Induced-charge electro-osmosis of polymer-containing fluid around a metallic rod

Induced-charge electro-osmotic (ICEO) flow of polymer-containing electrolyte solution around a cylindrical gold-coated stainless steel rod under AC electric field is measured by micro-particle image velocimetry (micro-PIV) for the first time. The ICEO flows as functions of the amount of non-ionic PEG (polyethylene glycol), cationic PDADMA (polydiallyldimethylammonium chloride), and anionic PVSASS (polyvinylsulfonic acid sodium salt) polymers added into the salt solution, frequency, and strength of the AC electric field are measured. The ICEO flow of polymer-containing fluid around the rod is quadrupolar with four vortices and is proportional to the square of imposed electric field. The ICEO flow velocity exponentially decreases with an increased concentration of neutral PEG. Ionic polyelectrolytes significantly increase ICEO velocities due to the enriched net charge within the induced electric double layer arising from the electrostatic interaction between the polarized rod’s surface and the charged polyelectrolytes in ionic polymer solution. In addition, polymer concentration affects the dependence of the ICEO flow on the frequency of AC electric field.     

Experiments on opto-electrically generated microfluidic vortices

Strong microfluidic vortices are generated when a near-infrared (1,064 nm) laser beam is focused within a microchannel and an alternating current (AC) electric field is simultaneously applied. The electric field is generated from a parallel-plate, indium tin oxide (ITO) electrodes separated by 50 μm. We present the first μ-PIV analysis of the flow structure of such vortices. The vortices exhibit a sink-type behavior in the plane normal to the electric field and the flow speeds are characterized as a function of the electric field strength and biasing AC signal frequency. At a constant AC frequency of 100 kHz, the fluid velocity increases as the square of the electric field strength. At constant electric field strength fluid velocity does not change appreciably in the 30–50 kHz range and it decreases at larger frequencies (>1 MHz) until at approximately 5 MHz when Brownian motion dominates the movement of the 300 nm μ-PIV tracer particles. Presence of strongly focused laser beams in an interdigitated-electrode configuration can also lead to strong microfluidic vortices. When the center of the illumination is focused in the middle of an electrode strip, particles experiencing negative dielectrophoresis are carried towards the illumination and aggregate in this area.     

A cell electrofusion microfluidic chip with micro-cavity microelectrode array

A new cell electrofusion microfluidic chip with 19,000 pairs of micro-cavity structures patterned on vertical sidewalls of a serpentine-shaped microchannel has been designed and fabricated. In each micro-cavity structure, the two sidewalls perpendicular to the microchannel are made of SiO₂ insulator, and that parallel to the microchannel is made of silicon as the microelectrode. One purpose of the design with micro-cavity microelectrode array is to obtain high membrane voltage occurring at the contact point of two paired cells, where cell fusion takes place. The device was tested to electrofuse NIH3T3 and myoblast cells under a relatively low voltage (~9 V). Under an AC electric field applied between the pair of microelectrodes positioned in the opposite micro-cavities, about 85–90 % micro-cavities captured cells, and about 60 % micro-cavities are effectively capable of trapping the desired two-cell pairs. DC electric pulses of low voltage (~9 V) were subsequently applied between the micro-cavity microelectrode arrays to induce electrofusion. Due to the concentration of the local electric field near the micro-cavity structure, fusion efficiency reaches about 50 % of total cells loaded into the device. Multi-cell electrofusion and membrane rupture at the end of cell chains are eliminated through the present novel design.     

Droplet group production in an AC electro-flow-focusing microdevice

We report the production of droplet groups with a controlled number of drops in a microfluidic electro-flow-focusing device under the action of an AC electric field. This regime appears for moderate voltages (500–700 V peak-to-peak) and signal frequencies between 25 and 100 Hz, much smaller than the droplet production rate (\({\sim }\,{500}\) Hz). For this experimental condition the production frequency of a droplet package is twice the signal frequency. Since the continuous phase flow in the microchannel is a Hagen–Poiseuille flow, the smaller droplets of a group move faster than the bigger ones leading to droplet clustering downstream.     

On-demand particle enrichment in a microfluidic channel by a locally controlled floating electrode

A flexible strategy for the on-demand control of the particle enrichment and positioning in a microfluidic channel is proposed and demonstrated by the use of a locally controlled floating metal electrode attached to the channel bottom wall. The channel is subjected to an axially acting global DC electric field, but the degree of charge polarization of the floating electrode is governed largely by a local control of the voltage applied to two micron-sized control electrodes (CEs) on either side of the floating electrode (FE). This strategy allows an independent tuning of the electrokinetic phenomena engendered by the floating electrode regardless of the global electric field across the channel, thus making the method for particle manipulation far more versatile and flexible. In contrast to a dielectric microchannel wall possessing a nearly uniform surface charge (or zeta potential), the patterned metal strip (floating electrode) is polarized under electric field resulting in a non-uniform distribution of the induced surface charge with a zero net surface charge, and accordingly induced-charge electro-osmotic (ICEO) flow. The ICEO flow can be regulated by the control electric field through tuning the magnitude and polarity of the voltage applied to the CEs, which in turn affects both the hydrodynamic field as well as the particle motion. By controlling the control electric field, on-demand control of the particle enrichment and its position inside a microfluidic channel has been experimentally demonstrated.     

Dynamic aspects of electroosmotic flow

This article presents an analysis of the frequency and time dependent electroosmotic flow in open-end and closed-end microchannels of arbitrary cross-section shape. In the numerical model, the modified Navier–Stokes equation governing the AC electroosmotic flow is solved using the control volume method. The iterative approach is used to determine the induced backpressure gradient. The potential distribution of the EDL in the channel is obtained by solving the non-linear 2D Poisson–Blotzmann equation. The comparison between the control volume formulation and the Green’s function method for the case of a rectangular microchannel shows a good agreement. The time evolution of the electroosmotic flow and the effect of a frequency-dependent AC electric field on the oscillating electroosmotic flow are also examined. The effect of the induced backpressure gradient with the frequency of the applied electric field is also shown.     

Particle focusing in a contactless dielectrophoretic microfluidic chip with insulating structures

The focusing of biological and synthetic particles in microfluidic devices is a crucial step for the construction of many microstructured materials as well as for medical applications. The present study examines the feasibility of using contactless dielectrophoresis (cDEP) in an insulator-based dielectrophoretic (iDEP) microdevice to effectively focus particles. Particles 10?μm in diameter were introduced into the microchannel and pre-confined hydrodynamically by funnel-shaped insulating structures near the inlet. The particles were repelled toward the center of the microchannel by the negative DEP forces generated by the insulating structures. The microchip was fabricated based on the concept of cDEP. The electric field in the main microchannel was generated using electrodes inserted into two conductive micro-reservoirs, which were separated from the main microchannel by 20-μm-thick insulating barriers made of polydimethylsiloxane (PDMS). The impedance spectrum of the thin insulating PDMS barrier was measured to investigate its capacitive behavior. Experiments employing polystyrene particles were conducted to demonstrate the feasibility of the proposed microdevice. Results show that the particle focusing performance increased with increasing frequency of the applied AC voltage due to the reduced impedance of PDMS barriers at high frequencies. When the frequency was above 800?kHz, most particles were focused into a single file. The smallest width of focused particles distributed at the outlet was about 13.1?μm at a frequency of 1?MHz. Experimental results also show that the particle focusing performance improved with increasing applied electric field strength and decreasing inlet flow rate. The usage of the cDEP technique makes the proposed microchip mechanically robust and chemically inert.     

An efficient micromixer actuated by induced-charge electroosmosis using asymmetrical floating electrodes

Efficient microfluid mixing is an important process for various microfluidic-based biological and chemical reactions. Herein we propose an efficient micromixer actuated by induced-charge electroosmosis (ICEO). The microchannel of this device is easy to fabricate for its simple straight channel structure. Importantly, unlike previous design featuring complicated three-dimensional conducting posts, we utilize the simpler asymmetrical planar floating-electrodes to induce asymmetrical microvortices. For evaluating the mixing performance of this micromixer, we conducted a series of simulations and experiments. The mixing performance was quantified using the mixing index, specifically, the mixing efficiency can reach 94.7% at a flow rate of 1500 µm/s under a sinusoidal wave with a peak voltage of 14 V and a frequency of 400 Hz. Finally, we compared this micromixer with different micromixing devices using a comparative mixing index, demonstrating that this micromixer remains competitive among these existing designs. Therefore, the method proposed herein can offer a simple solution for efficient fluids mixing in microfluidic systems.     

Effect of doping ferrocene in the working fluid of electrohydrodynamic (EHD) micropumps

The effect of doping ferrocene in the working fluid of electrohydrodynamic micropumps was investigated under the application of DC electric fields. The micropump consisted of 100 planar electrode pairs that were embedded along the bottom wall of a 100-micron-high, 5-mm-wide and 26-mm-long microchannel. The width of the emitter and collector electrodes was 20 and 40 µm, respectively, with inter-electrode spacing of 30 µm. A redox dopant, ferrocene, was diffused homogeneously into the working fluid HFE-7100 at 0.05, 0.1 and 0.2 % concentration by weight. The static pressure head generation and flow rate at different back pressure conditions were measured under different applied DC voltages. The current and pressure generated with the doped working fluid were significantly higher than with pure HFE-7100 under an applied DC field. A maximum static pressure of 6.7 kPa and flow rate of 0.47 mL/min at no back pressure were achieved at 700 V.     

Electrophoresis of a finite rod along the axis of a long cylindrical microchannel filled with Carreau fluids

The boundary effect on the electrophoretic behavior of a particle in a non-Newtonian fluid is studied by considering the electrophoresis of a finite rod along the axis of a cylindrical microchannel filled with shear-thinning Carreau fluids, which include both Newtonian and power-law fluids as special cases. Under the conditions of low surface potential and weak applied electric field, the influences of the radius of the microchannel, the aspect ratio of the rod, the thickness of double layer, and the nature of the Carreau fluid on the mobility of the rod are investigated. We show that due to the shear-thinning effect, the mobility of the rod in the present case can be significantly larger than that in the corresponding Newtonian case; the former is more sensitive to the variation in the thickness of double layer than the latter, and the difference between the two increases with decreasing thickness of double layer. The shear-thinning effect is important under the following conditions: the double layer is thin, the boundary effect is important, and/or the aspect ratio is large. We show that increasing the aspect ratio can either raise or lessen its mobility, which is not found previously, and can play an important role in electrophoresis measurement.     

Localized flow control in microchannels using induced-charge electroosmosis near conductive obstacles

In this paper, we investigate the use of induced-charge electroosmosis (ICEO) as a means of providing localized flow control near conductive obstacles within bulk pressure-driven flow. In an experimental device, this ICEO flow was induced by an on/off switchable AC field applied across a section containing gold post(s). A simple numerical model, adapted from Levitan et al. (Colloids Surf A 267:122–132, 2005), was implemented and used to provide guidance for the design of the experimental devices. The induced flow was combined with an applied bulk pressure-driven flow to modify flow patterns. We have specifically observed single and multiple stream patterns downstream of the posts in the experimental devices, suggesting the presence of ICEO flow in the experimental system. The custom devices were obtained using a fabrication process that relies on relatively standard steps in the MEMS community, however, unlike other fabrication processes, it has been shown to create fully conductive posts with vertical sidewalls. Utilizing various combinations of number(s) of post(s), geometry and position, useful flow patterns can be created.     

Improved particle concentration by cascade AC electroosmotic flow

The importance of electrokinetics in microfluidic technology has been growing owing to its versatility and simplicity in fabrication, implementation, and handling. Alternating-current electroosmosis (ACEO), which is the motion of fluid due to the ion movement by an interaction between AC electric field and an electrical double layer on the electrode surface, has a potential for a particle concentration method to detecti rare samples flowing in a microchannel. This study investigates an improved ACEO-based particle concentration by cascade electrokinetic approach. Flow field induced by ACEO and accumulation behavior of particles were parametrically measured to discuss the concentrating mechanism. The accumulation of particles by ACEO can be explained by a balance between the attenuating electroosmotic flow to transport particles and the inherent diffusive motion of the particles, which is hindered due to the near-wall location. Although a parallel double-gap electrode geometry enables particles to be collected at the center of electrode very sharply, it has scattering zones with accumulated particles at sidewalls of the channel. This drawback can be overcome by applying sheath flow or introducing cascade electrode pattern upstream of the focusing zone. As a result, total concentration efficiency was 98.4 % for all the particles flowing in the cascade device. The resultant concentrated particles exist on the electrode surface within 5 μm, and three-dimensional concentration of particle with the concentration factor as large as 700 is possible using a monolithic channel, co-planar electrode, and sheathless solution feeding. This cascade electrokinetic method provides a new and effective preconcentrator for ultra-sensitive detection of rare samples.     

Effects of applied electric field and microchannel wetted perimeter on electroosmotic velocity

Parameters which affect electroosmotic flow (EOF) behavior need to be determined for characterizing flow in miniature biological and chemical experimental processes. Several parameters like buffer pH, ionic concentration, applied electric field and channel dimensions influence the magnitude of the electroosmotic flow. We conducted numerical and experimental investigations to determine the impact of electric field strength and wetted microchannel perimeter on EOF in straight microchannels of rectangular cross-section. Deviation from theoretical behavior was also investigated. In the numerical model, we solved the continuity and Navier–Stokes equations for the fluid flow and the Gauss law equation for the electric field. Computational results were validated against experimental data for PDMS-glass channels of different wetted perimeters over a range of applied electric fields. Results show that increasing the applied electric field at constant wetted perimeter caused the electroosmotic mobility, the ratio of electroosmotic velocity to applied electric field, to increase nonlinearly. It was also found that increasing the wetted perimeter at constant applied electric field decreased the electroosmotic flow. These findings will be useful in determining the optimum value of the electric field required to produce a desired electroosmotic flow rate in a channel of a particular dimension. Alternately, these will also be useful in determining the optimum channel dimensions to provide a desired electroosmotic flow rate at a specified value of the electric field.     

Investigation of active interface control of pressure driven two-fluid flow in microchannels

We report a novel concept to control the interface location of a pressure-driven multi-phase flow in a microchannel by using electroosmotic flow effects. This concept has potential applications in flow switching and cell sorting in bio-analytical systems. In an H-shaped microchannel structure, aqueous sodium chloride (NaCl) solution and glycerol diluted with water were pumped through two inlets at the same flow rate. The electric field was applied on the electrolyte solution side. Adjusting the magnitude and direction of electric field has successfully controlled the interface position between the two phases. This technique provides a new approach to control the interface position between the two fluids.     

Analytical formulation of electric field and dielectrophoretic force for moving dielectrophoresis using Fourier series

Electrokinetics manipulation and separation of living cells employing microfluidic devices require good knowledge of the strength and distribution of electric field in such devices. AC dielectrophoresis is performed by generating non-uniform electric field using microsize electrodes. Among the several applications of dielectrophoretic phenomenon, this present study considers the recently introduced phenomenon of moving dielectrophoresis. An analytical solution using Fourier series is presented for the electric field distribution and dielectrophoretic force generated inside a microchannel. The potential at the upper part of the microchannel has been found by solving the governing equation of the electric potential with specific boundary conditions. The solutions for the electric field and dielectrophoretic force show excellent agreement with the numerical results. Microdevices were fabricated and experiments were carried out with living cells confirming and validating the analytical solutions.     

Droplet transport through dielectrophoretic actuation using line electrode

We explore a novel transverse line electrode configuration for droplet transport through dielectrophoretic actuation with potential lab-on-chip applications. Using a lumped electromechanical model, we show a weak dependence of DEP actuation force on electrode spacing in this configuration. The configuration successfully triggers translational drop motion with minimal changes in contact angle at considerably low voltages. Two sessile, deionized water drops placed horizontally apart on a indium-tin–oxide-coated glass with additional coatings of polydimethylsiloxane, and a thin layer of Teflon is merged by applying an AC field (88 V_rms at 150 kHz) through a common horizontal wire electrode. A lateral motion of two drops is induced along the horizontal electrode, eventually leading to coalescence. The drop motion is unique compared to electrowetting in its near-constant dynamic contact angle, and irreversibility on withdrawal of electric field. The effect of frequency on the drop behavior is examined through a parametric study on single drops within the range of 2–200 kHz. It is interesting to observe a switch-over from DEP behavior at high frequency to EWOD behavior at low frequency around a critical frequency (Jones in Langmuir 18:4437–4443, 2002).     

Liquid flow through a diverging microchannel

In this work, experiments and three-dimensional numerical calculations of fluid flow through diverging microchannels were carried out with the aim of bringing out differences between flow in uniform and nonuniform passages. Deionized water was used as the working fluid in the experiments where the effects of mass flow rate (8.33 × 10^?6 to 8.33 × 10^?5 kg/s), microchannel hydraulic diameter (118–177 µm), length (10–30 mm) and divergence angle (4°–16°) on pressure drop were studied. The results are analyzed in detail with the help of numerical data. The pressure drop exhibits a linear dependence on the mass flow rate, whereas it is inversely proportional to the divergence angle and square of the hydraulic diameter. The pressure drop increases anomalously at 16°, suggesting that flow reversal occurs between 12° and 16°, which agrees with the corresponding value at the conventional scale. For the purpose of predicting pressure drop using straight microchannel theory, an equivalent hydraulic diameter was defined. It is observed that the equivalent hydraulic diameter, located at one-third of the diverging microchannel length from the inlet, becomes mostly independent of the mass flow rate, microchannel hydraulic diameter, length and divergence angle. The pressure drop for a diverging microchannel becomes equal to an equivalent hydraulic diameter uniform cross-section microchannel, suggesting that conventional correlations for straight microchannels can also be applied to diverging microchannels. The data presented in this work are of fundamental importance and can help in optimization of diffuser design used for example in valveless micropumps.     

Analysis of combined pressure-driven electroosmotic flow through square microchannels

In this paper three-dimensional single-phase liquid flow through microchannels with a square-shaped cross-section driven by simultaneous application of pressure gradient and electroosmotic pumping mechanism is studied. The governing system of equations consists of the electric potential field and flow field equations. The solution procedure involves three steps. First, the net charge distribution on the cross-section of the microchannel is computed by solving two-dimensional Poisson–Boltzmann equation using the finite element method. Then, using the computed fluid’s charge distribution, the magnitude of the resulting body force due to interaction of an external electric field with the charged fluid particles is calculated along the microchannel. In the third step, the flow equations are solved by considering three-dimensional Navier–Stokes equations with an electrokinetic body force. The computations reveal that the flow pattern in the microchannel is significantly different from the parabolic velocity profile of the laminar pressure-driven flow. The effect of the liquid bulk ionic concentration and the external electric field strength on flow patterns through the square-shaped microchannels is also investigated.     

Configurable AC electroosmotic generated in-plane microvortices and pumping flow in microchannels

This paper proposes the patterned AC electroosmotic flows (AC-EOF) by simply grouping discrete electrodes together to form various electrode configurations for generation of in-plane microvortices with clockwise/counter-clockwise rotation, and pumping flow in a microchannel. The rotational direction of in-plane microvortices and pumping flow direction can be controlled using the same electrode pattern by simple switching of the voltages on the electrodes. Microparticle image velocimetry (μPIV) was used to characterize the flow fields of the generated in-plane microvortices and pumping flow. The rotational strength of microvortices and flow rates of pumping flows were found to increase with the increase of the applied voltages, and an optimal value was achieved at an appropriately applied frequency. Moreover, the dependency of the applied voltages, frequencies, and the heights of the measured planes on the rotational strength of in-plane microvortices between the interdigitated and discrete electrode configurations were examined. The discrepancy in electrode geometry results in a small performance reduction, whereas it can be compensated for the ability of switching the rotational direction of in-plane microvortices using the same device. The configurable in-plane microvortices and pumping flow in microchannels provide the potential for micromixing applications and for the integration into a lab-on-a-chip system.     

Application of astigmatism μ-PTV to analyze the vortex structure of AC electroosmotic flows

The three-dimensional (3D) flow due to AC electroosmotic (ACEO) forcing on an array of interdigitated symmetric electrodes in micro-channels is experimentally analyzed using astigmatism micro-particle tracking velocimetry (astigmatism μ-PTV). Upon application of the AC electric field with a frequency of 1,000 Hz and a voltage of 2 Volts peak–peak, the obtained 3D particle trajectories exhibit a vortical structure of ACEO flow above the electrodes. Two alternating time delays (0.03 and 0.37 s) were used to measure the flow field with a wide range of velocities, including error analysis. Presence and properties of the vortical flow were quantified. The steady nature and the quasi-2D character of the vortices can combine the results from a series of measurements into one dense data set. This facilitates accurate evaluation of the velocity field by data-processing methods. The primary circulation of the vortices due to ACEO forcing is given in terms of the spanwise component of vorticity. The outline of the vortex boundary is determined via the eigenvalues of the strain-rate tensor. Overall, astigmatism μ-PTV is proven to be a reliable tool for quantitative analysis of ACEO flow.