Chaotic mixing in microchannels via low frequency switching transverse electroosmotic flow generated on integrated microelectrodes

Non-Newtonian Solute Mixing via Protonic Exchange of a Polyelectrolyte Layer: Unveiling Formation of Electroosmotic Vortices

Biochemical and medical diagnostics are two main fields in which vortex generation in microfluidic devices has several applications. Therefore, the aim of the present endeavor is to investigate the characteristics of a non-Newtonian vortex under the influence of a pH-sensitive polyelectrolyte layer (PEL)-modulated electroosmotic effect in a microchannel. Additionally, it is considered that the bulk solution pH (pHb0) and ionic concentration of the solution influence the zeta potential. Accordingly, the corresponding mathematical framework is constructed by using a numerical solver based on the finite element method and is subsequently verified against available experimental data in limiting conditions. Within the range of pHb0 and rheological parameters─Carreau number and flow behavior index─we critically analyze the PEL space charge density, net body force, and flow pattern. The current findings indicate that the existence of discrete net electrical body force patterns yields specific flow structures that enable substantial variation in the flow rate and mixing efficiency. The dominance of the basic PEL group protonic exchange at lower pHb0 and acidic PEL group protonic exchange at higher pHb0, respectively, permits positive and negative PEL space charge densities. Consequently, it is evident that the net electrical body force in PEL is extremely pHb0-dependent. Therefore, with smaller pHb0, the anticlockwise vortex with a negative flow rate is identified, whereas the clockwise vortex with a positive flow rate is predicted for larger pHb0. In turn, five distinct flow pattern regimes appear when the bulk solution pH pivots from 3 to 11. Remarkably, mixing efficiency exceeds 90% for greater diffusive Peclet numbers in highly acidic liquids. Overall, the outcomes of this study may significantly impact the design of microfluidic devices that mix and transport non-Newtonian liquids at particular pHb0 values.

Scalable acoustic virtual stirrer for enhanced interfacial enzymatic nucleic acid reactions

Enzymatic nucleic acid reaction is a fundamental tool in molecular biology. However, high-complexity enzymatic DNA reactions and assays are still challenging due to the difficulties in integrating and scaling up microscale reaction units and mixing tools. Here, we present scalable acoustofluidic platform featuring acoustic virtual stirrer (AVS) arrays, serving as stirrers to increase the efficiency of interfacial enzymatic nucleic acid reactions. Analogous to magnetic stirrers, AVS arrays perturb the fluid through oscillating pressure nodes, controllable in terms of speeds and amplitudes via modulation. By optimizing the kinetics of surface-tethered DNA and enzymes via AVS, we achieve a 7.74% improvement in the stepwise yield of enzymatic DNA synthesis. In addition, the AVS enhanced DNA logic gate architecture can complete responses within 2 minutes, achieving average speed enhancement of 8.58 times compared to the non-AVS configuration. With its tunability, ease of integration, and efficiency, this technology holds promises for applications in biology and chemistry.

Electroneutral Layer Dynamics Drive Ion Migration in Low Frequency AC Electrophoresis Below the Water Electrolysis Threshold

Low-frequency AC electrophoresis lies in a regime between DC microchannel electrophoresis and dielectrophoresis, which typically utilizes frequencies above 1000 Hz. Although few electrophoretic methods have been reported in this ≤100 Hz range, traveling wave electrophoresis (TWE) and transverse AC electrophoresis (TrACE) operate in this frequency range, and use low voltages to avoid bubble formation from water electrolysis. TWE provides molecular separations with enhanced control and TrACE provides multiplexed, multiparameter particle characterization. However, two related fundamental questions remain about the mechanisms of electrophoretic migration in these systems. First, particle electrophoresis in TrACE is largely captured by a simple model that combines the alternating electric field with DC electrokinetics, but a deviation from the model is observed for applied square electric field waves that increases with decreasing frequency. Second, although electrode charging is believed to drive ion migration in TWE, the estimated electrode charging time is about 2-3 orders of magnitude faster than the wave period. In this study, a 1D finite numerical model that excludes Faradaic reactions simulates ion and particle migration across the microchannel width in TrACE. The 1D model results show good agreement with both particle and ion migration in TrACE systems. Furthermore, although ion migration between the pair of electrodes slows during each excursion of a 1 Hz square wave, there is substantial ion migration throughout the 0.5 s half-period. This modeling result agrees with experimental observations in TWE. Therefore, the clarification of the mechanisms of ion migration in these low-frequency and low-voltage AC electrophoresis is expected to expand their applications.

Measuring the electrophoretic mobility and size of single particles using microfluidic transverse AC electrophoresis (TrACE)

The ability to measure the charge and size of single particles is essential to understanding particle adhesion and interaction with their environment. Characterizing the physical properties of biological particles, like cells, can be a powerful tool in studying the association between the changes in physical properties and disease development. Currently, measuring charge via the electrophoretic mobility (μep) of individual particles remains challenging, and there is only one prior report of simultaneously measuring μep and size. We introduce microfluidic transverse AC electrophoresis (TrACE), a novel technique that combines particle tracking velocimetry (PTV) and AC electrophoresis. In TrACE, electric waves with 0.75 to 1.5 V amplitude are applied transversely to the bulk flow and cause the particles to oscillate. PTV records the particles' oscillating trajectories as pressure drives bulk flow through the microchannel. A simple quasi-equilibrium model agrees well with experimental measurements of frequency, amplitude, and phase, indicating that particle motion is largely described by DC electrophoresis. The measured μep of polystyrene particles (0.53, 0.84, 1, and 2 μm diameter) are consistent with ELS measurements, and precision is enhanced by averaging ∼100 measurements per particle. Particle size is simultaneously measured from Brownian motion quantified from the trajectory for particles <2 μm or image analysis for particles ≥2 μm. Lastly, the ability to analyze intact mammalian cells is demonstrated with B cells. TrACE systems are expected to be highly suitable as fieldable tools to measure the μep and size of a broad range of individual particles.

Mechanism study of how lipid vesicle electroformation is suppressed by the presence of sodium chloride

Giant lipid vesicles (GLVs) are usually adopted as models of cell membranes and electroformation is the most commonly used method for GLV formation. However, GLV electroformation are known to be suppressed by the presence of salt and the mechanism is not clear so far. In this paper, the lipid hydration and GLV electroformation were investigated as a function of the concentration of sodium chloride by depositing the lipids on the bottom substrates and top substrates. In addition, the electrohydrodynamic force generated by the electroosmotic flow (EOF) on the lipid phase was calculated with COMSOL Multiphysics. It was found that the mechanisms for the failure of GLV electroformation in salt solutions are: 1) the presence of sodium chloride decreases the membrane permeability to aqueous solution by accelerating the formation of well-packed membranes, suppressing the swelling and detachment of the lipid membranes; 2) the presence of sodium chloride decreased the electrohydrodynamic force by increasing the medium conductivity.

Mixing Mechanism of Microfluidic Mixer with Staggered Virtual Electrode Based on Light-Actuated AC Electroosmosis

In this paper, we present a novel microfluidic mixer with staggered virtual electrode based on light-actuated AC electroosmosis (LACE). We solve the coupled system of the flow field described by Navier–Stokes equations, the described electric field by a Laplace equation, and the concentration field described by a convection–diffusion equation via a finite-element method (FEM). Moreover, we study the distribution of the flow, electric, and concentration fields in the microchannel, and reveal the generating mechanism of the rotating vortex on the cross-section of the microchannel and the mixing mechanism of the fluid sample. We also explore the influence of several key geometric parameters such as the length, width, and spacing of the virtual electrode, and the height of the microchannel on mixing performance; the relatively optimal mixer structure is thus obtained. The current micromixer provides a favorable fluid-mixing method based on an optical virtual electrode, and could promote the comprehensive integration of functions in modern microfluidic-analysis systems.

Phase-controlled field-effect micromixing using AC electroosmosis

The exploration and application of electrokinetic techniques in micro total analysis systems have become ubiquitous in recent years, and scientists are expanding the use of such techniques in areas where comparable active or passive methods are not as successful. In this work, for the first time, we utilize the concept of AC electroosmosis to design a phase-controlled field-effect micromixer that benefits from a three-finger sinusoidally shaped electrodes. Analogous to field-effect transistor devices, the principle of operation for the proposed micromixer is governed by the source-gate and source-drain voltage potentials that are modulated by introducing a phase lag between the driving electrodes. At an optimized flow rate and biasing scheme, we demonstrate that the source, gate, and drain voltage phase relations can be configured such that the micromixer switches from an unmixed state (phase shift of 0°) to a mixed state (phase shift of 180°). High mixing efficiencies beyond 90% was achieved at a volumetric flow rate of 4 µL/min corresponding to ~13.9 mm/s at optimized voltage excitation conditions. Finally, we employed the proposed micromixer for the synthesis of nanoscale lipid-based drug delivery vesicles through the process of electrohydrodynamic-mediated nanoprecipitation. The phase-controlled electrohydrodynamic mixing utilized for the nanoprecipitation technique proved that nanoparticles of improved monodispersity and concentration can be produced when mixing efficiency is enhanced by tuning the phase shifts between electrodes.

Investigating the impact of metal ions and 3D printed droplet microfluidics chip geometry on the luminol‑potassium periodate chemiluminescence system for estimating total phenolic content in olive oil

The impact of the chip design and the mixing mechanisms using six different 3D printed microfluidic chips were investigated. The study was conducted using novel 3D printed droplet based microfluidics. A multi-mixing approach was utilized to enhance the CL signal of the CL system under investigation. The approach is based on droplet formation, droplet mixing and droplets merging in the 3D printed microfluidic chip. A 154% higher CL signal intensity was obtained using this approach compared to the CL signal obtained using the serpentine chip commonly used for improving the mixing inside droplet microfluidics. This chip was exploited to study the role of three metal ions: Co²⁺, Mn²⁺ and Fe²⁺ on catalyzing the luminol‑potassium periodate chemiluminescence (CL) reaction with selected phenolic compounds in basic media was carefully investigated. Furthermore, the luminol‑potassium periodate-metal ions system was optimized for all metal ions using gallic acid as the reference standard. Despite the popularity of luminol systems in estimating antioxidant activity or total phenolic content (TPC), the results of this study revealed the necessity of careful and vigilant attention when applying it to complex matrices. The only metal ion that showed quenching behavior with all 20 of the tested phenolic compounds was Fe²⁺, while Co²⁺and Mn²⁺ showed both quenching and enhancement in the CL signal. The luminol‑potassium periodate-Fe²⁺ system was applied to estimate TPC in olive oil extracts.

Electromagnetophoretic Micro-convection around a Droplet in a Capillary

The electromagnetophoretic behavior of organic droplets in an electrolyte solution was investigated in a silica capillary cell using a superconducting bulk magnet (3.5 T) and a magnetic circuit (2.7 T). The initially dispersed emulsion droplets of dodecane migrated to the wall of the capillary, responding to the direction of an electric current, and coalesced to form smaller and larger droplets after some repeated migrations. When the electric current was applied continuously, the larger droplets became arranged with regular intervals on the wall, and smaller droplets rotated around the larger droplets. These interesting behaviors were analyzed while taking into account the local electric current density determined by the flow velocity of the ionic current around a droplet, which was lowest on the electrode sides of the droplet. The difference in the local electric current density generated the Lorentz-force difference in the medium, which lead to local micro-convection around the droplet, and also the alignment of larger droplets by a repelling effect between the adjacent micro-convections.

A Novel Electroosmotic Micromixer with Asymmetric Lateral Structures and DC Electrode Arrays

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.

Frequency-Dependent Electroformation of Giant Unilamellar Vesicles in 3D and 2D Microelectrode Systems

A giant unilamellar vesicle (GUV), with similar properties to cellular membrane, has been widely studied. Electroformation with its simplicity and accessibility has become the most common method for GUV production. In this work, GUV electroformation in devices with traditional 3D and new 2D electrode structures were studied with respect to the applied electric field. An optimal frequency (10 kHz in the 3D and 1 kHz in the 2D systems) was found in each system. A positive correlation was found between GUV formation and applied voltage in the 3D electrode system from 1 to 10 V. In the 2D electrode system, the yield of the generated GUV increased first but decreased later as voltage increased. These phenomena were further confirmed by numerically calculating the load that the lipid film experienced from the generated electroosmotic flow (EOF). The discrepancy between the experimental and numerical results of the 3D electrode system may be because the parameters that were adopted in the simulations are quite different from those of the lipid film in experiments. The lipid film was not involved in the simulation of the 2D system, and the numerical results matched well with the experiments.

Continuous-flow sorting of stem cells and differentiation products based on dielectrophoresis

This paper presents a continuous-flow microfluidic device for sorting stem cells and their differentiation progenies. The principle of the device is based on the accumulation of multiple dielectrophoresis (DEP) forces to deflect cells laterally in conjunction with the alternating on/off electric field to manipulate the cell trajectories. The microfluidic device containing a large array of oblique interdigitated electrodes was fabricated using a combination of standard and soft lithography techniques to generate a PDMS-gold electrode construct. Experimental testing with human mesenchymal stem cells (hMSC) and their differentiation progenies (osteoblasts) was carried out at different flow rates, and clear separation of the two populations was achieved. Most of the osteoblasts experiencing stronger DEP forces were deflected laterally and continuously, following zig-zag trajectories, and moved towards the desired collection outlet, whereas most of the hMSCs remained on the original trajectory due to weaker DEP forces. The experimental measurements were characterized and evaluated quantitatively, and consistent performance was demonstrated. Collection efficiency up to 92% and 67% for hMSCs and osteoblasts, respectively, along with purity up to 84% and 87% was obtained. The experimental results established the feasibility of our microfluidic DEP sorting device for continuous, label-free sorting of stem cells and their differentiation progenies.

Electroosmotic flow hysteresis for dissimilar ionic solutions

Electroosmotic flow (EOF) with two or more fluids is commonly encountered in various microfluidics applications. However, no investigation has hitherto been conducted to investigate the hysteretic or flow direction-dependent behavior during the displacement flow of solutions with dissimilar ionic species. In this investigation, electroosmotic displacement flow involving dissimilar ionic solutions was studied experimentally through a current monitoring method and numerically through finite element simulations. The flow hysteresis can be characterized by the turning and displacement times; turning time refers to the abrupt gradient change of current-time curve while displacement time is the time for one solution to completely displace the other solution. Both experimental and simulation results illustrate that the turning and displacement times for a particular solution pair can be directional-dependent, indicating that the flow conditions in the microchannel are not the same in the two different flow directions. The mechanics of EOF hysteresis was elucidated through the theoretical model which includes the ionic mobility of each species, a major governing parameter. Two distinct mechanics have been identified as the causes for the EOF hysteresis involving dissimilar ionic solutions: the widening/sharpening effect of interfacial region between the two solutions and the difference in ion concentration distributions (and thus average zeta potentials) in different flow directions. The outcome of this investigation contributes to the fundamental understanding of flow behavior in microfluidic systems involving solution pair with dissimilar ionic species.

Concurrent DNA Preconcentration and Separation in Bipolar Electrode-Based Microfluidic Device

This paper presents a bipolar electrode (BPE) device in a microfluidic dual-channel design for concurrent preconcentration and separation of composite DNA containing samples. The novelty of the present effort relies on the combination of BPE-induced ion concentration polarization (ICP) and end-labeled free-solution electrophoresis (ELFSE). The ion concentration polarization effect arising from the faradaic reaction on the BPE is utilized to exert opposing electrophoretic and electroosmotic forces on the DNA samples. Meanwhile, end-labeled free-solution electrophoresis alters the mass-charge ratio to enable simultaneous DNA separation in free solution. The microfluidic device was fabricated using standard and soft lithography techniques to form gold-on-glass electrode capped with a PDMS microfluidic channel. Experimental testing with various DNA samples was carried out over a range of applied electric field. Concentration ratios up to 285× within 5 minutes for a 102-mer DNA, and concurrent preconcentration and free-solution separation of binary mixture of free and bound 102-mer DNA within 6 minutes was demonstrated. The effect of applied electric field was also interrogated with respect to pertinent performance metrics of preconcentration and separation.

A microfluidic impedance flow cytometer for identification of differentiation state of stem cells

This paper presents a microfluidic electrical impedance flow cytometer (FC) for identifying the differentiation state of single stem cells. This device is comprised of a novel dual micropore design, which not only enhances the processing throughput, but also allows the associated electrodes to be used as a reference for one another. A signal processing algorithm, based on the support vector machine (SVM) theory, and a data classification method were developed to automate the identification of sample types and cell differentiation state based on measured impedance values. The device itself was fabricated using a combination of standard and soft lithography techniques to generate a PDMS-gold electrode construct. Experimental testing with non-biological particles and mouse embryonic carcinoma cells (P19, undifferentiated and differentiated) was carried out using a range of excitation frequencies. The effects of the frequency and the interrogation parameters on sample identification performance were investigated. It was found that the real and imaginary part of the detected impedance signal were adequate for distinguishing the undifferentiated P19 cells from non-biological polystyrene beads at all tested frequencies. A higher frequency and an opacity index were required to resolve the undifferentiated and differentiated P19 cells by capturing capacitive changes in electrophysiological properties arising from differentiation. The experimental results demonstrated salient accuracy of the device and algorithm, and established its feasibility for non-invasive, label-free identification of the differentiation state of the stem cells.

A flexible layout design method for passive micromixers

This paper discusses a flexible layout design method of passive micromixers based on the topology optimization of fluidic flows. Being different from the trial and error method, this method obtains the detailed layout of a passive micromixer according to the desired mixing performance by solving a topology optimization problem. Therefore, the dependence on the experience of the designer is weaken, when this method is used to design a passive micromixer with acceptable mixing performance. Several design disciplines for the passive micromixers are considered to demonstrate the flexibility of the layout design method for passive micromixers. These design disciplines include the approximation of the real 3D micromixer, the manufacturing feasibility, the spacial periodic design, and effects of the Péclet number and Reynolds number on the designs obtained by this layout design method. The capability of this design method is validated by several comparisons performed between the obtained layouts and the optimized designs in the recently published literatures, where the values of the mixing measurement is improved up to 40.4% for one cycle of the micromixer.

Integrated microfluidic chip for rapid DNA digestion and time-resolved capillary electrophoresis analysis

An integrated microfluidic chip is proposed for rapid DNA digestion and time-resolved capillary electrophoresis (CE) analysis. The chip comprises two gel-filled chambers for DNA enrichment and purification, respectively, a T-form micromixer for DNA/restriction enzyme mixing, a serpentine channel for DNA digestion reaction, and a CE channel for on-line capillary electrophoresis analysis. The DNA and restriction enzyme are mixed electroomostically using a pinched-switching DC field. The experimental and numerical results show that a mixing performance of 97% is achieved within a distance of 1 mm from the T-junction when a driving voltage of 90 V/cm and a switching frequency of 4 Hz are applied. Successive mixing digestion and capillary electrophoresis operation clearly present the changes on digesting φx-174 DNA in different CE runs. The time-resolved electropherograms show that the proposed device enables a φx-174 DNA sample comprising 11 fragments to be concentrated and analyzed within 24 min. Overall, the results presented in this study show that the proposed microfluidic chip provides a rapid and effective tool for DNA digestion and CE analysis applications.

Continuous microfluidic DNA and protein trapping and concentration by balancing transverse electrokinetic forces

Sample pre-concentration can be a critical element to improve sensitivity of integrated microchip assays. In this work a converging Y-inlet microfluidic channel with integrated coplanar electrodes was used to investigate transverse DNA and protein migration under uniform direct current (DC) electric fields to assess the ability to concentrate a sample prior to other enzymatic modifications or capillary electrophoretic separations. Employing a pressure-driven flow to perfuse the microchannel, negatively charged samples diluted in low and high ionic strength buffers were co-infused with a receiving buffer of the same ionic strength into a main daughter channel. Experimental results demonstrated that, depending of the buffer selection, different DNA migration and accumulation dynamics were seen. Charged analytes could traverse the channel width and accumulate at the positive bias electrode in a low electroosmotic mobility, high electrophoretic mobility, high ionic strength buffer or migrated towards an equilibrium position within the channel in a high electroosmotic mobility, high electrophoretic mobility, low ionic strength buffer. The various migration behaviours are the result of a balance between the electrophoretic force and a drag force induced by a recirculating electroosmotic flow generated across the channel width due to the bounding walls. Under continuous flow conditions, DNA samples were concentrated several-fold by balancing these transverse electrokinetic forces. The electrokinetic trapping technique presented here is a simple technique which could be expanded to concentrate or separate other analytes as a preconditioning step for downstream processes.

Anomalous mixing behaviour in rotationally actuated microfluidic devices

We analyse the characteristics of two-fluid mixing in T-shaped microchannels on rotating platforms (Lab-on-a-Compact-Disk framework). Three regimes of mixing were identified based on the distinct flow behaviour in each of these regimes. A diffusion-based mixing regime was obtained for low rotation speeds. A Coriolis force based mixing regime was observed for intermediate rotation speeds, which introduced some nontrivial aspects in the mixing behaviour, which was explained through scaling analysis. At very high rotational speeds, rapid mixing close to the junction was achieved by exploiting flow instabilities (instability based mixing). A good agreement between the theoretical calculations and the experimental observations was obtained.

Electrokinetic secondary-flow behavior in a curved microchannel under dissimilar surface conditions

The curved channel appears to be indispensable for the lab-on-chips systems because it provides a convenient scheme for increasing the channel length per unit chip area in the direction of net flow. A secondary Dean flow in curved rectangular microchannels is examined by applying the finite-volume scheme with a semi-implicit method for pressure-linked equations (SIMPLE) algorithm for the pressure-driven electrokinetic transport. This framework is based on the theoretical model coupled with the full Poisson-Boltzmann, Navier-Stokes, and the Nernst-Planck principles of net charge conservation [Yun et al., Phys. Fluids 22, 052004 (2010)]. The effect of a dissimilar wall condition on the secondary flow at the turn is explored by considering different configurations of channel wall having complementary aspect ratios (i.e., ratio of the channel height to the channel width, H/W = 0.25 and 4.0) with combinations of hydrophilic glass and hydrophobic polydimethylsiloxane surfaces. Simulation results exhibit that, contrary to the case of general narrow-bore channels, the streamwise axial velocity tends to shift toward the inner wall caused by a stronger effect of the spanwise pressure gradient, according to a sufficiently low Dean number. The increasing rate of this shift with increasing curvature ratio is more significant in the shallow (or low-aspect-ratio) channel, due to the effect of greater distance traveled by the fluid along the outer wall. The curvature introduces the presence of pairs of counter-rotating vortices perpendicular to the flow direction. Comparing between shallow and deep (or high-aspect-ratio) channels allows us to identify that the patterns of axial velocity and vorticity are altered by the heterogeneity effect of surfaces occupying a large area. The total magnitude of vorticity at the cross section of the channel increases with increasing slip length, due to the contribution of enhanced axial velocity driven by the slip, while there is no fluid-slip dependency for the slip length of less than about 50 nm.

Optimal frequency for microfluidic mixing across a fluid interface

A new analytical tool for determining the optimum frequency for a micromixing strategy to mix two fluids across their interface is presented. The frequency dependence of the flux is characterized in terms of a Fourier transform related to the apparatus geometry. Illustrative microfluidic mixing examples based on electromagnetic forcing and fluid pumping strategies are presented.