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Interfaces in Microfluidics

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

Deadline for manuscript submissions: closed (31 October 2022) | Viewed by 25861

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Special Issue Editors

Dr. Sangjin Ryu

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Guest Editor

Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0526, USA
Interests: interfacial fluid dynamics; microfluidics; cell mechanics; experimental fluid mechanics
Special Issues, Collections and Topics in MDPI journals

Dr. Moeto Nagai

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Guest Editor

Department of Mechanical Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan
Interests: single-cell processing; intracellular delivery; bioprinting; microorganisms-driven system

Dr. Seunghee Kim

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Guest Editor

Department of Civil and Environmental Engineering, University of Nebraska-Lincoln, Omaha, NE 68182-0178, USA
Interests: fluid flow in porous media; poroelasticity; carbon utilization and storage; energy geo-storage

Special Issue Information

Dear Colleagues,

Microfluidics has found numerous applications in various fields, including multiphase fluid dynamics, biomedical engineering, geosciences, petroleum engineering, and geotechnical engineering. One common aspect of these microfluidics systems is that they involve microscale interfaces that may develop between fluids (e.g., oil drops in water for enhanced oil recovery; air bubbles in water during compressed air energy storage; water-CO₂ for geologic carbon storage) or between liquids and solids (e.g., interfaces between fluids and microchannels, small particles, or microorganisms). The understanding of the interface has expanded the range of applications and it is desirable to understand and advance interfacial phenomena.

For this Special Issue, microfluidics is broadly defined to encompass traditional microchannel devices as well as various systems involving microscale transport phenomena. In this Special Issue, we aim to collate relevant studies to highlight recent progress and emerging challenges with “Interfaces in Microfluidics” and thus to inform readers of the current status-of-the-art. Some subject examples for this Special Issue are:

Hele-Shaw cell for interfacial fluid dynamics
Droplet dispensing for thermal inkjet printing
Micro-droplets as micro-reactors
Multiphase transport in biological systems
Microvalves and micropumps
Interface in microfluidics for energy conversion, fuel cells, geologic energy storage, energy-geotechnics, and geologic carbon/waste sequestration

Original research papers, short communications, and review articles on theoretical, numerical, and experimental progress are welcomed. We look forward to your submissions.

Dr. Sangjin Ryu
Dr. Moeto Nagai
Dr. Seunghee Kim
Guest Editors

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Keywords

Interfacial transport phenomena
Microorganisms
Droplet microfluidics
Particulate and multiphase flow phenomena
Electrokinesis
Fluid flow in porous media
Hydro-thermo-chemo-mechanical coupling
Particle science and technology

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Published Papers (11 papers)

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Research

18 pages, 4069 KiB

Open AccessArticle

Numerical Simulation of a Lab-on-Chip for Dielectrophoretic Separation of Circulating Tumor Cells

by Abdallah M. Alkhaiyat and Mohamed Badran

Micromachines 2023, 14(9), 1769; https://doi.org/10.3390/mi14091769 - 15 Sep 2023

Cited by 2 | Viewed by 1539

Abstract

Circulating tumor cells (CTCs) are cancer cells detached from tumors that enter the bloodstream with the rest of the blood cells before settling on remote organs and growing. CTCs play a major role as a target for cancer diagnosis. This study aims to propose and simulate a lab-on-chip (LOC) design that separates CTCs from white blood cells (WBCs) and blood platelets (PLTs) using low-voltage dielectrophoretic separation with high efficiency. The proposed design include two stages a passive and an active one cascaded in a compact package. Numerical simulations are performed on the COMSOL Multiphysics^® software package to optimize the geometric parameters of the LOC, such as the width and length of the microchannel and the number of electrodes and their arrangements. Moreover, the effects of adjusting the applied voltage values as well as buffer inlet velocity are investigated. The proposed LOC design uses four electrodes at ±2 V to achieve 100% separation efficiency for the three cell types in simulation. The 919 µm × 440 µm LOC has a channel width of 40 µm. The inlet velocities for the blood-carrying cells and buffer are 134 and 850 µm/s, respectively. The proposed LOC can be used for the early detection of CTCs, which can be beneficial in cancer diagnosis and early treatment. In addition, it can be used in cancer prognosis, treatment monitoring and personalizing medicine. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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18 pages, 7827 KiB

Open AccessArticle

Experimental Study: The Effect of Pore Shape, Geometrical Heterogeneity, and Flow Rate on the Repetitive Two-Phase Fluid Transport in Microfluidic Porous Media

by Seunghee Kim, Jingtao Zhang and Sangjin Ryu

Micromachines 2023, 14(7), 1441; https://doi.org/10.3390/mi14071441 - 18 Jul 2023

Cited by 2 | Viewed by 1451

Abstract

Geologic subsurface energy storage, such as porous-media compressed-air energy storage (PM-CAES) and underground hydrogen storage (UHS), involves the multi-phase fluid transport in structurally disordered or heterogeneous porous media (e.g., soils and rocks). Furthermore, such multi-phase fluid transport is likely to repeatedly occur due to successive fluid injections and extractions, thus, resulting in cyclic drainage–imbibition processes. To complement our preceding study, we conducted a follow-up study with microfluidic pore-network devices with a square solid shape (Type II) to further advance our understanding on the effect of the pore shape (aspect ratio, Type I: 5–6 > Type II: ~1), pore-space heterogeneity (coefficient of variation, COV = 0, 0.25, and 0.5), and flow rates (Q = 0.01 and 0.1 mL/min) on the repetitive two-phase fluid flow in general porous media. The influence of pore shape and pore-space heterogeneity were observed to be more prominent when the flow rate was low (e.g., Q = 0.01 mL/min in this study) on the examined outcomes, including the drainage and imbibition patterns, the similarity of those patterns between repeated steps, the sweep efficiency and residual saturation of the nonwetting fluid, and fluid pressure. On the other hand, a higher flow rate (e.g., Q = 0.1 mL/min in this study) appeared to outweigh those factors for the Type II structure, owing to the low aspect ratio (~1). It was also suggested that the flow morphology, sweep efficiency, residual saturation, and required pressure gradient may not severely fluctuate during the repeated drainage-–imbibition processes; instead, becoming stabilized after 4–5 cycles, regardless of the aspect ratio, COV, and Q. Implications of the study results for PM-CAES and UHS are discussed as a complementary analysis at the end of this manuscript. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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13 pages, 3566 KiB

Open AccessArticle

Nearly Reversible Expansion and Shrinkage of Casein Microparticles Triggered by Extreme pH Changes

by Ronald Gebhardt, Thomas Pütz and Jann Schulte

Micromachines 2023, 14(3), 678; https://doi.org/10.3390/mi14030678 - 19 Mar 2023

Cited by 2 | Viewed by 1759

Abstract

Solvent flows in the fL/s range across the total surface of a casein microparticle cause its expansion and shrinkage. Microparticles prepared from the milk protein casein have a porous and flexible inner structure with water-filled channels and cavities. Solvent uptake occurs in two phases and results in disintegration if de-swelling is not triggered by acidification. So far, nothing is known about the reversibility of the swelling/de-swelling steps. We performed pH jump experiments between pH 11 and pH 1 on a single micro-particle and analyzed the swelling-induced size changes with system dynamics modeling. Both the swelling steps and the subsequent de-swelling process proceed reversibly and at an unchanged rate over a sequence of at least three pH exchange cycles. We observed that the duration of the first swelling step increased during the sequence, while the second step became shorter. Both of the time intervals are negatively correlated, while a statistical evaluation of only one swelling cycle for an ensemble of microparticles with different stabilities did not reveal any significant correlation between the two parameters. Our results indicate that the pH-induced swelling/shrinkage of casein microparticles is, to a large extent, reversible and only slightly influenced by the acid-induced decomposition of colloidal calcium phosphate. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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24 pages, 12442 KiB

Open AccessArticle

Mixing Performance of a Planar Asymmetric Contraction-and-Expansion Micromixer

by Daigo Natsuhara, Ryogo Saito, Shunya Okamoto, Moeto Nagai and Takayuki Shibata

Micromachines 2022, 13(9), 1386; https://doi.org/10.3390/mi13091386 - 25 Aug 2022

Cited by 5 | Viewed by 3831

Abstract

Micromixers are one of the critical components in microfluidic devices. They significantly affect the efficiency and sensitivity of microfluidics-based lab-on-a-chip systems. This study introduces an efficient micromixer with a simple geometrical feature that enables easy incorporation in a microchannel network without compromising the original design of microfluidic devices. The study proposes a newly designed planar passive micromixer, termed a planar asymmetric contraction-and-expansion (P-ACE) micromixer, with asymmetric vertical obstacle structures. Numerical simulation and experimental investigation revealed that the optimally designed P-ACE micromixer exhibited a high mixing efficiency of 80% or more within a microchannel length of 10 mm over a wide range of Reynolds numbers (0.13 ≤ Re ≤ 13), eventually attaining approximately 90% mixing efficiency within a 20 mm microchannel length. The highly asymmetric geometric features of the P-ACE micromixers enhance mixing because of their synergistic effects. The flow velocities and directions of the two fluids change differently while alternately crossing the longitudinal centerline of the microchannel, with the obstacle structures asymmetrically arranged on both sidewalls of the rectangular microchannel. This flow behavior increases the interfacial contact area between the two fluids, thus promoting effective mixing in the P-ACE micromixer. Further, the pressure drops in the P-ACE micromixers were experimentally investigated and compared with those in a serpentine micromixer with a perfectly symmetric mixing unit. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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17 pages, 3080 KiB

Open AccessArticle

Enhancing Mixing Performance in a Rotating Disk Mixing Chamber: A Quantitative Investigation of the Effect of Euler and Coriolis Forces

by Jihyeong Lee, Saebom Lee, Minki Lee, Ritesh Prakash, Hyejeong Kim, Gyoujin Cho and Jinkee Lee

Micromachines 2022, 13(8), 1218; https://doi.org/10.3390/mi13081218 - 29 Jul 2022

Cited by 7 | Viewed by 2616

Abstract

Lab-on-a-CD (LOCD) is gaining importance as a diagnostic platform due to being low-cost, easy-to-use, and portable. During LOCD usage, mixing and reaction are two processes that play an essential role in biochemical applications such as point-of-care diagnosis. In this paper, we numerically and experimentally investigate the effects of the Coriolis and Euler forces in the mixing chamber during the acceleration and deceleration of a rotating disk. The mixing performance is investigated under various conditions that have not been reported, such as rotational condition, chamber aspect ratio at a constant volume, and obstacle arrangement in the chamber. During disk acceleration and deceleration, the Euler force difference in the radial direction causes rotating flows, while the Coriolis force induces perpendicular vortices. Increasing the maximum rotational velocity improves the maximum rotational displacement, resulting in better mixing performance. A longer rotational period increases the interfacial area between solutions and enhances mixing. Mixing performance also improves when there is a substantial difference between Euler forces at the inner and outer radii. Furthermore, adding obstacles in the angular direction also passively promotes or inhibits mixing by configuration. This quantitative investigation provides valuable information for designing and developing high throughput and multiplexed point-of-care LOCDs. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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10 pages, 1895 KiB

Open AccessArticle

The Dark Annulus of a Drop in a Hele-Shaw Cell Is Caused by the Refraction of Light through Its Meniscus

by Sangjin Ryu, Haipeng Zhang and Carson Emeigh

Micromachines 2022, 13(7), 1021; https://doi.org/10.3390/mi13071021 - 28 Jun 2022

Cited by 1 | Viewed by 1496

Abstract

Knowing the meniscus shape of confined drops is important for understanding how they make first contact and then coalesce. When imaged from the top view by brightfield microscopy, a liquid drop (e.g., corn syrup) confined in a Hele-Shaw cell, surrounded by immiscible liquid (e.g., mineral oil), had a dark annulus, and the width of the annulus decreased with increasing concentration of corn syrup. Since the difference in the annulus width was presumed to be related to the meniscus shape of the drops, three-dimensional images of the drops with different concentrations were obtained using confocal fluorescence microscopy, and their cross-sectional meniscus profile was determined by image processing. The meniscus of the drops remained circular despite varying concentration. Since the refractive index of corn syrup increased with concentration, while the surface tension coefficient between corn syrup and mineral oil remained unchanged, the observed change in the annulus width was then attributed to the refraction of light passing through the drop’s meniscus. As such, a ray optics model was developed, which predicted that the annulus width of the drop would decrease as the refractive index of the drop approached that of the surrounding liquid. Therefore, the dark annulus of the drops in the Hele-Shaw cell was caused by the refraction of light passing through the circular meniscus of the drop. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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18 pages, 4498 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

The Thickness and Structure of Dip-Coated Polymer Films in the Liquid and Solid States

by Zhao Zhang, Fei Peng and Konstantin G. Kornev

Micromachines 2022, 13(7), 982; https://doi.org/10.3390/mi13070982 - 22 Jun 2022

Cited by 11 | Viewed by 3023

Abstract

Films formed by dip coating brass wires with dilute and semi-dilute solutions of polyvinyl butyral in benzyl alcohol were studied in their liquid and solid states. While dilute and semi-dilute solutions behaved as Maxwell viscoelastic fluids, the thickness of the liquid films followed the Landau-Levich-Derjaguin prediction for Newtonian fluids. At a very slow rate of coating, the film thickness was difficult to evaluate. Therefore, the dynamic contact angle was studied in detail. We discovered that polymer additives preserve the advancing contact angle at its static value while the receding contact angle follows the Cox–Voinov theory. In contrast, the thickness of solid films does not correlate with the Landau-Levich-Derjaguin predictions. Only solutions of high-molecular-weight polymers form smooth solid films. Solutions of low-molecular-weight polymers may form either solid films with an inhomogeneous roughness or solid polymer domains separated by the dry substrate. In technological applications, very dilute polymer solutions of high-molecular-weight polymers can be used to avoid inhomogeneities in solid films. These solutions form smooth solid films, and the film thickness can be controlled by the experimental coating conditions. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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15 pages, 8741 KiB

Open AccessArticle

Investigation of Multiphase Flow in a Trifurcation Microchannel—A Benchmark Problem

by Eugen Chiriac, Marioara Avram and Corneliu Balan

Micromachines 2022, 13(6), 974; https://doi.org/10.3390/mi13060974 - 20 Jun 2022

Cited by 4 | Viewed by 2594

Abstract

The evolution of an interface between two immiscible liquids in a three-branch symmetric microchannel is numerically and experimentally investigated. The main goals of the paper are to correlate the numeric data with the experimental results for the tested flow case and to assess the quality of the VOF procedure to trace the interface using the Fluent commercial code. The focus of the experiments was to characterize the dynamics of the oil–water interface formed in the vicinity of the bifurcation, at the entrance in the main microchannel of 400 microns width and 50 microns height. The oil core surrounded by water is visualized and micro-PIV measurements are performed in water. Experimental results qualitatively and quantitatively confirm the 3D numerical simulations. We propose the present investigated flow as a benchmark case for the study of the interface in a branching microchannel geometry. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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15 pages, 3859 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Contributions of Red Blood Cell Sedimentation in a Driving Syringe to Blood Flow in Capillary Channels

by Yang Jun Kang

Micromachines 2022, 13(6), 909; https://doi.org/10.3390/mi13060909 - 8 Jun 2022

Cited by 5 | Viewed by 2702

Abstract

The erythrocyte sedimentation rate (ESR), which has been commonly used to detect physiological and pathological diseases in clinical settings, has been quantified using an interface in a vertical tube. However, previous methods do not provide biophysical information on blood during the ESR test. Therefore, it is necessary to quantify the individual contributions in terms of viscosity and pressure. In this study, to quantify RBC sedimentation, the image intensity (I_b) and interface (β) were obtained by analyzing the blood flow in the microfluidic channels. Based on threshold image intensity, the corresponding interfaces of RBCs (I_b > 0.15) and diluent (I_b < 0.15) were employed to obtain the viscosities (µ_b, µ₀) and junction pressures (P_b, P₀). Two coefficients (CH₁, CH₂) obtained from the empirical formulas (µ_b = µ₀ [1 + CH₁], P_b = P₀ [1 + CH₂]) were calculated to quantify RBC sedimentation. The present method was then adopted to detect differences in RBC sedimentation for various suspended blood samples (healthy RBCs suspended in dextran solutions or plasma). Based on the experimental results, four parameters (µ₀, P₀, CH₁, and CH₂) are considered to be effective for quantifying the contributions of the hematocrit and diluent. Two coefficients exhibited more consistent trends than the conventional ESR method. In conclusion, the proposed method can effectively detect RBC sedimentation. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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13 pages, 20026 KiB

Open AccessArticle

Removing Gas from a Closed-End Small Hole by Irradiating Acoustic Waves with Two Frequencies

by Yuta Matsumoto, Yuki Mizushima and Toshiyuki Sanada

Micromachines 2022, 13(1), 109; https://doi.org/10.3390/mi13010109 - 10 Jan 2022

Cited by 2 | Viewed by 1494

Abstract

Filling microstructures in the air with liquid or removing trapped gases from a surface in a liquid are required in processes such as cleaning, bonding, and painting. However, it is difficult to deform the gas–liquid interface to fill a small hole with liquid when surface tension has closed one end. Therefore, it is necessary to have an efficient method of removing gas from closed-end holes in liquids. Here, we demonstrate the gas-removing method using acoustic waves from small holes. We observed gas column oscillation by changing the hole size, wettability, and liquid surface tension to clarify the mechanism. First, we found that combining two different frequencies enabled complete gas removal in water within 2 s. From high-speed observation, about half of the removal was dominated by droplet or film formation caused by oscillating the gas column. The other half was dominated by approaching and coalescing the divided gas column. We conclude that the natural frequency of both the air column and the bubbles inside the tube are important. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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13 pages, 1988 KiB

Open AccessArticle

Electrostatic Potential Analysis in Polyelectrolyte Brush-Grafted Microchannels Filled with Polyelectrolyte Dispersion

by Byoungjin Chun and Myung-Suk Chun

Micromachines 2021, 12(12), 1475; https://doi.org/10.3390/mi12121475 - 29 Nov 2021

Cited by 1 | Viewed by 1927

Abstract

In this study, the model framework that includes almost all relevant parameters of interest has been developed to quantify the electrostatic potential and charge density occurring in microchannels grafted with polyelectrolyte brushes and simultaneously filled with polyelectrolyte dispersion. The brush layer is described by the Alexander-de Gennes model incorporated with the monomer distribution function accompanying the quadratic decay. Each ion concentration due to mobile charges in the bulk and fixed charges in the brush layer can be determined by multi-species ion balance. We solved 2-dimensional Poisson–Nernst–Planck equations adopted for simulating electric field with ion transport in the soft channel, by considering anionic polyelectrolyte of polyacrylic acid (PAA). Remarkable results were obtained regarding the brush height, ionization, electrostatic potential, and charge density profiles with conditions of brush, dispersion, and solution pH. The Donnan potential in the brush channel shows several times higher than the surface potential in the bare channel, whereas it becomes lower with increasing PAA concentration. Our framework is fruitful to provide comparative information regarding electrostatic interaction properties, serving as an important bridge between modeling and experiments, and is possible to couple with governing equations for flow field. Full article

(This article belongs to the Special Issue Interfaces in Microfluidics)

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