Special Issue "Inertial Microfluidics"

A special issue of Micromachines (ISSN 2072-666X). This special issue belongs to the section "A:Physics".

Deadline for manuscript submissions: closed (31 December 2020).

Special Issue Editors

Dr. Soojung Claire Hur
E-Mail Website
Guest Editor
Department of Mechanical Engineering and Oncology, Johns Hopkins University 3400 N Charles St, Latrobe 221, Baltimore, MD 21211, USA
Interests: microfluidics; cell purtification; electroporation; cell engineering; personalized medicine
Special Issues and Collections in MDPI journals
Dr. Wonhee Lee
E-Mail Website
Guest Editor
Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, 291 Daehak-ro E6-6, Yuseong-gu, Daejeon 34141, Korea
Interests: inertial microfluidic; thermal biosensor; nanofluidic; microfabrication

Special Issue Information

Dear Colleagues,

During the past decade, inertial microfluidics has been attracting much attention from researchers in diverse fields, including fluid mechanics, bioengineering, and biomedical sciences. The ability of inertial microfluidics to handle fluids and particles with extremely high-throughput has enabled many practical applications such as cell separation/enrichment, flow shaping, single-cell manipulation/analysis, and unconventional microparticle fabrications. Recently, there have been notable increases in publications for the application of inertial microfluidic techniques in bioanalytical research and medical sample collection and diagnostics. Yet, interesting topics for understanding the underlying physics of inertial microfluidics and the development of novel fluid and particle manipulation techniques are still actively studied to achieve higher throughput and efficiency. This Special Issue seeks original research papers, short communications, and review articles, illustrating the capabilities of inertial microfluidic technology not only for fundamental research but also for diverse applications.

We look forward to receiving your contributions.

Dr. Soojung Claire Hur
Dr. Wonhee Lee
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Micromachines is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Inertial microfluidics
  • finite reflow
  • cell separation
  • inertial focusing
  • single cell analysis
  • microparticle
  • flow cytometer
  • elasto-inertial flow
  • Dean flow
  • mixing

Published Papers (11 papers)

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Editorial

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Editorial
Editorial for the Special Issue on Inertial Microfluidics
Micromachines 2021, 12(6), 587; https://doi.org/10.3390/mi12060587 - 21 May 2021
Viewed by 421
Abstract
The growing demands for label-free, high throughput processing of biological, environmental, and industrial samples have instigated technical innovations for inflow particle manipulations with better resolution and purity [...] Full article
(This article belongs to the Special Issue Inertial Microfluidics)

Research

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Article
Inertial Migration of Neutrally Buoyant Spherical Particles in Square Channels at Moderate and High Reynolds Numbers
Micromachines 2021, 12(2), 198; https://doi.org/10.3390/mi12020198 - 14 Feb 2021
Cited by 2 | Viewed by 674
Abstract
The inertial migration of particles in microchannel flows has been deeply investigated in the last two decades. In spite of numerous reports on the inertial focusing patterns in a square channel, the particle inertial focusing and longitudinal ordering processes remain unclear at high [...] Read more.
The inertial migration of particles in microchannel flows has been deeply investigated in the last two decades. In spite of numerous reports on the inertial focusing patterns in a square channel, the particle inertial focusing and longitudinal ordering processes remain unclear at high Reynolds numbers (>200) in square microchannels smaller than 100 µm in width. Thus, in this work, in situ visualization of particles flowing in square micro-channels at Reynolds numbers Re ranging from 5 to 280 has been conducted and their migration behaviors have been analyzed. The obtained results confirm that new equilibrium positions appear above a critical Re depending on the particle to channel size ratio and the particle volume fraction. It is also shown that, for a given channel length, an optimal Reynolds number can be identified, for which the ratio of particles located on equilibrium positions is maximal. Moreover, the longitudinal ordering process, i.e., the formation of trains of particles on equilibrium positions and the characterization of their length, has also been analyzed for the different flow conditions investigated in this study. Full article
(This article belongs to the Special Issue Inertial Microfluidics)
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Article
Inertial Microfluidics-Based Separation of Microalgae Using a Contraction–Expansion Array Microchannel
Micromachines 2021, 12(1), 97; https://doi.org/10.3390/mi12010097 - 19 Jan 2021
Cited by 3 | Viewed by 994
Abstract
Microalgae separation technology is essential for both executing laboratory-based fundamental studies and ensuring the quality of the final algal products. However, the conventional microalgae separation technology of micropipetting requires highly skilled operators and several months of repeated separation to obtain a microalgal single [...] Read more.
Microalgae separation technology is essential for both executing laboratory-based fundamental studies and ensuring the quality of the final algal products. However, the conventional microalgae separation technology of micropipetting requires highly skilled operators and several months of repeated separation to obtain a microalgal single strain. This study therefore aimed at utilizing microfluidic cell sorting technology for the simple and effective separation of microalgae. Microalgae are characterized by their various morphologies with a wide range of sizes. In this study, a contraction–expansion array microchannel, which utilizes these unique properties of microalgae, was specifically employed for the size-based separation of microalgae. At Reynolds number of 9, two model algal cells, Chlorella vulgaris (C. vulgaris) and Haematococcus pluvialis (H. pluvialis), were successfully separated without showing any sign of cell damage, yielding a purity of 97.9% for C. vulgaris and 94.9% for H. pluvialis. The result supported that the inertia-based separation technology could be a powerful alternative to the labor-intensive and time-consuming conventional microalgae separation technologies. Full article
(This article belongs to the Special Issue Inertial Microfluidics)
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Article
Numerical Study of Multivortex Regulation in Curved Microchannels with Ultra-Low-Aspect-Ratio
Micromachines 2021, 12(1), 81; https://doi.org/10.3390/mi12010081 - 14 Jan 2021
Cited by 2 | Viewed by 739
Abstract
The field of inertial microfluidics has been significantly advanced in terms of application to fluid manipulation for biological analysis, materials synthesis, and chemical process control. Because of their superior benefits such as high-throughput, simplicity, and accurate manipulation, inertial microfluidics designs incorporating channel geometries [...] Read more.
The field of inertial microfluidics has been significantly advanced in terms of application to fluid manipulation for biological analysis, materials synthesis, and chemical process control. Because of their superior benefits such as high-throughput, simplicity, and accurate manipulation, inertial microfluidics designs incorporating channel geometries generating Dean vortexes and helical vortexes have been studied extensively. However, existing technologies have not been studied by designing low-aspect-ratio microchannels to produce multi-vortexes. In this study, an inertial microfluidic device was developed, allowing the generation and regulation of the Dean vortex and helical vortex through the introduction of micro-obstacles in a semicircular microchannel with ultra-low aspect ratio. Multi-vortex formations in the vertical and horizontal planes of four dimension-confined curved channels were analyzed at different flow rates. Moreover, the regulation mechanisms of the multi-vortex were studied systematically by altering the micro-obstacle length and channel height. Through numerical simulation, the regulation of dimensional confinement in the microchannel is verified to induce the Dean vortex and helical vortex with different magnitudes and distributions. The results provide insights into the geometry-induced secondary flow mechanism, which can inspire simple and easily built planar 2D microchannel systems with low-aspect-ratio design with application in fluid manipulations for chemical engineering and bioengineering. Full article
(This article belongs to the Special Issue Inertial Microfluidics)
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Article
Changes of Inertial Focusing Position in a Triangular Channel Depending on Droplet Deformability and Size
Micromachines 2020, 11(9), 839; https://doi.org/10.3390/mi11090839 - 07 Sep 2020
Cited by 3 | Viewed by 710
Abstract
Studies on cell separation with inertial microfluidics are often carried out with solid particles initially. When this condition is applied for actual cell separations, the efficiency typically becomes lower because of the polydispersity and deformability of cells. Therefore, the understanding of deformability-induced lift [...] Read more.
Studies on cell separation with inertial microfluidics are often carried out with solid particles initially. When this condition is applied for actual cell separations, the efficiency typically becomes lower because of the polydispersity and deformability of cells. Therefore, the understanding of deformability-induced lift force is essential to achieve highly efficient cell separation. We investigate the inertial focusing positions of viscous droplets in a triangular channel while varying Re, deformability, and droplet size. With increasing Re and decreasing droplet size, the top focusing position splits and shifts along the sidewalls. The threshold size of the focusing position splitting increases for droplets with larger deformability. The overall path of the focusing position shifts with increasing Re also has a strong dependency on deformability. Consequently, droplets of the same size can have different focusing positions depending on their deformability. The feasibility of deformability-based cell separation is shown by different focusing positions of MCF10a and MCF7 cells. Full article
(This article belongs to the Special Issue Inertial Microfluidics)
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Article
Optimal Control of Colloidal Trajectories in Inertial Microfluidics Using the Saffman Effect
Micromachines 2020, 11(6), 592; https://doi.org/10.3390/mi11060592 - 15 Jun 2020
Cited by 1 | Viewed by 1010
Abstract
In inertial microfluidics colloidal particles in a Poiseuille flow experience the Segré-Silberberg lift force, which drives them to specific positions in the channel cross section. An external force applied along the microchannel induces a cross-streamline migration to a new equilibrium position because of [...] Read more.
In inertial microfluidics colloidal particles in a Poiseuille flow experience the Segré-Silberberg lift force, which drives them to specific positions in the channel cross section. An external force applied along the microchannel induces a cross-streamline migration to a new equilibrium position because of the Saffman effect. We apply optimal control theory to design the time protocol of the axial control force in order to steer a single particle as precisely as possible from a channel inlet to an outlet at a chosen target position. We discuss the influence of particle radius and channel length and show that optimal steering is cheaper than using a constant control force. Using a single optimized control-force protocol, we demonstrate that even a pulse of particles spread along the channel axis can be steered to a target and that particles of different radii can be separarted most efficiently. Full article
(This article belongs to the Special Issue Inertial Microfluidics)
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Article
Investigation on Inertial Sorter Coupled with Magnetophoretic Effect for Nonmagnetic Microparticles
Micromachines 2020, 11(6), 566; https://doi.org/10.3390/mi11060566 - 31 May 2020
Cited by 3 | Viewed by 852
Abstract
The sizes of most prokaryotic cells are several microns. It is very difficult to separate cells with similar sizes. A sorter with a contraction–expansion microchannel and applied magnetic field is designed to sort microparticles with diameters of 3, 4 and 5 microns. To [...] Read more.
The sizes of most prokaryotic cells are several microns. It is very difficult to separate cells with similar sizes. A sorter with a contraction–expansion microchannel and applied magnetic field is designed to sort microparticles with diameters of 3, 4 and 5 microns. To evaluate the sorting efficiency of the designed sorter, numerical simulations for calculating the distributions of microparticles with similar sizes were carried out for various magnetic fields, inlet velocities, sheath flow ratios and structural parameters. The numerical results indicate that micro-particles with diameters of 3, 4 and 5 microns can be sorted efficiently in such a sorter within appropriate parameters. Furthermore, it is shown that a bigger particle size and more powerful magnetic field can result in a greater lateral migration of microparticles. The sorting efficiency of microparticles promotes a lower inlet velocity and greater sheath flow ratios. A smaller contraction–expansion ratio can induce a greater space between particle-bands. Finally, the micro particle image velocity (micro-PIV) experiments were conducted to obtain the bandwidths and spaces between particle-bands. The comparisons between the numerical and experimental results show a good agreement and make the validity of the numerical results certain. Full article
(This article belongs to the Special Issue Inertial Microfluidics)
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Article
Differential Sorting of Microparticles Using Spiral Microchannels with Elliptic Configurations
Micromachines 2020, 11(4), 412; https://doi.org/10.3390/mi11040412 - 14 Apr 2020
Cited by 4 | Viewed by 1119
Abstract
Label-free, size-dependent cell-sorting applications based on inertial focusing phenomena have attracted much interest during the last decade. The separation capability heavily depends on the precision of microparticle focusing. In this study, five-loop spiral microchannels with a height of 90 µm and a width [...] Read more.
Label-free, size-dependent cell-sorting applications based on inertial focusing phenomena have attracted much interest during the last decade. The separation capability heavily depends on the precision of microparticle focusing. In this study, five-loop spiral microchannels with a height of 90 µm and a width of 500 µm are introduced. Unlike their original spiral counterparts, these channels have elliptic configurations of varying initial aspect ratios, namely major axis to minor axis ratios of 3:2, 11:9, 9:11, and 2:3. Accordingly, the curvature of these configurations increases in a curvilinear manner through the channel. The effects of the alternating curvature and channel Reynolds number on the focusing of fluorescent microparticles with sizes of 10 and 20 µm in the prepared suspensions were investigated. At volumetric flow rates between 0.5 and 3.5 mL/min (allowing separation), each channel was tested to collect samples at the designated outlets. Then, these samples were analyzed by counting the particles. These curved channels were capable of separating 20 and 10 µm particles with total yields up to approximately 95% and 90%, respectively. The results exhibited that the level of enrichment and the focusing behavior of the proposed configurations are promising compared to the existing microfluidic channel configurations. Full article
(This article belongs to the Special Issue Inertial Microfluidics)
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Article
High-Efficiency Small Sample Microparticle Fractionation on a Femtosecond Laser-Machined Microfluidic Disc
Micromachines 2020, 11(2), 151; https://doi.org/10.3390/mi11020151 - 30 Jan 2020
Cited by 1 | Viewed by 1223
Abstract
The fabrication and testing of microfluidic spinning compact discs with embedded trapezoidal microchambers for the purpose of inertial microparticle focusing is reported in this article. Microparticle focusing channels require small features that cannot be easily fabricated in acrylic sheets and are complicated to [...] Read more.
The fabrication and testing of microfluidic spinning compact discs with embedded trapezoidal microchambers for the purpose of inertial microparticle focusing is reported in this article. Microparticle focusing channels require small features that cannot be easily fabricated in acrylic sheets and are complicated to realize in glass by traditional lithography techniques; therefore, the fabrication of microfluidic discs with femtosecond laser ablation is reported for the first time in this paper. It could be demonstrated that high-efficiency inertial focusing of 5 and 10 µm particles is achieved in a channel with trapezoidal microchambers regardless of the direction of disc rotation, which correlates to the dominance of inertial forces over Coriolis forces. To achieve the highest throughput possible, the suspension concentration was increased from 0.001% (w/v) to 0.005% (w/v). The focusing efficiency was 98.7% for the 10 µm particles and 93.75% for the 5 µm particles. Full article
(This article belongs to the Special Issue Inertial Microfluidics)
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Review

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Review
Transport of Non-Spherical Particles in Square Microchannel Flows: A Review
Micromachines 2021, 12(3), 277; https://doi.org/10.3390/mi12030277 - 07 Mar 2021
Cited by 1 | Viewed by 683
Abstract
Understanding the behavior of a single particle flowing in a microchannel is a necessary step in designing and optimizing efficient microfluidic devices for the separation, concentration, counting, detecting, sorting, or mixing of particles in suspension. Although the inertial migration of spherical particles has [...] Read more.
Understanding the behavior of a single particle flowing in a microchannel is a necessary step in designing and optimizing efficient microfluidic devices for the separation, concentration, counting, detecting, sorting, or mixing of particles in suspension. Although the inertial migration of spherical particles has been deeply investigated in the last two decades, most of the targeted applications involve shaped particles whose behavior in microflows is still far from being completely understood. While traveling in a channel, a particle both rotates and translates: it translates in the streamwise direction driven by the fluid flow but also in the cross-section perpendicular to the streamwise direction due to inertial effects. In addition, particles’ rotation and translation motions are coupled. Most of the existing works investigating the transport of particles in microchannels decouple their rotational and lateral migration behaviors: particle rotation is mainly studied in simple shear flows, whereas lateral migration is neglected, and studies on lateral migration mostly focus on spherical particles whose rotational behavior is simple. The aim of this review is to provide a summary of the different works existing in the literature on the inertial migration and the rotational behavior of non-spherical particles with a focus and discussion on the remaining scientific challenges in this field. Full article
(This article belongs to the Special Issue Inertial Microfluidics)
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Review
Inertial Microfluidics Enabling Clinical Research
Micromachines 2021, 12(3), 257; https://doi.org/10.3390/mi12030257 - 03 Mar 2021
Cited by 2 | Viewed by 1155
Abstract
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 [...] Read more.
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 downstream 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 streamlines. Several distinct microfluidic channel geometries (e.g., straight, curved, spiral, contraction-expansion 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. Full article
(This article belongs to the Special Issue Inertial Microfluidics)
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