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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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Characterization of the critical lift-off of a single flat-plate microchip particle in straight rectangular microchannel flows
Hydrodynamic sorting of microchip particles in microchannels is essential in microfluidic systems used for applications requiring particle-based multiplexing. Understanding the forces acting on the particle, as well as the dependencies of the forces on channel and fluid flow parameters, allows for prediction of the flow conditions needed to initiate particle movement, or lift-off. This study presents the experimental characterization of the lift-off of a single, flat-plate, non-neutrally buoyant microchip particle initially sedimented near the inlet of straight, rectangular microfluidic channels of different channel sizes and solvents at moderate Archimedes number of 191 to 2820. The critical shear Reynolds number, corresponding to the minimum required for lift-off, was found to increase with larger Archimedes number and the relationship was found to exhibit particle-channel size dependency. The observed critical lift-off for the flat-plate particle was lower than that predicted using a previous generalized lift-off model based on modified particle Reynolds and Archimedes numbers which may be explained by entrance effects and fluid film lubrication pressure under the particle. Numerical evaluations of the hydrodynamic forces acting on the particle revealed that electrostatic forces are significant. A remodified Archimedes number, based on the channel width, particle diameter, and solvent relative permittivity, is introduced as a correction to the generalized lift-off model to account for hydrodynamics and electrostatics affecting the lift-off of a flat-plate particle. This model is in good agreement with the generalized particle lift-off model and allows for prediction of flat-plate particle lift-off in microfluidic channels for a moderate range of Archimedes numbers.
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- Award ID(s):
- 2131428
- PAR ID:
- 10665333
- Publisher / Repository:
- International Journal of Multiphase Flow
- Date Published:
- Journal Name:
- International Journal of Multiphase Flow
- Volume:
- 193
- Issue:
- C
- ISSN:
- 0301-9322
- Page Range / eLocation ID:
- 105355
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.ijmultiphaseflow.2025.105355
