Experiments are conducted exploring the flow of Carbopol past obstacles in a narrow slot and compared with predictions of a model based on the Herschel-Bulkley constitutive law and the conventional Hele-Shaw approximation. Although Carbopol is often assumed to be a relatively simple yield-stress fluid, the flow pattern around an obstacle markedly lacks the fore-aft symmetry expected theoretically. Such asymmetry has been observed previously for viscoplastic flows past obstacles in unconfined geometries, but the narrowness of the Hele-Shaw cell ensures that the stress state is very different, placing further constraints on the underlying origin. The asymmetry is robust, as demonstrated by varying the shape and number of the obstacles, the surfaces of the cell walls, and the steadiness of the flow rate. The results suggest that rheological hysteresis near the yield point may be the cause of the asymmetry.

The endothelial glycocalyx is a dynamic structure integral to blood vessel hemodynamics and capable of tightly regulating a range of biological processes (ie, innate immunity, inflammation, and coagulation) through dynamic changes in its composition of the brush structure. Evaluating the specific roles of the endothelial glycocalyx under a range of pathophysiologic conditions has been a challenge in vitro as it is difficult to generate functional glycocalyces using commonly employed 2D cell culture models. We present a new multi-height microfluidic platform that promotes the growth of functional glycocalyces by eliciting unique shear stress forces over a continuous human umbilical vein endothelial cell monolayer at magnitudes that recapitulate the physical environment in arterial, capillary and venous regions of the vasculature. Following 72 hours of shear stress, unique glycocalyx structures formed within each region that were distinct from that observed in short (3 days) and long-term (21 days) static cell culture. The model demonstrated glycocalyx-specific properties that match the characteristics of the endothelium in arteries, capillaries and veins, with respect to surface protein expression, platelet adhesion, lymphocyte binding and nanoparticle uptake. With artery-to-capillary-to-vein transition on a 2 of 14 | SIREN Et al.

Turbulent drag reduction has been observed to occur over a wide range of additive systems, such as solution of synthetic or natural polymers, and fibre suspensions. In this study, the influence of softwood kraft pulp fibres and synthetic polymer additives on turbulent drag reduction (DR) in a hydrocyclone is investigated. It was demonstrated that cellulose fibre suspensions and aqueous polymeric solutions reduce the fluid energy losses in comparison to water during hydrocyclone operation within the range of reject ratios studied. A maximum drag reduction of 58% and 55% was found to occur at a volume split fraction of 50% for a 0.9% fibre suspension and a 300 ppm anionic polyacrylamide solution, respectively. Polymer degradation or polymer chain decay displayed adverse effects for 100 ppm and 150 ppm solutions after 22 min of run time at 11.2 kW pumping power and a reject ratio of 25%. Synergistic effects were observed with pulp suspensions containing both cationic and anionic polyacrylamide (CPAM and APAM, respectively); a maximum DR of 41% was observed for a 0.7% fibre suspension containing 100-300 ppm of polymer at a reject ratio of 50%. Similarly to aqueous polymer solutions, the degradation of 300 ppm APAM or CPAM in a 0.7% fibre suspension decreased the observed DR up to 38% after 30 min of run time at 11.2 kW pumping power and a reject ratio of 25%. The near 43% reduction in CPAM concentration, due to surface adsorption, when present in a 0.7% fibre suspension assisted in quantifying the DR variations observed between suspensions containing APAM or CPAM.

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The focus of the present work is an experimental study of the behaviour of semi-dilute, opaque fibre suspensions in fully developed cylindrical pipe flows. Measurements of the normal and turbulent shear stress components and the mean flow were acquired using phase-contrast magnetic resonance velocimetry. Two fibre types, namely, pulp fibre and nylon fibre, were considered in this work and are known to differ in elastic modulus. In total, three different mass concentrations and seven Reynolds numbers were tested to investigate the effects of fibre interactions during the transition from the plug flow to fully turbulent flow. It was found that in fully turbulent flows of nylon fibres, the normal, ⟨uzuz⟩+, and shear, ⟨uzur⟩+ (note that ⟨·⟩ is the temporal average, u is the fluctuating velocity, z is the axial or streamwise component, and r is the radial direction), turbulent stresses increased with Reynolds number regardless of the crowding number (a concentration measure). For pulp fibre, the turbulent stresses increased with Reynolds number when a fibre plug was present in the flow and were spatially similar in magnitude when no fibre plug was present. Pressure spectra revealed that the stiff, nylon fibre reduced the energy in the inertial-subrange with an increasing Reynolds and crowding number, whereas the less stiff pulp fibre effectively cuts the energy cascade prematurely when the network was fully dispersed.