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X-fluidics at the Micro/Nanoscale

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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 (1 November 2022) | Viewed by 23129

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Special Issue Editors

Prof. Dr. Xiangchun Xuan

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Guest Editor

Department of Mechanical Engineering, Clemson University, Clemson, SC 29634, USA
Interests: microfluidics; electrokinetics; magnetofluidics; viscoelasticity
Special Issues, Collections and Topics in MDPI journals

Dr. Hui Zhao

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Guest Editor

Department of Mechanical Engineering, University of Nevada, Las Vegas, NV 89154, USA
Interests: micro and nano fluidics; lab-on-chip technology; nanotechnology and micro/nano energy conversion

Special Issue Information

Lab-on-a-chip devices have the potential to revolutionalize point-of-care technologices. They rely on the transport and control of liquid samples (e.g., chemical solutions and biologial fluids) in micro/nano systems. A variety of force fields have been demonstrated to accomplish this task, ranging from the pressure field (e.g., inertial microfluidics) to electric (via electrokinetic phemonema), acoustic (i.e., acoustofluidics), magnetic (i.e., magnetofluidics), optical (i.e., optofluidics), acceleration (e.g., centrifugal microfluidics), and surface tension (e.g., capillary and Marrangoni flow) fields, etc. We would like to invite you to contribute to this Special Issue of Micromachines original research or review articles on the diverse manipulation of fluids and samples (e.g., particles and cells) in micro/nano systems using any kind of force (not necessarily limited to those mentioned above). Both experimental and theoretical (inculding numerical) works are welcome on either the fundamentals or applications of X-fluidics at the micro/nanoscale.

Prof. Dr. Xiangchun Xuan
Dr. Hui Zhao
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 submissions that pass pre-check are 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 2600 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

Electrokinetic microfluidics
Acoustofluidics
Optofluidics
Magnetofluidics
Inertial microfluidics
Capillary microfluidics
Centrifugal microfluidics

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Micro/Nano-Chip Electrokinetics, Volume II in Micromachines (23 articles)
Micro/Nano-Chip Electrokinetics, Volume III in Micromachines (16 articles)

Published Papers (10 papers)

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Research

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17 pages, 7694 KiB

Open AccessArticle

Analysis of Sequential Micromixing Driven by Sinusoidally Shaped Induced-Charge Electroosmotic Flow

by Haizhen Sun, Ziyi Li, Yongji Wu, Xinjian Fan, Minglu Zhu, Tao Chen and Lining Sun

Micromachines 2022, 13(11), 1985; https://doi.org/10.3390/mi13111985 - 16 Nov 2022

Cited by 1 | Viewed by 1421

Abstract

Multi-fluid micromixing, which has rarely been explored, typically represents a highly sought-after technique in on-chip biochemical and biomedical assays. Herein, we propose a novel micromixing approach utilizing induced-charge electroosmosis (ICEO) to implement multicomplex mixing between parallel streams. The variations of ICEO microvortices above a sinusoidally shaped floating electrode (SSFE) are first investigated to better understand the microvortex development and the resultant mixing process within a confined channel. On this basis, a mathematical model of the vortex index is newly developed to predict the mixing degree along the microchannel. The negative exponential distribution obtained between the vortex index and mixing index demonstrates an efficient model to describe the mixing performance without solving the coupled diffusion and momentum equations. Specifically, sufficient mixing with a mixing index higher than 0.9 can be achieved when the vortex index exceeds 51, and the mixing efficiency reaches a plateau at an AC frequency close to 100 Hz. Further, a rectangle floating electrode (RFE) is deposited before SSFE to enhance the controlled sequence for three-fluid mixing. One side fluid can fully mix with the middle fluid with a mixing index of 0.623 above RFE in the first mixing stage and achieve entire-channel mixing with a mixing index of 0.983 above SSFE in the second mixing stage, thereby enabling on-demand sequential mixing. As a proof of concept, this work can provide a robust alternative technique for multi-objective issues and structural design related to mixers. Full article

(This article belongs to the Special Issue X-fluidics at the Micro/Nanoscale)

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13 pages, 2974 KiB

Open AccessArticle

Electrochemical Detection of Ascorbic Acid in Finger-Actuated Microfluidic Chip

by Xing Liu, Mi Li, Jiahui Zheng, Xiaoling Zhang, Junyi Zeng, Yanjian Liao, Jian Chen, Jun Yang, Xiaolin Zheng and Ning Hu

Micromachines 2022, 13(9), 1479; https://doi.org/10.3390/mi13091479 - 06 Sep 2022

Cited by 5 | Viewed by 2270

Abstract

The traditional quantitative analysis methods of ascorbic acid (AA), which require expensive equipment, a large amount of samples and professional technicians, are usually complex and time-consuming. A low-cost and high-efficiency AA detection device is reported in this work. It integrates a three-electrode sensor module prepared by screen printing technology, and a microfluidic chip with a finger-actuated micropump peeled from the liquid-crystal display (LCD) 3D printing resin molds. The AA detection process on this device is easy to operate. On-chip detection has been demonstrated to be 2.48 times more sensitive than off-chip detection and requires only a microliter-scale sample volume, which is much smaller than that required in traditional electrochemical methods. Experiments show that the sample and buffer can be fully mixed in the microchannel, which is consistent with the numerical simulation results wherein the mixing efficiency is greater than 90%. Commercially available tablets and beverages are also tested, and the result shows the reliability and accuracy of the device, demonstrating its broad application prospects in the field of point-of-care testing (POCT). Full article

(This article belongs to the Special Issue X-fluidics at the Micro/Nanoscale)

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11 pages, 2656 KiB

Open AccessArticle

Growth of Laser-Induced Microbubbles inside Capillary Tubes Affected by Gathered Light-Absorbing Particles

by Jia-Wen He, Hao-Dong Wang, Bo-Wei Li, Wen Bai, Dong Chen and Min-Cheng Zhong

Micromachines 2022, 13(5), 740; https://doi.org/10.3390/mi13050740 - 06 May 2022

Cited by 2 | Viewed by 1609

Abstract

Microbubbles have important applications in optofluidics. The generation and growth of microbubbles is a complicated process in microfluidic channels. In this paper, we use a laser to irradiate light-absorbing particles to generate microbubbles in capillary tubes and investigate the factors affecting microbubble size. The results show that the key factor is the total area of the light-absorbing particles gathered at the microbubble bottom. The larger the area of the particles at bottom, the larger the size of the microbubbles. Furthermore, the area is related to capillary tube diameter. The larger the diameter of the capillary tube, the more particles gathered at the bottom of the microbubbles. Numerical simulations show that the Marangoni convection is stronger in a capillary tube with a larger diameter, which can gather more particles than that in a capillary tube with a smaller diameter. The calculations show that the particles in contact with the microbubbles will be in a stable position due to the surface tension force. Full article

(This article belongs to the Special Issue X-fluidics at the Micro/Nanoscale)

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21 pages, 3936 KiB

Open AccessArticle

Electroosmotic Flow Hysteresis for Fluids with Dissimilar pH and Ionic Species

by An Eng Lim and Yee Cheong Lam

Micromachines 2021, 12(9), 1031; https://doi.org/10.3390/mi12091031 - 28 Aug 2021

Cited by 13 | Viewed by 1650

Abstract

Electroosmotic flow (EOF) involving displacement of multiple fluids is employed in micro-/nanofluidic applications. There are existing investigations on EOF hysteresis, i.e., flow direction-dependent behavior. However, none so far have studied the solution pair system of dissimilar ionic species with substantial pH difference. They exhibit complicated hysteretic phenomena. In this study, we investigate the EOF of sodium bicarbonate (NaHCO₃, alkaline) and sodium chloride (NaCl, slightly acidic) solution pair via current monitoring technique. A developed slip velocity model with a modified wall condition is implemented with finite element simulations. Quantitative agreements between experimental and simulation results are obtained. Concentration evolutions of NaHCO₃–NaCl follow the dissimilar anion species system. When NaCl displaces NaHCO₃, EOF reduces due to the displacement of NaHCO₃ with high pH (high absolute zeta potential). Consequently, NaCl is not fully displaced into the microchannel. When NaHCO₃ displaces NaCl, NaHCO₃ cannot displace into the microchannel as NaCl with low pH (low absolute zeta potential) produces slow EOF. These behaviors are independent of the applied electric field. However, complete displacement tends to be achieved by lowering the NaCl concentration, i.e., increasing its zeta potential. In contrast, the NaHCO₃ concentration has little impact on the displacement process. These findings enhance the understanding of EOF involving solutions with dissimilar pH and ion species. Full article

(This article belongs to the Special Issue X-fluidics at the Micro/Nanoscale)

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19 pages, 14888 KiB

Open AccessArticle

Flow of Non-Newtonian Fluids in a Single-Cavity Microchannel

by Mahmud Kamal Raihan, Purva P. Jagdale, Sen Wu, Xingchen Shao, Joshua B. Bostwick, Xinxiang Pan and Xiangchun Xuan

Micromachines 2021, 12(7), 836; https://doi.org/10.3390/mi12070836 - 18 Jul 2021

Cited by 14 | Viewed by 3973

Abstract

Having a basic understanding of non-Newtonian fluid flow through porous media, which usually consist of series of expansions and contractions, is of importance for enhanced oil recovery, groundwater remediation, microfluidic particle manipulation, etc. The flow in contraction and/or expansion microchannel is unbounded in the primary direction and has been widely studied before. In contrast, there has been very little work on the understanding of such flow in an expansion–contraction microchannel with a confined cavity. We investigate the flow of five types of non-Newtonian fluids with distinct rheological properties and water through a planar single-cavity microchannel. All fluids are tested in a similarly wide range of flow rates, from which the observed flow regimes and vortex development are summarized in the same dimensionless parameter spaces for a unified understanding of the effects of fluid inertia, shear thinning, and elasticity as well as confinement. Our results indicate that fluid inertia is responsible for developing vortices in the expansion flow, which is trivially affected by the confinement. Fluid shear thinning causes flow separations on the contraction walls, and the interplay between the effects of shear thinning and inertia is dictated by the confinement. Fluid elasticity introduces instability and asymmetry to the contraction flow of polymers with long chains while suppressing the fluid inertia-induced expansion flow vortices. However, the formation and fluctuation of such elasto-inertial fluid vortices exhibit strong digressions from the unconfined flow pattern in a contraction–expansion microchannel of similar dimensions. Full article

(This article belongs to the Special Issue X-fluidics at the Micro/Nanoscale)

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12 pages, 20086 KiB

Open AccessArticle

Fine-Tuning Electrokinetic Injections Considering Nonlinear Electrokinetic Effects in Insulator-Based Devices

by Abbi Miller, Nicole Hill, Kel Hakim and Blanca H. Lapizco-Encinas

Micromachines 2021, 12(6), 628; https://doi.org/10.3390/mi12060628 - 28 May 2021

Cited by 8 | Viewed by 2014

Abstract

The manner of sample injection is critical in microscale electrokinetic (EK) separations, as the resolution of a separation greatly depends on sample quality and how the sample is introduced into the system. There is a significant wealth of knowledge on the development of EK injection methodologies that range from simple and straightforward approaches to sophisticated schemes. The present study focused on the development of optimized EK sample injection schemes for direct current insulator-based EK (DC-iEK) systems. These are microchannels that contain arrays of insulating structures; the presence of these structures creates a nonuniform electric field distribution when a potential is applied, resulting in enhanced nonlinear EK effects. Recently, it was reported that the nonlinear EK effect of electrophoresis of the second kind plays a major role in particle migration in DC-iEK systems. This study presents a methodology for designing EK sample injection schemes that consider the nonlinear EK effects exerted on the particles being injected. Mathematical modeling with COMSOL Multiphysics was employed to identify proper voltages to be used during the EK injection process. Then, a T-microchannel with insulating posts was employed to experimentally perform EK injection and separate a sample containing two types of similar polystyrene particles. The quality of the EK injections was assessed by comparing the resolution (Rs) and number of plates (N) of the experimental particle separations. The findings of this study establish the importance of considering nonlinear EK effects when planning for successful EK injection schemes. Full article

(This article belongs to the Special Issue X-fluidics at the Micro/Nanoscale)

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13 pages, 4055 KiB

Open AccessArticle

A Continuous Cell Separation and Collection Approach on a Microfilter and Negative Dielectrophoresis Combined Chip

by Qiong Wang, Xiaoling Zhang, Danfen Yin, Jinan Deng, Jun Yang and Ning Hu

Micromachines 2020, 11(12), 1037; https://doi.org/10.3390/mi11121037 - 26 Nov 2020

Cited by 8 | Viewed by 1751

Abstract

Cell separation plays an important role in the fields of analytical chemistry and biomedicine. To solve the blockage problem and improve the separation throughput in the traditional microstructure filtration-based separation approach, a continuous cell separation and collection approach via micropost array railing on a microfilter and negative dielectrophoresis combined chip is proposed. By tilting the micropost array at a certain angle, microparticles or cells enter the collection area under micropost array railing. The effects of the inclination angle of the micropost array and the electrode distance on the microparticle collection efficiency were investigated. Based on the optimized microfluidic chip structure, 37- and 16.3-μm particles were collected with 85% and 89% efficiencies, respectively. Additionally, algal cells were separated and collected by using the optimized microchip. The chip also had good separation and collection effects on biological samples, which effectively solved the blockage problem and improved the separation throughput, laying a foundation for subsequent microstructure filtration separation-based research and application. Full article

(This article belongs to the Special Issue X-fluidics at the Micro/Nanoscale)

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17 pages, 6043 KiB

Open AccessArticle

Numerical Investigation of Nanostructure Orientation on Electroosmotic Flow

by An Eng Lim and Yee Cheong Lam

Micromachines 2020, 11(11), 971; https://doi.org/10.3390/mi11110971 - 29 Oct 2020

Cited by 13 | Viewed by 1814

Abstract

Electroosmotic flow (EOF) is fluid flow induced by an applied electric field, which has been widely employed in various micro-/nanofluidic applications. Past investigations have revealed that the presence of nanostructures in microchannel reduces EOF. Hitherto, the angle-dependent behavior of nanoline structures on EOF has not yet been studied in detail and its understanding is lacking. Numerical analyses of the effect of nanoline orientation angle θ on EOF to reveal the associated mechanisms were conducted in this investigation. When θ increases from 5° to 90° (from parallel to perpendicular to the flow direction), the average EOF velocity decreases exponentially due to the increase in distortion of the applied electric field distribution at the structured surface, as a result of the increased apparent nanolines per unit microchannel length. With increasing nanoline width W, the decrease of average EOF velocity is fairly linear, attributed to the simultaneous narrowing of nanoline ridge (high local fluid velocity region). While increasing nanoline depth D results in a monotonic decrease of the average EOF velocity. This reduction stabilizes for aspect ratio D/W > 0.5 as the electric field distribution distortion within the nanoline trench remains nearly constant. This investigation reveals that the effects on EOF of nanolines, and by extrapolation for any nanostructures, may be directly attributed to their effects on the distortion of the applied electric field distribution within a microchannel. Full article

(This article belongs to the Special Issue X-fluidics at the Micro/Nanoscale)

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18 pages, 9797 KiB

Open AccessArticle

Modifying Wicking Speeds in Paper-Based Microfluidic Devices by Laser-Etching

by Brent Kalish, Mick Kyle Tan and Hideaki Tsutsui

Micromachines 2020, 11(8), 773; https://doi.org/10.3390/mi11080773 - 14 Aug 2020

Cited by 15 | Viewed by 3305

Abstract

Paper-based microfluidic devices are an attractive platform for developing low-cost, point-of-care diagnostic tools. As paper-based devices’ detection chemistries become more complex, more complicated devices are required, often entailing the sequential delivery of different liquids or reagents to reaction zones. Most research into flow control has been focused on introducing delays. However, delaying the flow can be problematic due to increased evaporation leading to sample loss. We report the use of a CO₂ laser to uniformly etch the surface of the paper to modify wicking speeds in paper-based microfluidic devices. This technique can produce both wicking speed increases of up to 1.1× faster and decreases of up to 0.9× slower. Wicking speeds can be further enhanced by etching both sides of the paper, resulting in wicking 1.3× faster than unetched channels. Channels with lengthwise laser-etched grooves were also compared to uniformly etched channels, with the most heavily grooved channels wicking 1.9× faster than the fastest double-sided etched channels. Furthermore, sealing both sides of the channel in packing tape results in the most heavily etched channels, single-sided, double-sided, and grooved, wicking over 13× faster than unetched channels. By selectively etching individual channels, different combinations of sequential fluid delivery can be obtained without altering any channel geometry. Laser etching is a simple process that can be integrated into the patterning of the device and requires no additional materials or chemicals, enabling greater flow control for paper-based microfluidic devices. Full article

(This article belongs to the Special Issue X-fluidics at the Micro/Nanoscale)

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Review

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20 pages, 2219 KiB

Open AccessFeature PaperEditor’s ChoiceReview

Autonomously Propelled Colloids for Penetration and Payload Delivery in Complex Extracellular Matrices

by Shrishti Singh and Jeffrey L. Moran

Micromachines 2021, 12(10), 1216; https://doi.org/10.3390/mi12101216 - 06 Oct 2021

Cited by 1 | Viewed by 2301

Abstract

For effective treatment of diseases such as cancer or fibrosis, it is essential to deliver therapeutic agents such as drugs to the diseased tissue, but these diseased sites are surrounded by a dense network of fibers, cells, and proteins known as the extracellular matrix (ECM). The ECM forms a barrier between the diseased cells and blood circulation, the main route of administration of most drug delivery nanoparticles. Hence, a stiff ECM impedes drug delivery by limiting the transport of drugs to the diseased tissue. The use of self-propelled particles (SPPs) that can move in a directional manner with the application of physical or chemical forces can help in increasing the drug delivery efficiency. Here, we provide a comprehensive look at the current ECM models in use to mimic the in vivo diseased states, the different types of SPPs that have been experimentally tested in these models, and suggest directions for future research toward clinical translation of SPPs in diverse biomedical settings. Full article

(This article belongs to the Special Issue X-fluidics at the Micro/Nanoscale)

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