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1. Elasto-Inertial Focusing Mechanisms of Particles in Shear-Thinning Viscoelastic Fluid in Rectangular Microchannels

   https://doi.org/10.3390/mi13122131  

   Naderi, Mohammad Moein ; Barilla, Ludovica ; Zhou, Jian ; Papautsky, Ian ; Peng, Zhangli ( December 2022 , Micromachines)

   Growth of the microfluidics field has triggered numerous advances in focusing and separating microparticles, with such systems rapidly finding applications in biomedical, chemical, and environmental fields. The use of shear-thinning viscoelastic fluids in microfluidic channels is leading to evolution of elasto-inertial focusing. Herein, we showed that the interplay between the elastic and shear-gradient lift forces, as well as the secondary flow transversal drag force that is caused by the non-zero second normal stress difference, lead to different particle focusing patterns in the elasto-inertial regime. Experiments and 3D simulations were performed to study the effects of flowrate, particle size, and the shear-thinning extent of the fluid on the focusing patterns. The Giesekus constitutive equation was used in the simulations to capture the shear-thinning and viscoelastic behaviors of the solution used in the experiments. At low flowrate, with Weissenberg number Wi ~ O(1), both the elastic force and secondary flow effects push particles towards the channel center. However, at a high flowrate, Wi ~ O(10), the elastic force direction is reversed in the central regions. This remarkable behavior of the elastic force, combined with the enhanced shear-gradient lift at the high flowrate, pushes particles away from the channel center. Additionally, a precise prediction of the focusing position can only be made when the shear-thinning extent of the fluid is correctly estimated in the modeling. The shear-thinning also gives rise to the unique behavior of the inertial forces near the channel walls which is linked with the ‘warped’ velocity profile in such fluids. 

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2. Inertial Microfluidics Enabling Clinical Research

   https://doi.org/https://doi.org/10.3390/ mi12030257  

   Kalyan, Srivathsan ; Torabi, Corinna Torabi ; Khoo, Harrison ; Sung, Hyun Woo ; Choi, Sung-Eun ; Wang, Wenzhao ; Treutler, Benjamin ; Kim, Dohyun ; Hur, Soojung Claire ( March 2021 , Micromachines)

   null (Ed.)

   Fast and accurate interrogation of complex samples containing diseased cells or pathogens is important to make informed decisions on clinical and public health issues. Inertial microfluidics has been increasingly employed for such investigations to isolate target bioparticles from liquid samples with size and/or deformability-based manipulation. This phenomenon is especially useful for the clinic, owing to its rapid, label-free nature of target enrichment that enables further down- stream assays. Inertial microfluidics leverages the principle of inertial focusing, which relies on the balance of inertial and viscous forces on particles to align them into size-dependent laminar stream- lines. Several distinct microfluidic channel geometries (e.g., straight, curved, spiral, contraction-ex- pansion array) have been optimized to achieve inertial focusing for a variety of purposes, including particle purification and enrichment, solution exchange, and particle alignment for on-chip assays. In this review, we will discuss how inertial microfluidics technology has contributed to improving accuracy of various assays to provide clinically relevant information. This comprehensive review expands upon studies examining both endogenous and exogenous targets from real-world samples, highlights notable hybrid devices with dual functions, and comments on the evolving outlook of the field. 

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3. Electro-elastic migration of particles in viscoelastic fluid flows

   https://doi.org/10.1063/5.0167571  

   Li, Di ; Xuan, Xiangchun ( September 2023 , Physics of Fluids)

   Microfluidic manipulation of particles usually relies on their cross-stream migration. A center- or wall-directed motion has been reported for particles leading or lagging the Poiseuille flow of viscoelastic polyethylene oxide (PEO) solution via positive or negative electrophoresis. Such electro-elastic migration is exactly opposite to the electro-inertial migration of particles in a Newtonian fluid flow. We demonstrate here through the top- and side-view imaging that the leading and lagging particles in the electro-hydrodynamic flow of PEO solution migrate toward the centerline and corners of a rectangular microchannel, respectively. Each of these electro-elastic particle migrations is reduced in the PEO solution with shorter polymers though neither of them exhibits a strong dependence on the particle size. Both phenomena can be reasonably explained by the theory in terms of the ratios of the forces involved in the process. Decreasing the PEO concentration causes the particle migration to shift from the viscoelastic mode to the Newtonian mode, for which the magnitude of the imposed electric field is found to play an important role.

    

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4. 3D particle transport in multichannel microfluidic networks with rough surfaces

   https://doi.org/10.1038/s41598-020-70728-1  

   Ryan, Duncan P. ; Chen, Yu ; Nguyen, Phong ; Goodwin, Peter M. ; Carey, J. William ; Kang, Qinjun ; Werner, James H. ; Viswanathan, Hari S. ( August 2020 , Scientific Reports)

   The transport of particles and fluids through multichannel microfluidic networks is influenced by details of the channels. Because channels have micro-scale textures and macro-scale geometries, this transport can differ from the case of ideally smooth channels. Surfaces of real channels have irregular boundary conditions to which streamlines adapt and with which particle interact. In low-Reynolds number flows, particles may experience inertial forces that result in trans-streamline movement and the reorganization of particle distributions. Such transport is intrinsically 3D and an accurate measurement must capture movement in all directions. To measure the effects of non-ideal surface textures on particle transport through complex networks, we developed an extended field-of-view 3D macroscope for high-resolution tracking across large volumes ($$25\,\hbox {mm} \times 25\,\hbox {mm} \times 2\,\hbox {mm}$$$25\text{mm}\times 25\text{mm}\times 2\text{mm}$) and investigated a model multichannel microfluidic network. A topographical profile of the microfluidic surfaces provided lattice Boltzmann simulations with a detailed feature map to precisely reconstruct the experimental environment. Particle distributions from simulations closely reproduced those observed experimentally and both measurements were sensitive to the effects of surface roughness. Under the conditions studied, inertial focusing organized large particles into an annular distribution that limited their transport throughout the network while small particles were transported uniformly to all regions.

    

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5. A Novel Fluid–Structure Interaction Framework for Flapping, Flexible Wings

   https://doi.org/10.1115/1.4044268  

   Schwab, Ryan ; Johnson, Erick ; Jankauski, Mark ( December 2019 , Journal of Vibration and Acoustics)

   Fluid–structure interaction (FSI) plays a significant role in the deformation of flapping insect wings. However, many current FSI models are high-order and rely on direct computational methods, thereby limiting parametric studies as well as insights into the physics governing wing dynamics. We develop a novel flapping wing FSI framework that accommodates general wing geometry and fluid loading. We use this framework to study the unilaterally coupled FSI of an idealized hawkmoth forewing considering two fluid models: Reynolds-averaged Navier–Stokes computational fluid dynamics (RANS CFD) and blade element theory (BET). We first compare aerodynamic modal forces estimated by the low-order BET model to those calculated via high fidelity RANS CFD. We find that for realistic flapping kinematics, BET estimates modal forces five orders of magnitude faster than CFD within reasonable accuracy. Over the range flapping kinematics considered, BET and CFD estimated modal forces vary maximally by 350% in magnitude and approximately π/2 radians in phase. The large reduction in computational time offered by BET facilitates high-dimensional parametric design of flapping-wing-based technologies. Next, we compare the contributions of aerodynamic and inertial forces to wing deformation. Under the unilateral coupling assumption, aerodynamic and inertial-elastic forces are on the same order of magnitude—however, inertial-elastic forces primarily excite the wing’s bending mode whereas aerodynamic forces primarily excite the wing’s torsional mode. This suggests that, via conscientious sensor placement and orientation, biological wings may be able to sense independently inertial and aerodynamic forces. 

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