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1. Dynamics of rigid achiral magnetic microswimmers in shear-thinning fluids

   https://doi.org/10.1063/5.0167307  

   Quashie, David; Wang, Qi; Jermyn, Sophie; Katuri, Jaideep; Ali, Jamel (September 2023, Physics of Fluids)

   Here, we use magnetically driven self-assembled achiral swimmers made of two to four superparamagnetic micro-particles to provide insight into how swimming kinematics develop in complex, shear-thinning fluids. Two model shear-thinning polymer fluids are explored, where measurements of swimming dynamics reveal contrasting propulsion kinematics in shear-thinning fluids vs a Newtonian fluid. When comparing the velocity of achiral swimmers in polymer fluids to their dynamics in water, we observe kinematics dependent on (1) no shear-thinning, (2) shear-thinning with negligible elasticity, and (3) shear-thinning with elasticity. At the step-out frequency, the fluidic environment's viscoelastic properties allow swimmers to propel faster than their Newtonian swimming speed, although their swimming gait remains similar. Micro-particle image velocimetry is also implemented to provide insight into how shear-thinning viscosity fluids with elasticity can modify the flow fields of the self-assembled magnetic swimmers. Our findings reveal that flow asymmetry can be created for symmetric swimmers through either the confinement effect or the Weissenberg effect. For pseudo-chiral swimmers in shear-thinning fluids, only three bead swimmers show swimming enhancement, while four bead swimmers always have a decreased step-out frequency velocity compared to their dynamics in water. 

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2. Effect of shear-thinning rheology on the dynamics and pressure distribution of a single rigid ellipsoidal particle in viscous fluid flow

   https://doi.org/10.1063/5.0242953  

   Awenlimobor, A; Smith, D E (December 2024, Physics of Fluids)

   This paper evaluates the behavior of a single rigid ellipsoidal particle suspended in homogeneous viscous flow with a power-law generalized Newtonian fluid rheology using a custom-built finite element analysis (FEA) simulation. The combined effects of the shear-thinning fluid rheology, the particle aspect ratio, the initial particle orientation, and the shear-extensional rate factor in various homogeneous flow regimes on the particles dynamics and surface pressure evolution are investigated. The shear-thinning fluid behavior was found to modify the particle’s trajectory and alter the particle’s kinematic response. Moreover, the pressure distribution over the particle’s surface is significantly reduced by the shear-thinning fluid rheology. The FEA model is validated by comparing results of the Newtonian case with results obtained from the well-known Jeffery’s analytical model. Furthermore, Jeffery’s model is extended to define the particle’s trajectory in a special class of homogeneous Newtonian flows with combined extension and shear rate components typically found in axisymmetric nozzle flow contractions. The findings provide an improved understanding of key transport phenomenon related to physical processes involving fluid–structure interaction such as that which occurs within the flow field developed during material extrusion–deposition additive manufacturing of fiber reinforced polymeric composites. These results provide insight into important microstructural formations within the print beads. 

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3. Universal non-monotonic drainage in large bare viscous bubbles

   https://doi.org/10.1038/s41467-023-36397-0  

   Bartlett, Casey; Oratis, Alexandros T.; Santin, Matthieu; Bird, James C. (February 2023, Nature Communications)

   Abstract Bubbles will rest at the surface of a liquid bath until their spherical cap drains sufficiently to spontaneously rupture. For large film caps, the memory of initial conditions is believed to be erased due to a visco-gravitational flow, whose velocity increases from the top of the bubble to its base. Consequently, the film thickness has been calculated to be relatively uniform as it thins, regardless of whether the drainage is regulated by shear or elongation. Here, we demonstrate that for large bare bubbles, the film thickness is highly nonuniform throughout drainage, spanning orders of magnitude from top to base. We link the film thickness profile to a universal non-monotonic drainage flow that depends on the bubble thinning rate. These results highlight an unexpected coupling between drainage velocity and bubble thickness profiles and provide critical insight needed to understand the retraction and breakup dynamics of these bubbles upon rupture. 

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4. Numerical modelling on polymer composites prepared via large area extrusion-deposition additive manufacturing: fibre orientation, material inhomogeneity, and thermal-mechanical responses during material loading process

   https://doi.org/10.1080/14658011.2023.2194463  

   Wang, Zhaogui; Fang, Zhenyu; Liu, Xirui; Smith, Douglas E (September 2023, Plastics, Rubber and Composites)

   The flow-induced fibre orientation formed during polymer extrusions causes the composite to exhibit non-homogeneous thermal-mechanical behaviours during Large Area extrusion deposition Additive Manufacturing (LAAM) processes. This study numerically evaluates the fibre orientation state of a 20 wt.% short carbon fibre reinforced polyethylenimine fabricated by LAAM. The fibre orientation state of the solidified deposited bead is determined by a fully coupled flow/orientation simulation approach. The material properties of deposited composites are computed by assuming that the deposited bead has heterogeneous regions with varying local fibre orientation states. A finite element simulation is performed to model the LAAM process of a thin-wall structure, where the predicted inhomogeneous material properties are employed. Computed results show notable differences between simulations performed by employing homogenous properties and those obtained using heterogeneous properties. The bead-direction tensile stress contours computed under the heterogeneous assumption are comparable to experimental data in the literature, supporting our numerical approach. 

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5. Fluid Rheological Effects on the Flow of Polymer Solutions in a Contraction–Expansion Microchannel

   https://doi.org/10.3390/mi11030278  

   Jagdale, Purva P.; Li, Di; Shao, Xingchen; Bostwick, Joshua B.; Xuan, Xiangchun (March 2020, Micromachines)

   A fundamental understanding of the flow of polymer solutions through the pore spaces of porous media is relevant and significant to enhanced oil recovery and groundwater remediation. We present in this work an experimental study of the fluid rheological effects on non-Newtonian flows in a simple laboratory model of the real-world pores—a rectangular sudden contraction–expansion microchannel. We test four different polymer solutions with varying rheological properties, including xanthan gum (XG), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and polyacrylamide (PAA). We compare their flows against that of pure water at the Reynolds ( R e ) and Weissenburg ( W i ) numbers that each span several orders of magnitude. We use particle streakline imaging to visualize the flow at the contraction–expansion region for a comprehensive investigation of both the sole and the combined effects of fluid shear thinning, elasticity and inertia. The observed flow regimes and vortex development in each of the tested fluids are summarized in the dimensionless W i − R e and χ L − R e parameter spaces, respectively, where χ L is the normalized vortex length. We find that fluid inertia draws symmetric vortices downstream at the expansion part of the microchannel. Fluid shear thinning causes symmetric vortices upstream at the contraction part. The effect of fluid elasticity is, however, complicated to analyze because of perhaps the strong impact of polymer chemistry such as rigidity and length. Interestingly, we find that the downstream vortices in the flow of Newtonian water, shear-thinning XG and elastic PVP solutions collapse into one curve in the χ L − R e space. 

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