geometries. The EPP model is found to be significantly cheaper computationally in both geometries. Additionally, in the case of channel flow where analytical solutions exist, the EPP model is associated with a much smaller error than the regularised Bingham model. • The Elastic Perfectly Plastic (EPP) model for yield stress fluids is proposed. • Convected stress derivatives are discussed in the context of yield stress fluids. • Predictions of the new model are discussed in basic rheometric flows. • EPP and Bingham models are compared in channel and contraction flows. [ABSTRACT FROM AUTHOR]