Special Issue "Micro/Nano-Chip Electrokinetics, Volume II"

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

Deadline for manuscript submissions: closed (30 April 2018).

Special Issue Editors

Guest Editor
Prof. Dr. Xiangchun Xuan

Department of Mechanical Engineering, Clemson University, Clemson, SC 29634, USA
Website | E-Mail
Interests: microfluidics; electrokinetics; magnetofluidics; viscoelasticity
Guest Editor
Prof. Dr. Shizhi Qian

Department of Mechanical and Aerospace Engineering, Old Dominion University, Norfolk, VA 23529, USA
Website | E-Mail
Interests: micro/nanofluidics; electrokinetics; transport phenomena in micro and nanoscales

Special Issue Information

Dear Colleagues,

Micro/nanofluidic chips have found increasing applications in the analysis of chemical and biological samples over the past two decades. Electrokinetics has become the method of choice in these micro/nano-chips for transporting, manipulating and sensing ions, (bio)molecules, fluids and (bio)particles, etc., due to the high maneuverability, scalability, sensitivity, and integrability. The involved phenomena, which cover electroosmosis, electrophoresis, dielectrophoresis, electrohydrodynamics, electrothermal flow, diffusioosmosis, diffusiophoresis, streaming potential, current, etc., arise from either the inherent or the induced surface charge on the solid-liquid interface under DC and/or AC electric fields. To review the state-of-the-art of micro/nanochip electrokinetics, we welcome, in this Special Issue of Micromachines, all original research or review articles on the fundamentals and applications of the variety of electrokinetic phenomena in both microfluidic and nanofluidic devices.

Prof. Dr. Xiangchun Xuan
Prof. Dr. Shizhi Qian
Guest Editors

Manuscript Submission Information

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Keywords

  • electrokinetics
  • micro/nanofluidics
  • electroosmosis
  • electrophoresis
  • diffusioosmosis
  • diffusiophoresis
  • streaming potential/current
  • dielectrophoresis
  • induced charge electrokinetics
  • electrical sensing

Published Papers (23 papers)

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Editorial

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Open AccessEditorial
Editorial for the Special Issue on Micro/Nano-Chip Electrokinetics, Volume II
Micromachines 2018, 9(8), 383; https://doi.org/10.3390/mi9080383
Received: 25 July 2018 / Accepted: 30 July 2018 / Published: 2 August 2018
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(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)

Research

Jump to: Editorial

Open AccessArticle
Elucidating the Mechanisms of Two Unique Phenomena Governed by Particle-Particle Interaction under DEP: Tumbling Motion of Pearl Chains and Alignment of Ellipsoidal Particles
Micromachines 2018, 9(6), 279; https://doi.org/10.3390/mi9060279
Received: 7 May 2018 / Revised: 26 May 2018 / Accepted: 29 May 2018 / Published: 1 June 2018
Cited by 1 | PDF Full-text (2517 KB) | HTML Full-text | XML Full-text
Abstract
Particle-particle interaction plays a crucial role in determining the movement and alignment of particles under dielectrophoresis (DEP). Previous research efforts focus on studying the mechanism governing the alignment of spherical particles with similar sizes in a static condition. Different approaches have been developed [...] Read more.
Particle-particle interaction plays a crucial role in determining the movement and alignment of particles under dielectrophoresis (DEP). Previous research efforts focus on studying the mechanism governing the alignment of spherical particles with similar sizes in a static condition. Different approaches have been developed to simulate the alignment process of a given number of particles from several up to thousands depending on the applicability of the approaches. However, restricted by the simplification of electric field distribution and use of identical spherical particles, not much new understanding has been gained apart from the most common phenomenon of pearl chain formation. To enhance the understanding of particle-particle interaction, the movement of pearl chains under DEP in a flow condition was studied and a new type of tumbling motion with unknown mechanism was observed. For interactions among non-spherical particles, some preceding works have been done to simulate the alignment of ellipsoidal particles. Yet the modeling results do not match experimental observations. In this paper, the authors applied the newly developed volumetric polarization and integration (VPI) method to elucidate the underlying mechanism for the newly observed movement of pearl chains under DEP in a flow condition and explain the alignment patterns of ellipsoidal particles. The modeling results show satisfactory agreement with experimental observations, which proves the strength of the VPI method in explaining complicated DEP phenomena. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Approximate Solution for Electroosmotic Flow of Power-Law Fluids in a Planar Microchannel with Asymmetric Electrochemical Boundary Conditions
Micromachines 2018, 9(6), 265; https://doi.org/10.3390/mi9060265
Received: 3 May 2018 / Revised: 23 May 2018 / Accepted: 24 May 2018 / Published: 28 May 2018
Cited by 4 | PDF Full-text (2511 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Electroosmotic flow (EOF) is widely used in microfluidic systems and chemical analysis. It is driven by an electric force inside microchannel with highly charged boundary conditions. In practical applications, electrochemical boundary conditions are often inhomogeneous because different materials as walls are commonly utilized [...] Read more.
Electroosmotic flow (EOF) is widely used in microfluidic systems and chemical analysis. It is driven by an electric force inside microchannel with highly charged boundary conditions. In practical applications, electrochemical boundary conditions are often inhomogeneous because different materials as walls are commonly utilized in routine fabrication methods. In the present study, we focus on the analytic solutions of the EOF generated in a planar microchannel with asymmetric electrochemical boundary conditions for non-Newtonian fluids. The velocity profile and flow rate are approximated by employing the power-law model of fluids in the Cauchy momentum equation. The hydrodynamic features of the EOF under asymmetric zeta potentials are scrutinized as a function of the fluid behavior index of the power-law fluid, thickness of Debye length, and zeta potential ratios between planes. The approximate solutions of the power-law model are comparable to the numerically obtained solutions when the Debye length is small and the fluid behavior index is close to unity. This study provides insights into the electrical control of non-Newtonian fluids, such as biological materials of blood, saliva, and DNA solution, in lab-on-a-chip devices. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Electromagnetic Regulation of Electrolyte Solution Heat Convection in Microchannels
Micromachines 2018, 9(6), 262; https://doi.org/10.3390/mi9060262
Received: 27 April 2018 / Revised: 19 May 2018 / Accepted: 24 May 2018 / Published: 28 May 2018
Cited by 1 | PDF Full-text (3244 KB) | HTML Full-text | XML Full-text
Abstract
With the rapid development of microelectronics and micro-electromechanical system technology, the electronic components have become smaller and the performance has become higher. Under this condition, however, their energy consumption and heat production have also increased continuously, which poses a great challenge to heat [...] Read more.
With the rapid development of microelectronics and micro-electromechanical system technology, the electronic components have become smaller and the performance has become higher. Under this condition, however, their energy consumption and heat production have also increased continuously, which poses a great challenge to heat dissipation. In this paper, electromagnetic driving technology is applied to drive the electrolyte solution flow within a microchannel to realize efficient heat convection with microchannel walls. By changing the magnitude and direction of electric–magnetic field, the regulation of heat convection performance is studied. The results show that the Nu number of microchannel increases as the Ha number and magnetic direction angle increases, while it decreases as the potential difference increases. According to the average index of the four factors, it was determined that the electrolyte solution had the best heat convection performance with Ha = 0.05, Vb = 0.00006, Pe = 90, and α = 90°. After that, sensitivity analysis of the Ha number, potential difference and magnetic direction angle was used to regulate the heat convection performance. This paper may provide some theoretical support for the design of microelectronics and micro-electromechanical systems. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Numerical Investigation of DC Dielectrophoretic Deformable Particle–Particle Interactions and Assembly
Micromachines 2018, 9(6), 260; https://doi.org/10.3390/mi9060260
Received: 14 April 2018 / Revised: 20 May 2018 / Accepted: 22 May 2018 / Published: 25 May 2018
Cited by 2 | PDF Full-text (6537 KB) | HTML Full-text | XML Full-text
Abstract
In a non-uniform electric field, the surface charge of the deformable particle is polarized, resulting in the dielectrophoretic force acting on the surface of the particle, which causes the electrophoresis. Due to dielectrophoretic force, the two deformable particles approach each other, and distort [...] Read more.
In a non-uniform electric field, the surface charge of the deformable particle is polarized, resulting in the dielectrophoretic force acting on the surface of the particle, which causes the electrophoresis. Due to dielectrophoretic force, the two deformable particles approach each other, and distort the flow field between them, which cause the hydrodynamic force correspondingly. The dielectrophoresis (DEP) force and the hydrodynamic force together form the net force acting on the particles. In this paper, based on a thin electric double layer (EDL) assumption, we developed a mathematical model under the arbitrary Lagrangian–Eulerian (ALE) numerical approach method to simulate the flow field, electric field, and deformable particles simultaneously. Simulation results show that, when two deformable particles’ distances are in a certain range, no matter the initial position of the two particles immersed in the fluid field, the particles will eventually form a particle–particle chain parallel to the direction of the electric field. In actual experiments, the biological cells used are deformable. Compared with the previous study on the DEP motion of the rigid particles, the research conclusion of this paper provides a more rigorous reference for the design of microfluidics. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Electroosmotic Flow in Microchannel with Black Silicon Nanostructures
Micromachines 2018, 9(5), 229; https://doi.org/10.3390/mi9050229
Received: 2 April 2018 / Revised: 7 May 2018 / Accepted: 8 May 2018 / Published: 11 May 2018
Cited by 3 | PDF Full-text (4264 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Although electroosmotic flow (EOF) has been applied to drive fluid flow in microfluidic chips, some of the phenomena associated with it can adversely affect the performance of certain applications such as electrophoresis and ion preconcentration. To minimize the undesirable effects, EOF can be [...] Read more.
Although electroosmotic flow (EOF) has been applied to drive fluid flow in microfluidic chips, some of the phenomena associated with it can adversely affect the performance of certain applications such as electrophoresis and ion preconcentration. To minimize the undesirable effects, EOF can be suppressed by polymer coatings or introduction of nanostructures. In this work, we presented a novel technique that employs the Dry Etching, Electroplating and Molding (DEEMO) process along with reactive ion etching (RIE), to fabricate microchannel with black silicon nanostructures (prolate hemispheroid-like structures). The effect of black silicon nanostructures on EOF was examined experimentally by current monitoring method, and numerically by finite element simulations. The experimental results showed that the EOF velocity was reduced by 13 ± 7%, which is reasonably close to the simulation results that predict a reduction of approximately 8%. EOF reduction is caused by the distortion of local electric field at the nanostructured surface. Numerical simulations show that the EOF velocity decreases with increasing nanostructure height or decreasing diameter. This reveals the potential of tuning the etching process parameters to generate nanostructures for better EOF suppression. The outcome of this investigation enhances the fundamental understanding of EOF behavior, with implications on the precise EOF control in devices utilizing nanostructured surfaces for chemical and biological analyses. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessFeature PaperArticle
On Developing Field-Effect-Tunable Nanofluidic Ion Diodes with Bipolar, Induced-Charge Electrokinetics
Micromachines 2018, 9(4), 179; https://doi.org/10.3390/mi9040179
Received: 10 March 2018 / Revised: 7 April 2018 / Accepted: 10 April 2018 / Published: 12 April 2018
Cited by 3 | PDF Full-text (7413 KB) | HTML Full-text | XML Full-text
Abstract
We introduce herein the induced-charge electrokinetic phenomenon to nanometer fluidic systems; the design of the nanofluidic ion diode for field-effect ionic current control of the nanometer dimension is developed by enhancing internal ion concentration polarization through electrochemical transport of inhomogeneous inducing-counterions resulting from [...] Read more.
We introduce herein the induced-charge electrokinetic phenomenon to nanometer fluidic systems; the design of the nanofluidic ion diode for field-effect ionic current control of the nanometer dimension is developed by enhancing internal ion concentration polarization through electrochemical transport of inhomogeneous inducing-counterions resulting from double gate terminals mounted on top of a thin dielectric layer, which covers the nanochannel connected to microfluidic reservoirs on both sides. A mathematical model based on the fully-coupled Poisson-Nernst-Plank-Navier-Stokes equations is developed to study the feasibility of this structural configuration causing effective ionic current rectification. The effect of various physiochemical and geometrical parameters, such as the native surface charge density on the nanochannel sidewalls, the number of gate electrodes (GE), the gate voltage magnitude, and the solution conductivity, permittivity, and thickness of the dielectric coating, as well as the size and position of the GE pair of opposite gate polarity, on the resulted rectification performance of the presented nanoscale ionic device is numerically analyzed by using a commercial software package, COMSOL Multiphysics (version 5.2). Three types of electrohydrodynamic flow, including electroosmosis of 1st kind, induced-charge electroosmosis, and electroosmosis of 2nd kind that were originated by the Coulomb force within three distinct charge layers coexist in the micro/nanofluidic hybrid network and are shown to simultaneously influence the output current flux in a complex manner. The rectification factor of a contrast between the ‘on’ and ‘off’ working states can even exceed one thousand-fold in the case of choosing a suitable combination of several key parameters. Our demonstration of field-effect-tunable nanofluidic ion diodes of double external gate electrodes proves invaluable for the construction of a flexible electrokinetic platform for ionic current control and may help transform the field of smart, on-chip, integrated circuits. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Electroosmotic Flow of Viscoelastic Fluid in a Nanoslit
Micromachines 2018, 9(4), 155; https://doi.org/10.3390/mi9040155
Received: 7 March 2018 / Revised: 22 March 2018 / Accepted: 26 March 2018 / Published: 29 March 2018
Cited by 2 | PDF Full-text (12366 KB) | HTML Full-text | XML Full-text
Abstract
The electroosmotic flow (EOF) of viscoelastic fluid in a long nanoslit is numerically studied to investigate the rheological property effect of Linear Phan-Thien-Tanner (LPTT) fluid on the fully developed EOF. The non-linear Poisson-Nernst-Planck equations governing the electric potential and the ionic concentration distribution [...] Read more.
The electroosmotic flow (EOF) of viscoelastic fluid in a long nanoslit is numerically studied to investigate the rheological property effect of Linear Phan-Thien-Tanner (LPTT) fluid on the fully developed EOF. The non-linear Poisson-Nernst-Planck equations governing the electric potential and the ionic concentration distribution within the channel are adopted to take into account the effect of the electrical double layer (EDL), including the EDL overlap. When the EDL is not overlapped, the velocity profiles for both Newtonian and viscoelastic fluids are plug-like and increase sharply near the charged wall. The velocity profile resembles that of pressure-driven flow when the EDL is overlapped. Regardless of the EDL thickness, apparent increase of velocity is obtained for viscoelastic fluid of larger Weissenberg number compared to the Newtonian fluid, indicating the shear thinning behavior of the LPTT fluid. The effect of the Weissenberg number on the velocity distribution is less significant as the degree of EDL overlapping increases, due to the overall decrease of the shear rate. The increase (decrease) of polymer extensibility (viscosity ratio) also enhances the EOF of viscoelastic fluid. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Improving Electrophoretic Particle Motion Control in Electrophoretic Displays by Eliminating the Fringing Effect via Driving Waveform Design
Micromachines 2018, 9(4), 143; https://doi.org/10.3390/mi9040143
Received: 31 January 2018 / Revised: 19 March 2018 / Accepted: 20 March 2018 / Published: 23 March 2018
Cited by 1 | PDF Full-text (2133 KB) | HTML Full-text | XML Full-text
Abstract
Electrophoretic display is realized by controlling colored nanoparticles moving in micrometer spaces via electrophoresis. The quality of information display is therefore affected by the unsynchronized particle moving speed and the mismatched electric signal according to the crosstalk of the electric field and inhomogeneous [...] Read more.
Electrophoretic display is realized by controlling colored nanoparticles moving in micrometer spaces via electrophoresis. The quality of information display is therefore affected by the unsynchronized particle moving speed and the mismatched electric signal according to the crosstalk of the electric field and inhomogeneous material distribution. In this work, we analyzed the mechanism of a fringe phenomenon that affected the information display quality of electrophoretic displays (EPDs). Electrical driving waveforms (voltage signals) are designed to reduce the fringe phenomenon. By using the optimizing driving waveform, we proposed that the fringe phenomenon is quantified as gray value that can be diminished by 25.5, while keeping a response time of 200 ms. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Three-Dimensional Reservoir-Based Dielectrophoresis (rDEP) for Enhanced Particle Enrichment
Micromachines 2018, 9(3), 123; https://doi.org/10.3390/mi9030123
Received: 25 January 2018 / Revised: 24 February 2018 / Accepted: 9 March 2018 / Published: 10 March 2018
Cited by 8 | PDF Full-text (2301 KB) | HTML Full-text | XML Full-text
Abstract
Selective enrichment of target species is crucial for a wide variety of engineering systems for improved performance of subsequent processes. Dielectrophoresis (DEP) is a powerful electrokinetic method that can be used to focus, trap, concentrate, and separate a variety of species in a [...] Read more.
Selective enrichment of target species is crucial for a wide variety of engineering systems for improved performance of subsequent processes. Dielectrophoresis (DEP) is a powerful electrokinetic method that can be used to focus, trap, concentrate, and separate a variety of species in a label-free manner. The commonly employed methods for DEP suffer from limitations such as electrode fouling and high susceptibility to Joule heating effects. Recently, our group has demonstrated DEP-based manipulations of particles and cells using a novel method of reservoir-based dielectrophoresis (rDEP) which exploits the naturally produced electric field gradients at the reservoir-microchannel junction. Although this method reasonably addresses the limitations mentioned above while maintaining a high simplicity of fabrication, all of our demonstrations so far have used a two-dimensional rDEP, which limits the performance of the devices. This work aims to improve their performance further by making the DEP three-dimensional. Through detailed experimental and numerical analysis, we demonstrate a six-fold increase in the enrichment performance of latex beads and a significant reduction in the power consumption for the new devices, which would allow a more reliable integration of the same into micro-total analysis systems. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
On AC-Field-Induced Nonlinear Electroosmosis next to the Sharp Corner-Field-Singularity of Leaky Dielectric Blocks and Its Application in on-Chip Micro-Mixing
Micromachines 2018, 9(3), 102; https://doi.org/10.3390/mi9030102
Received: 7 February 2018 / Revised: 24 February 2018 / Accepted: 24 February 2018 / Published: 28 February 2018
Cited by 11 | PDF Full-text (9789 KB) | HTML Full-text | XML Full-text
Abstract
Induced-charge electroosmosis has attracted lots of attention from the microfluidic community over the past decade. Most previous researches on this subject focused on induced-charge electroosmosis (ICEO) vortex streaming actuated on ideally polarizable surfaces immersed in electrolyte solutions. Starting from this point, we conduct [...] Read more.
Induced-charge electroosmosis has attracted lots of attention from the microfluidic community over the past decade. Most previous researches on this subject focused on induced-charge electroosmosis (ICEO) vortex streaming actuated on ideally polarizable surfaces immersed in electrolyte solutions. Starting from this point, we conduct herein a linear asymptotic analysis on nonlinear electroosmotic flow next to leaky dielectric blocks of arbitrary electrical conductivity and dielectric permittivity in harmonic AC electric fields, and theoretically demonstrate that observable ICEO fluid motion can be generated at high field frequencies in the vicinity of nearly insulating semiconductors, a very low electrical conductivity, of which can evidently increase the double-layer relaxation frequency (inversely proportional to the solid permittivity) to be much higher than the typical reciprocal RC time constant for induced double-layer charging on ideally polarizable surfaces. A computational model is developed to study the feasibility of this high-frequency vortex flow field of ICEO for sample mixing in microfluidics, in which the usage of AC voltage signal at high field frequencies may be beneficial to suppress electrochemical reactions to some extent. The influence of various parameters for developing an efficient mixer is investigated, and an integrated arrangement of semiconductor block array is suggested for achieving a reliable mixing performance at relatively high sample fluxes. Our physical demonstration with high-frequency ICEO next to leaky dielectric blocks using a simple channel structure offers valuable insights into the design of high-throughput micromixers for a variety of lab-on-a-chip applications. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Electrowetting Using a Microfluidic Kelvin Water Dropper
Micromachines 2018, 9(3), 92; https://doi.org/10.3390/mi9030092
Received: 16 December 2017 / Revised: 15 February 2018 / Accepted: 22 February 2018 / Published: 25 February 2018
Cited by 1 | PDF Full-text (1677 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The Kelvin water dropper is an electrostatic generator that can generate high voltage electricity through water dripping. A conventional Kelvin water dropper converts the gravitational potential energy of water into electricity. Due to its low current output, Kelvin water droppers can only be [...] Read more.
The Kelvin water dropper is an electrostatic generator that can generate high voltage electricity through water dripping. A conventional Kelvin water dropper converts the gravitational potential energy of water into electricity. Due to its low current output, Kelvin water droppers can only be used in limited cases that demand high voltage. In the present study, microfluidic Kelvin water droppers (MKWDs) were built in house to demonstrate a low-cost but accurately controlled miniature device for high voltage generation. The performance of the MKWDs was characterized using different channel diameters and flow rates. The best performed MKWD was then used to conduct experiments of the electrowetting of liquid on dielectric surfaces. Electrowetting is a process that has been widely used in manipulating the wetting properties of a surface using an external electric field. Usually electrowetting requires an expensive DC power supply that outputs high voltage. However, in this research, it was demonstrated that electrowetting can be conducted by simply using an MKWD. Additionally, an analytic model was developed to simulate the electrowetting process. Finally, the model’s ability to well predict the liquid deformation during electrowetting using MKWDs was validated. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
On the Bipolar DC Flow Field-Effect-Transistor for Multifunctional Sample Handing in Microfluidics: A Theoretical Analysis under the Debye–Huckel Limit
Micromachines 2018, 9(2), 82; https://doi.org/10.3390/mi9020082
Received: 26 January 2018 / Revised: 11 February 2018 / Accepted: 13 February 2018 / Published: 16 February 2018
Cited by 6 | PDF Full-text (9806 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
We present herein a novel method of bipolar field-effect control on DC electroosmosis (DCEO) from a physical point of view, in the context of an intelligent and robust operation tool for stratified laminar streams in microscale systems. In this unique design of the [...] Read more.
We present herein a novel method of bipolar field-effect control on DC electroosmosis (DCEO) from a physical point of view, in the context of an intelligent and robust operation tool for stratified laminar streams in microscale systems. In this unique design of the DC flow field-effect-transistor (DC-FFET), a pair of face-to-face external gate terminals are imposed with opposite gate-voltage polarities. Diffuse-charge dynamics induces heteropolar Debye screening charge within the diffuse double layer adjacent to the face-to-face oppositely-polarized gates, respectively. A background electric field is applied across the source-drain terminal and forces the face-to-face counterionic charge of reversed polarities into induced-charge electroosmotic (ICEO) vortex flow in the lateral direction. The chaotic turbulence of the transverse ICEO whirlpool interacts actively with the conventional plug flow of DCEO, giving rise to twisted streamlines for simultaneous DCEO pumping and ICEO mixing of fluid samples along the channel length direction. A mathematical model in thin-layer approximation and the low-voltage limit is subsequently established to test the feasibility of the bipolar DC-FFET configuration in electrokinetic manipulation of fluids at the micrometer dimension. According to our simulation analysis, an integrated device design with two sets of side-by-side, but upside-down gate electrode pair exhibits outstanding performance in electroconvective pumping and mixing even without any externally-applied pressure difference. Moreover, a paradigm of a microdevice for fully electrokinetics-driven analyte treatment is established with an array of reversed bipolar gate-terminal pairs arranged on top of the dielectric membrane along the channel length direction, from which we can obtain almost a perfect liquid mixture by using a smaller magnitude of gate voltages for causing less detrimental effects at a small Dukhin number. Sustained by theoretical analysis, our physical demonstration on bipolar field-effect flow control for the microfluidic device of dual functionalities in simultaneous electroconvective pumping and mixing holds great potential in the development of fully-automated liquid-phase actuators in modern microfluidic systems. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Analytical Solution of Electro-Osmotic Peristalsis of Fractional Jeffreys Fluid in a Micro-Channel
Micromachines 2017, 8(12), 341; https://doi.org/10.3390/mi8120341
Received: 18 September 2017 / Revised: 8 November 2017 / Accepted: 19 November 2017 / Published: 23 November 2017
Cited by 8 | PDF Full-text (3122 KB) | HTML Full-text | XML Full-text
Abstract
The electro-osmotic peristaltic flow of a viscoelastic fluid through a cylindrical micro-channel is studied in this paper. The fractional Jeffreys constitutive model, including the relaxation time and retardation time, is utilized to describe the viscoelasticity of the fluid. Under the assumptions of long [...] Read more.
The electro-osmotic peristaltic flow of a viscoelastic fluid through a cylindrical micro-channel is studied in this paper. The fractional Jeffreys constitutive model, including the relaxation time and retardation time, is utilized to describe the viscoelasticity of the fluid. Under the assumptions of long wavelength, low Reynolds number, and Debye-Hückel linearization, the analytical solutions of pressure gradient, stream function and axial velocity are explored in terms of Mittag-Leffler function by Laplace transform method. The corresponding solutions of fractional Maxwell fluid and generalized second grade fluid are also obtained as special cases. The numerical analysis of the results are depicted graphically, and the effects of electro-osmotic parameter, external electric field, fractional parameters and viscoelastic parameters on the peristaltic flow are discussed. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Electrode Cooling Effect on Out-Of-Phase Electrothermal Streaming in Rotating Electric Fields
Micromachines 2017, 8(11), 327; https://doi.org/10.3390/mi8110327
Received: 23 September 2017 / Revised: 3 November 2017 / Accepted: 4 November 2017 / Published: 6 November 2017
Cited by 9 | PDF Full-text (4466 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
In this work, we focus on investigating electrothermal flow in rotating electric fields (ROT-ETF), with primary attention paid to the horizontal traveling-wave electrothermal (TWET) vortex induced at the center of the electric field. The frequency-dependent flow profiles in the microdevice are analyzed using [...] Read more.
In this work, we focus on investigating electrothermal flow in rotating electric fields (ROT-ETF), with primary attention paid to the horizontal traveling-wave electrothermal (TWET) vortex induced at the center of the electric field. The frequency-dependent flow profiles in the microdevice are analyzed using different heat transfer models. Accordingly, we address in particular the importance of electrode cooling in ROT-ETF as metal electrodes of high thermal conductivity, while substrate material of low heat dissipation capability is employed to develop such microfluidic chips. Under this circumstance, cooling of electrode array due to external natural convection on millimeter-scale electrode pads for external wire connection occurs and makes the internal temperature maxima shift from the electrode plane to a bit of distance right above the cross-shaped interelectrode gaps, giving rise to reversal of flow rotation from a typical repulsion-type to attraction-type induction vortex, which is in good accordance with our experimental observations of co-field TWET streaming at frequencies in the order of reciprocal charge relaxation time of the bulk fluid. These results point out a way to make a correct interpretation of out-of-phase electrothermal streaming behavior, which holds great potential for handing high-conductivity analytes in modern microfluidic systems. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
A Novel On-Chip Impedance Sensor for the Detection of Particle Contamination in Hydraulic Oil
Micromachines 2017, 8(8), 249; https://doi.org/10.3390/mi8080249
Received: 13 June 2017 / Revised: 25 July 2017 / Accepted: 7 August 2017 / Published: 14 August 2017
Cited by 9 | PDF Full-text (4285 KB) | HTML Full-text | XML Full-text
Abstract
A novel impedance sensor based on a microfluidic chip is presented. The sensor consists of two single-layer coils and a straight micro-channel, and can detect, not only ferromagnetic and non-ferromagnetic particles in oil as an inductive sensor, but also, water droplets and air [...] Read more.
A novel impedance sensor based on a microfluidic chip is presented. The sensor consists of two single-layer coils and a straight micro-channel, and can detect, not only ferromagnetic and non-ferromagnetic particles in oil as an inductive sensor, but also, water droplets and air bubbles in oil as a capacitive sensor. The experiments are carried out at different excitation frequencies, number of coil turns and particle sizes. For the inductance detection, the inductance signals are found to increase with the excitation frequency and the noise is constant; both the inductance signals and the noise increase with the number of coil turns, but because the noise increases at a faster rate than the signal, the signal-to-noise ratio decreases with the number of coil turns. We demonstrate the successful detection of 40 μm iron particles and 110 μm copper particles using the coil with 20 turns at the excitation frequency of 2 MHz. For the capacitance detection, capacitance signals decrease with the excitation frequency and the noise is constant; the capacitance signals decrease with the number of coil turns, while the noise increases, thus, the signal-to-noise ratio decreases with the number of coil turns. We can detect 100 μm water droplets and 180 μm bubbles successfully using the coil with 20 turns at the excitation frequency of 0.3 MHz. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Interfacial Electric Effects on a Non-Isothermal Electroosmotic Flow in a Microcapillary Tube Filled by Two Immiscible Fluids
Micromachines 2017, 8(8), 232; https://doi.org/10.3390/mi8080232
Received: 19 June 2017 / Revised: 18 July 2017 / Accepted: 25 July 2017 / Published: 27 July 2017
Cited by 4 | PDF Full-text (709 KB) | HTML Full-text | XML Full-text
Abstract
In this work, a non-isothermal electroosmotic flow of two immiscible fluids within a uniform microcapillary is theoretically studied. It is considered that there is an annular layer of a non-Newtonian liquid, whose behavior follows the power-law model, adjacent to the inside wall of [...] Read more.
In this work, a non-isothermal electroosmotic flow of two immiscible fluids within a uniform microcapillary is theoretically studied. It is considered that there is an annular layer of a non-Newtonian liquid, whose behavior follows the power-law model, adjacent to the inside wall of the capillary, which in turn surrounds an inner flow of a second conducting liquid that is driven by electroosmosis. The inner fluid flow exerts an interfacial force, dragging the annular fluid due to shear and Maxwell stresses at the interface between the two fluids. Because the Joule heating effect may be present in electroosmotic flow (EOF), temperature gradients can appear along the microcapillary, making the viscosity coefficients of both fluids and the electrical conductivity of the inner fluid temperature dependent. The above makes the variables of the flow field in both fluids, velocity, pressure, temperature and electric fields, coupled. An additional complexity of the mathematical model that describes the electroosmotic flow is the nonlinear character due to the rheological behavior of the surrounding fluid. Therefore, based on the lubrication theory approximation, the governing equations are nondimensionalized and simplified, and an asymptotic solution is determined using a regular perturbation technique by considering that the perturbation parameter is associated with changes in the viscosity by temperature effects. The principal results showed that the parameters that notably influence the flow field are the power-law index, an electrokinetic parameter (the ratio between the radius of the microchannel and the Debye length) and the competition between the consistency index of the non-Newtonian fluid and the viscosity of the conducting fluid. Additionally, the heat that is dissipated trough the external surface of the microchannel and the sensitivity of the viscosity to temperature changes play important roles, which modify the flow field. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Simulation Analysis of Improving Microfluidic Heterogeneous Immunoassay Using Induced Charge Electroosmosis on a Floating Gate
Micromachines 2017, 8(7), 212; https://doi.org/10.3390/mi8070212
Received: 7 June 2017 / Revised: 2 July 2017 / Accepted: 3 July 2017 / Published: 4 July 2017
Cited by 5 | PDF Full-text (6626 KB) | HTML Full-text | XML Full-text
Abstract
On-chip immuno-sensors are a hot topic in the microfluidic community, which is usually limited by slow diffusion-dominated transport of analytes in confined microchannels. Specifically, the antigen-antibody binding reaction at a functionalized area cannot be provided with enough antigen source near the reaction surface, [...] Read more.
On-chip immuno-sensors are a hot topic in the microfluidic community, which is usually limited by slow diffusion-dominated transport of analytes in confined microchannels. Specifically, the antigen-antibody binding reaction at a functionalized area cannot be provided with enough antigen source near the reaction surface, since a small diffusion flux cannot match with the quick rate of surface reaction, which influences the response time and sensitivity of on-chip heterogeneous immunoassay. In this work, we propose a method to enhance the transportation of biomolecules to the surface of an antibody-immobilized electrode with induce charge electroosmotic (ICEO) convection in a low concentration suspension, so as to improve the binding efficiency of microfluidic heterogeneous immunoassays. The circular stirring fluid motion of ICEO on the surface of a floating gate electrode at the channel bottom accelerates the transport of freely suspended antigen towards the wall-immobilized antibodies. We investigate the dependence of binding efficiency on voltage magnitude and field frequency of the applied alternate current (AC) electrical field. The binding rate yields a factor of 5.4 higher binding for an applied voltage of 4 V at 10 Hz when the Damkohler number is 1000. The proposed microfluidic immuno-sensor technology of a simple electrode structure using ICEO convective fluid flow around floating conductors could offer exciting opportunities for diffusion-limited on-chip bio-microfluidic sensors. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Parametric Study of an Electroosmotic Micromixer with Heterogeneous Charged Surface Patches
Micromachines 2017, 8(7), 199; https://doi.org/10.3390/mi8070199
Received: 4 May 2017 / Revised: 15 June 2017 / Accepted: 19 June 2017 / Published: 23 June 2017
Cited by 6 | PDF Full-text (7702 KB) | HTML Full-text | XML Full-text
Abstract
A T-shaped micromixer featuring electroosmotic flow with heterogeneous charged surface patches on the channel walls was analyzed, and an improved design was proposed to enhance mixing performance. Numerical analysis was performed using steady Navier-Stokes equations with an additional electrokinetic body force. The numerical [...] Read more.
A T-shaped micromixer featuring electroosmotic flow with heterogeneous charged surface patches on the channel walls was analyzed, and an improved design was proposed to enhance mixing performance. Numerical analysis was performed using steady Navier-Stokes equations with an additional electrokinetic body force. The numerical results for species concentration were validated with available experimental data. A parametric analysis of the micromixer was performed by varying channel height, channel width, patch width, and externally applied voltage. The effects of these parameters on the flow structure and mixing performance were analyzed in detail. A quantitative measurement based upon the mass variance was employed to quantify the mixing performance. Numerical results of the parametric study were used to propose an improved micromixer design with spacing between adjacent charged patches. The proposed design provided a more favorable flow structure to allow for enhanced mixing performance. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Electroosmotic Flow in a Rough Nanochannel with Surface Roughness Characterized by Fractal Cantor
Micromachines 2017, 8(6), 190; https://doi.org/10.3390/mi8060190
Received: 6 May 2017 / Revised: 7 June 2017 / Accepted: 15 June 2017 / Published: 19 June 2017
Cited by 4 | PDF Full-text (2097 KB) | HTML Full-text | XML Full-text
Abstract
Molecular dynamics simulation is applied to study the electroosmotic flow in rough nanochannels, with particular attention given to the fluid–solid interactions. In the simulation, the surface roughness is characterized by a fractal Cantor. The roles of roughness height and fractal dimension on nanoscale [...] Read more.
Molecular dynamics simulation is applied to study the electroosmotic flow in rough nanochannels, with particular attention given to the fluid–solid interactions. In the simulation, the surface roughness is characterized by a fractal Cantor. The roles of roughness height and fractal dimension on nanoscale electroosmotic flow are examined and analyzed. The concentration distributions, zeta potential and electroosmotic velocity are presented and investigated. The results indicate that surface roughness plays a significant role in the fluid–solid interaction and nanoscale electroosmotic flow. The distribution of dipole angle for water molecules in both the near-wall region and middle region is almost unaffected by surface roughness; however, a significant difference of dipole angle distribution is observed in the fluid region away from the wall. Interestingly, the concentration distributions, electroosmotic velocity and zeta potential are highly affected by the surface fractal dimension, even with the same roughness height. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Electroosmotic Flows of Power-Law Fluids with Asymmetric Electrochemical Boundary Conditions in a Rectangular Microchannel
Micromachines 2017, 8(5), 165; https://doi.org/10.3390/mi8050165
Received: 5 April 2017 / Revised: 11 May 2017 / Accepted: 17 May 2017 / Published: 20 May 2017
Cited by 4 | PDF Full-text (6627 KB) | HTML Full-text | XML Full-text
Abstract
In this paper, a systematic study of a fully developed electroosmotic flow of power-law fluids in a rectangular microchannel bounded by walls with different zeta potentials is described. Because the upper and lower layers of most microchannels are made of different materials, it [...] Read more.
In this paper, a systematic study of a fully developed electroosmotic flow of power-law fluids in a rectangular microchannel bounded by walls with different zeta potentials is described. Because the upper and lower layers of most microchannels are made of different materials, it is necessary to study the flow characteristics for cases in which the microchannels have different zeta potentials at each wall. The electrical potential and momentum equations were solved numerically using a finite element analysis. The velocity profiles and flow rates were studied parametrically by varying the fluid behavior index, channel aspect ratio, and electrochemical properties of the liquid and the bounding walls. The calculated volumetric flow rates in a rectangular microchannel were compared with those between two infinite parallel plates. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
Fluid Flow and Mixing Induced by AC Continuous Electrowetting of Liquid Metal Droplet
Micromachines 2017, 8(4), 119; https://doi.org/10.3390/mi8040119
Received: 11 March 2017 / Revised: 27 March 2017 / Accepted: 6 April 2017 / Published: 9 April 2017
Cited by 5 | PDF Full-text (2478 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
In this work, we proposed a novel design of a microfluidic mixer utilizing the amplified Marangoni chaotic advection induced by alternating current (AC) continuous electrowetting of a metal droplet situated in electrolyte solution, due to the linear and quadratic voltage-dependence of flow velocity [...] Read more.
In this work, we proposed a novel design of a microfluidic mixer utilizing the amplified Marangoni chaotic advection induced by alternating current (AC) continuous electrowetting of a metal droplet situated in electrolyte solution, due to the linear and quadratic voltage-dependence of flow velocity at small or large voltages, respectively. Unlike previous researchers exploiting the unidirectional surface stress with direct current (DC) bias at droplet/medium interface for pumping of electrolytes where the resulting flow rate is linearly proportional to the field intensity, dominance of another kind of dipolar flow pattern caused by local Marangoni stress at the drop surface in a sufficiently intense AC electric field is demonstrated by both theoretical analysis and experimental observation, which exhibits a quadratic growth trend as a function of the applied voltage. The dipolar shear stress merely appears at larger voltages and greatly enhances the mixing performance by inducing chaotic advection between the neighboring laminar flow. The mixer design developed herein, on the basis of amplified Marangoni chaotic advection around a liquid metal droplet at larger AC voltages, has great potential for chemical reaction and microelectromechanical systems (MEMS) actuator applications because of generating high-throughput and excellent mixing performance at the same time. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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Open AccessArticle
A Novel Electroosmotic Micromixer with Asymmetric Lateral Structures and DC Electrode Arrays
Micromachines 2017, 8(4), 105; https://doi.org/10.3390/mi8040105
Received: 28 February 2017 / Revised: 21 March 2017 / Accepted: 27 March 2017 / Published: 29 March 2017
Cited by 6 | PDF Full-text (8882 KB) | HTML Full-text | XML Full-text
Abstract
We present a novel electroosmotic micromixer that consists of arrays of direct current (DC) asymmetric electrode and asymmetric lateral structures. By embedding asymmetric electrode arrays on the top and bottom walls of a rectangular microchannel appropriately, the flow perturbations and vortexes can be [...] Read more.
We present a novel electroosmotic micromixer that consists of arrays of direct current (DC) asymmetric electrode and asymmetric lateral structures. By embedding asymmetric electrode arrays on the top and bottom walls of a rectangular microchannel appropriately, the flow perturbations and vortexes can be induced when a DC electric field is imposed. An efficient lateral structure is then sequentially combined with the rectangular microchannel, which enhances the mixing effect significantly. The effects of operational parameters such as the Reynolds number, the applied potential, and the Peclet number on the mixing performance are analyzed in detail by numerical simulations. The results indicate that an enhanced mixing performance can be achieved with low applied potential. The novel method proposed in this paper provides a simple solution for mixing in the field of micro-total-analysis systems. Full article
(This article belongs to the Special Issue Micro/Nano-Chip Electrokinetics, Volume II)
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