Remotely powered distributed microfluidic pumps and mixers based on miniature diodes

We demonstrate new principles of microfluidic pumping and mixing by electronic components integrated into a microfluidic chip. The miniature diodes embedded into the microchannel walls rectify the voltage induced between their electrodes from an external alternating electric field. The resulting electroosmotic flows, developed in the vicinity of the diode surfaces, were utilized for pumping or mixing of the fluid in the microfluidic channel. The flow velocity of liquid pumped by the diodes facing in the same direction linearly increased with the magnitude of the applied voltage and the pumping direction could be controlled by the pH of the solutions. The transverse flow driven by the localized electroosmotic flux between diodes oriented oppositely on the microchannel was used in microfluidic mixers. The experimental results were interpreted by numerical simulations of the electrohydrodynamic flows. The techniques may be used in novel actively controlled microfluidic-electronic chips.     

A linear analysis of the effect of Faradaic currents on traveling-wave electroosmosis

Net fluid flow of electrolytic solutions induced by a traveling-wave potential applied to an array of co-planar interdigitated microelectrodes has been reported. At low applied voltages the flow is driven in the direction of the traveling-wave potential, as expected by linear and weakly nonlinear theoretical studies. The flow is driven at the surfaces of the electrodes by electrical forces acting in the diffuse electrical double layer. The pumping mechanism has been analyzed theoretically under the assumption of perfectly polarizable electrodes. Here we extend these studies to include the effect of Faradaic currents on the electroosmotic slip velocity generated at the electrode/electrolyte interface. We integrate the electrokinetic equations under the thin-double-layer and low-potential approximations. Finally, we analyze the pumping of electrolyte induced by a traveling-wave signal applied to a microelectrode array using this linear model.     

An integrated AC electrokinetic pump in a microfluidic loop for fast and tunable flow control

We have built a dedicated lab on a chip to study the performance of an integrated electrokinetic micropump, driven by a low voltage AC signal. This micropump consists of an array of interdigitated electrodes and is here integrated in a microfluidic loop. We demonstrate that this device can pump continuously and reproducibly electrolyte solutions of low to moderate ionic strength. The pumping speed reaches up to 500 [micro sign]m s(-1) in 20 [micro sign]m deep and 100 [micro sign]m wide channels with a driving signal in the 1-10 kHz range and an amplitude of only a few volts. In addition, we have observed an interesting reversal of the pumping direction at higher frequencies (50-100 kHz). Our device permits a systematic and automated exploration of the influence of the ionic strength thanks to an integrated micromixer.     

Numerical investigation of field-effect control on hybrid electrokinetics for continuous and position-tunable nanoparticle concentration in microfluidics

We introduce herein an effective way for continuous delivery and position-switchable trapping of nanoparticles via field-effect control on hybrid electrokinetics (HEK). Flow field-effect transistor exploiting HEK delicately combines horizontal linear electroosmosis and transversal nonlinear electroosmosis of a shiftable flow stagnation line (FSL) on gate terminals under DC-biased AC forcing. The microfluidic nanoparticle concentrator proposed herein makes use of a simple device geometry, in which an individual or a series of planar metal strips serving as gate electrode (GE) are subjected to a hybrid gate voltage signal and arranged in parallel between a pair of 3D driving electrodes. On the application of a DC-biased AC electric field across channel length direction, all the GE are electrochemically polarized, and the action of imposed hybrid electric field on the multiple-frequency bipolar counterions within the composite-induced double layer generates two counter-rotating induced-charge electroosmotic (ICEO) micro-vortices on top of each GE. Symmetry breaking in ICEO flow profile occurs once the gate voltage deviates from natural floating potential of corresponding GE. The gate voltage offset not only results in an additional pump motion of working fluid for enhanced electroosmotic transport but also directly changes the location of FSL where nanoparticles are preferentially collected by field-effect HEK. Our results of field-effect control on HEK are supposed to guide an elaborate design of flexible electrokinetic frameworks embedding coplanar metal strips for a high degree of freedom analyte manipulation in modern micro-total-analytical systems.     

Electroosmotic flow in channels with step changes in zeta potential and cross section

We consider the effects that step changes in zeta potential and cross section have on electroosmosis in long-and-narrow channels with arbitrary cross-sectional shapes. The Stokes equation of flow is solved analytically utilizing the thin Debye layer approximation to provide effective slip velocities on the channel walls. The effects of channel dimensions, surface potentials, applied pressure drop, and applied voltage are discussed. One anecdotal case, a two-region rectangular channel, is presented to illustrate the solution. The flow in each region is a combination of a uniform electroosmotic flow and a nonuniform pressure-driven flow. The electroosmotic pumping causes the pressure gradient in each region to adjust so that the flow rate is the same in each region and the overall applied pressure drop is met, resulting in convex velocity profiles in some regions and concave velocity profiles in other regions. By appropriate choice of the applied pressure drop, flat velocity profiles may be achieved in one or more regions.     

Investigation of pumping mechanism for non‐Newtonian blood flow with AC electrothermal forces in a microchannel by hybrid boundary element method and immersed boundary‐lattice Boltzmann method

Efficient pumping of blood flow in a microfluidic device is essential for rapid detection of bacterial bloodstream infections (BSI) using alternating current (AC) electrokinetics. Compared with AC electro‐osmosis (ACEO) phenomenon, the advantage of AC electrothermal (ACET) mechanism is its capability of pumping biofluids with high electrical conductivities at a relatively high AC voltage frequency. In the current work, the microfluidic pumping of non‐Newtonian blood flow using ACET forces is investigated in detail by modeling its multi‐physics process with hybrid boundary element method (BEM) and immersed boundary‐lattice Boltzmann method (IB‐LBM). The Carreau–Yasuda model is used to simulate the realistic rheological behavior of blood flow. The ACET pumping efficiency of blood flow is studied in terms of different AC voltage magnitudes and frequencies, thermal boundary conditions of electrodes, electrode configurations, channel height, and the channel length per electrode pair. Besides, the effect of rheological behavior on the blood flow velocity is theoretically analyzed by comparing with the Newtonian fluid flow using scaling law analysis under the same physical conditions. The results indicate that the rheological behavior of blood flow and its frequency‐dependent dielectric property make the pumping phenomenon of blood flow different from that of the common Newtonian aqueous solutions. It is also demonstrated that using a thermally insulated electrode could enhance the pumping efficiency dramatically. Besides, the results conclude that increasing the AC voltage magnitude is a more economical pumping approach than adding the number of electrodes with the same energy consumption when the Joule heating effect is acceptable.     

AC electroosmotic pump with bubble-free palladium electrodes and rectifying polymer membrane valves

We present the design, test and theoretical analysis of a novel micropump. The purpose is to make a pump with large flow rate (approximately 10 microL min-1) and high pressure capacity (approximately 1 bar) powered by a low voltage DeltaV<30 V. The pump is operated in AC mode with an electroosmotic actuator in connection with a full wave rectifying valve system. Individual valves are based on a flexible membrane with a slit. Bubble-free palladium electrodes are implemented in order to increase the range of applications and reduce maintenance.     

单级高压微流量电渗泵的研究

设计了一种高压微流量电渗泵。泵体主要由高压电源、电渗柱、毛细通道、导电空心电极以及气泡去除器、压力传感器等构成。单级电渗泵可以给出0～20MPa范围的输出压力和nL～μL级输出流量。输出压强和输出流量取决于电压、填充柱阻力和流体性质。     

AC electroosmotic micromixer for chemical processing in a microchannel

A rapid micromixer of fluids in a microchannel is presented. The mixer uses AC electroosmotic flow, which is induced by applying an AC voltage to a pair of coplanar meandering electrodes configured in parallel to the channel. To demonstrate performance of the mixer, dilution experiments were conducted using a dye solution in a channel of 120 microm width. Rapid mixing was observed for flow velocity up to 12 mm s(-1). The mixing time was 0.18 s, which was 20-fold faster than that of diffusional mixing without an additional mixing mechanism. Compared with the performance of reported micromixers, the present mixer worked with a shorter mixing length, particularly at low Peclet numbers (Pe < 2 x 10(3)).     

Dielectrophoresis switching with vertical sidewall electrodes for microfluidic flow cytometry

A novel dielectrophoresis switching with vertical electrodes in the sidewall of microchannels for multiplexed switching of objects has been designed, fabricated and tested. With appropriate electrode design, lateral DEP force can be generated so that one can dynamically position particulates along the width of the channel. A set of interdigitated electrodes in the sidewall of the microchannels is used for the generation of non-uniform electrical fields to generate negative DEP forces that repel beads/cells from the sidewalls. A countering DEP force is generated from another set of electrodes patterned on the opposing sidewall. These lateral negative DEP forces can be adjusted by the voltage and frequency applied. By manipulating the coupled DEP forces, the particles flowing through the microchannel can be positioned at different equilibrium points along the width direction and continue to flow into different outlet channels. Experimental results for switching biological cells and polystyrene microbeads to multiple outlets (up to 5) have been achieved. This novel particle switching technique can be integrated with other particle detection components to enable microfluidic flow cytometry systems.     

An optically driven pump for microfluidics

We demonstrate a method for generating flow within a microfluidic channel using an optically driven pump. The pump consists of two counter rotating birefringent vaterite particles trapped within a microfluidic channel and driven using optical tweezers. The transfer of spin angular momentum from a circularly polarised laser beam rotates the particles at up to 10 Hz. We show that the pump is able to displace fluid in microchannels, with flow rates of up to 200 microm(3) s(-1) (200 fL s(-1)). The direction of fluid pumping can be reversed by altering the sense of the rotation of the vaterite beads. We also incorporate a novel optical sensing method, based upon an additional probe particle, trapped within separate optical tweezers, enabling us to map the magnitude and direction of fluid flow within the channel. The techniques described in the paper have potential to be extended to drive an integrated lab-on-chip device, where pumping, flow measurement and optical sensing could all be achieved by structuring a single laser beam.     

Electroosmotic flow in microchannels with arbitrary geometry and arbitrary distribution of wall charge

General solutions are developed for direct current (DC) and alternating current (AC) electroosmotic flows in microfluidic channels with arbitrary cross-sectional geometry and arbitrary distribution of wall charge (zeta potential). The applied AC electric field can also be of arbitrary waveform. By proposing a nondimensional time scale varpi defined as the ratio of the diffusion time of momentum across the electric double-layer thickness to the period of the applied electric field, we demonstrate analytically that the Helmholtz-Smoluchowski electroosmotic velocity is an appropriate slip condition for AC electroosmotic flows in typical microfluidic applications. With this slip condition approach, electroosmotic flows in rectangular and asymmetric trapezoidal microchannels with nonuniform wall charge, as examples, are investigated. The unknown constants in the proposed general solutions are numerically determined with a least-squares method through matching the boundary conditions. We find that the wall charge affects significantly the electroosmotic flow while the channel geometry does not. Moreover, the flow feature is characterized by another nondimensional time scale Omega defined as the ratio of the diffusion time of momentum across the channel hydraulic radius to the period of the applied electric field. The onset of phase shift between AC electroosmotic velocity and applied electric field is also examined analytically.     

Bi-directional flow induced by an AC electroosmotic micropump with DC voltage bias

This paper discusses the principle of biased alternating current electroosmosis (ACEO) and its application to move the bulk fluid in a microchannel, as an alternative to mechanical pumping methods. Previous EO-driven flow research has looked at the effect of electrode asymmetry and transverse traveling wave forms on the performance of electroosmotic pumps. This paper presents an analysis that was conducted to assess the effect of combining an AC signal with a DC (direct current) bias when generating the electric field needed to impart electroosmosis (EO) within a microchannel. The results presented here are numerical and experimental. The numerical results were generated through simulations performed using COMSOL 3.5a. Currently available theoretical models for EO flows were embedded in the software and solved numerically to evaluate the effects of channel geometry, frequency of excitation, electrode array geometry, and AC signal with a DC bias on the flow imparted on an electrically conducting fluid. Simulations of the ACEO flow driven by a constant magnitude of AC voltage over symmetric electrodes did not indicate relevant net flows. However, superimposing a DC signal over the AC signal on the same symmetric electrode array leads to a noticeable net forward flow. Moreover, changing the polarity of electrical signal creates a bi-directional flow on symmetrical electrode array. Experimental flow measurements were performed on several electrode array configurations. The mismatch between the numerical and experimental results revealed the limitations of the currently available models for the biased EO. However, they confirm that using a symmetric electrode array excited by an AC signal with a DC bias leads to a significant improvement in flow rates in comparison to the flow rates obtained in an asymmetric electrode array configuration excited just with an AC signal.     

The effect of step height on the performance of three-dimensional ac electro-osmotic microfluidic pumps

Recent numerical and experimental studies have investigated the increase in efficiency of microfluidic ac electro-osmotic pumps by introducing nonplanar geometries with raised steps on the electrodes. In this study, we analyze the effect of the step height on ac electro-osmotic pump performance. AC electro-osmotic pumps with three-dimensional electroplated steps are fabricated on glass substrates and pumping velocities of low ionic strength electrolyte solutions are measured systematically using a custom microfluidic device. Numerical simulations predict an improvement in pump performance with increasing step height, at a given frequency and voltage, up to an optimal step height, which qualitatively matches the trend observed in experiment. For a broad range of step heights near the optimum, the observed flow is much faster than with existing planar pumps (at the same voltage and minimum feature size) and in the theoretically predicted direction of the "fluid conveyor belt" mechanism. For small step heights, the experiments also exhibit significant flow reversal at the optimal frequency, which cannot be explained by the theory, although the simulations predict weak flow reversal at higher frequencies due to incomplete charging. These results provide insight to an important parameter for the design of nonplanar electro-osmotic pumps and clues to improve the fundamental theory of ACEO.     

Bubble‐free electrokinetic flow with propylene carbonate

For electroosmotic pumping, a large direct‐current (DC) electric field (10+ V/cm) is applied across a liquid, typically an aqueous electrolyte. At these high voltages, water undergoes electrolysis to form hydrogen and oxygen, generating bubbles that can block the electrodes, cause pressure fluctuations, and lead to pump failure. The requirement to manage these gases constrains system designs. This article presents an alternative polar liquid for DC electrokinetic pumping, propylene carbonate (PC), which remains free of bubbles up to at least 10 kV/cm. This offers the opportunity to create electrokinetic devices in closed configurations, which we demonstrate with a fully sealed microfluidic hydraulic actuator. Furthermore, the electroosmotic velocity of PC is similar to that of water in PDMS microchannels. Thus, water could be substituted by PC in existing electroosmotic pumps.     

Electric field-decoupled electroosmotic pump for microfluidic devices

An electric field-free electroosmotic pump has been constructed and its pumping rate has been measured under various experimental conditions. The key component of the pump is an ion-exchange membrane grounding joint that serves two major functions: (i) to maintain fluid continuity between pump channels and microfluidic conduit and (ii) to ground the solution in the microfluidic channel at the joint through an external electrode, and hence to decouple the electric field applied to the pump channels from the rest of the microfluidic system. A theoretical model has been developed to calculate the pumping rates and its validity has been demonstrated.     

复杂电渗泵系统的输液特征

基于对电渗泵中压强产生原理的讨论, 对新兴的复杂串、并联电渗泵系统的流体输液特征加以研究. 在多级串联电渗泵系统中, 可以通过增加电渗泵的级数或操作电压来提高泵系统产生的压强, 但是单一流路的输液能力有限. 而在多级并联电渗泵系统中, 输液量也可以通过增加电渗泵的级数或操作电压得到提高. 在这一体系中, 尽管输液量的叠加有利于其在更为广泛的领域中应用, 但是因Joule热产生的电场强度与电渗流速度线性关系的偏离也使得其线性范围变小, 不利于操作条件的控制.     

Sodium silicate based sol–gel structures for generating pressure-driven flow in microfluidic channels

In this article, we report the design of a microchip based hydraulic pump that employs a sodium silicate derived sol–gel structure for generating pressure-driven flow within a microfluidic network. The reported sol–gel structure was fabricated in a chosen location of our device by selectively retaining sodium silicate solution within a sub-micrometer deep segment via capillary forces, and then providing the precursor material appropriate thermal treatment. It was shown that while the molecular weight cut-off for these membranes is at least an order of magnitude smaller than their photo-polymerized counterparts, their electrical conductance is significant. Moreover, unlike their polymeric counterparts these structures were found to be capable of blocking electroosmotic flow, thereby generating a pressure-gradient around their interface with an open microchannel upon application of an electric field across the microchannel–membrane junction. In this work, a fraction of the resulting hydrodynamic flow was successfully guided to an electric field-free analysis channel to implement a pressure-driven assay. Our experiments show that the pressure-driven velocity produced in the analysis channel of our device varied linearly with the voltage applied across the sol–gel membrane and was nearly independent of the cross-sectional dimensions of the membrane and the microfluidic channels. With our current design pressure-driven velocities up to 1.7 mm/s were generated for an applied voltage of 2 kV, which easily covers the range of flow speeds that can minimize the plate height in most microfluidic separations. Finally, the functionality of our device was demonstrated by implementing a reverse phase chromatographic separation in the analysis channel of our device using the pressure-driven flow generated on-chip.     

Electroosmotic pumping in microchips with nonhomogeneous distribution of electrolytes

A general equation to calculate the node pressure at a junction in a microfluidic network is presented. The node pressure is generated from both the hydrodynamic flow due to the external applied hydraulic pressures and the electrokinetic flow resulted from the applied electric field. Pure electroosmotic flow has a plug-flow profile and pressure flow has a parabolic flow profile. In a first order approximation, these two flows can be treated separately, and the total flow is the sum of the two. An externally applied pressure simply creates a constant offset in the node pressure as long as the flow resistances remain the same. In a nonhomogeneous microfluidic network, where the electrical resistivity or the electroosmotic mobility is not constant everywhere, the differences in electroosmotic flow in various sections of the network will create an electroosmotically induced pressure at the internal nodes. Our theoretical approach can easily be extended to networks with more than one internal node. One prediction of this theory is that any variation in electroosmotic mobility or solution resistivity in different network branches will generate a pressure, and can thus be used as a pump. As an example, we demonstrate electroosmotic pumping in a high-low buffer system.     

Bi‐directional ACET micropump for on‐chip biological applications

The ability to control and pump high ionic strength fluids inside microchannels forms a major advantage for clinical diagnostics and drug screening processes, where high conductive biological and physiological buffers are used. Despite the known potential of AC electro‐thermal (ACET) effect in different biomedical applications, comparatively little is known about controlling the velocity and direction of fluid inside the chip. Here, we proposed to discretize the conventional electrodes to form various asymmetric electrode structures in order to control the fluid direction by simple switching the appropriate electric potential applied to the discretized electrodes. The ACET pumping effect was numerically studied by solving electrical, thermal and hydrodynamic multi‐physic coupled equations to optimize the geometrical dimensions of the discretized system. PBS solutions with different ionic strength were seeded with 1 μm sized fluorescent particles and electrothermally driven fluid motion was observed inside the channel for different electrode structures. Experimental analyses confirm that the proposed micropump is efficient for a conductivity range between 0.1 and 1 S/m and the efficiency improves by increasing the voltage amplitude. Behavior of the proposed electrode–electrolyte system is discussed by lumped circuit model. Frequency response of system illustrated that the optimal frequency range increases by increasing the conductivity of medium. For 0.18 S/m PBS solution, the constant pumping effect was observed at frequency range between 100 kHz and 1 MHz, while frequency range of 100 kHz to 5 MHZ was observed for 0.42 S/m. The characteristics of experimental results were in good agreement with the theoretical model.