Induced-charge Electro-osmosis Research Articles

Induced-charge electrophoresis of a tilted metal nanowire near an insulating wall.

Electric fields are commonly used to control the orientation and motion of microscopic metal particles in aqueous suspensions. For example, metallodielectric Janus spheres are propelled by the induced-charge electro-osmotic flow occurring on their metallic side, the most common case in electrokinetics of exploiting symmetry breaking of surface properties for achieving net particle motion. In this work, we demonstrate that a homogeneous metal rod can translate parallel to a dielectric wall as a result of the hydrodynamic wall-particle interaction arising from the induced-charge electro-osmosis on the rod surface. The applied electric field could be either dc or low-frequency ac. The only requirement for a nonvanishing particle velocity is that the axis of the rod be inclined with respect to the wall, i.e., it cannot be neither parallel nor perpendicular. We show numerical results of the rod velocity as a function of rod orientation and distance to the wall. The maximum particle velocity is found for an orientation of between ∼30^{∘} and ∼50^{∘}, depending on the position and aspect ratio of the cylinder. Particle velocities of up to tens of µm/s are predicted for typical conditions in electrokinetic experiments.

Bidirectional and Stepwise Rotation of Cells and Particles Using Induced Charge Electroosmosis Vortexes.

The rotation of cells is of significant importance in various applications including bioimaging, biophysical analysis and microsurgery. Current methods usually require complicated fabrication processes. Herein, we proposed an induced charged electroosmosis (ICEO) based on a chip manipulation method for rotating cells. Under an AC electric field, symmetric ICEO flow microvortexes formed above the electrode surface can be used to trap and rotate cells. We have discussed the impact of ICEO and dielectrophoresis (DEP) under the experimental conditions. The capabilities of our method have been tested by investigating the precise rotation of yeast cells and K562 cells in a controllable manner. By adjusting the position of cells, the rotation direction can be changed based on the asymmetric ICEO microvortexes via applying a gate voltage to the gate electrode. Additionally, by applying a pulsed signal instead of a continuous signal, we can also precisely and flexibly rotate cells in a stepwise way. Our ICEO-based rotational manipulation method is an easy to use, biocompatible and low-cost technique, allowing rotation regardless of optical, magnetic or acoustic properties of the sample.

Controllable and Scaffold‐Free Formation of 3D Multicellular Architectures Using a Bipolar Electrode Array

AbstractThe natural environment of cells in vivo is a 3D network in which cellular responses are determined by influences from surrounding cells on all sides. This complex system is hard to be recapitulated in a conventional way. It is highly desirable to replace 2D models with 3D cell culture models. Microfluidic technology offers a variety of advantageous approaches for the long‐term 3D cell culture. Herein, a new cell patterning approach for generating cell clusters of specified size and shape by exploiting induced charge electroosmosis (ICEO) flow at a bipolar electrode array is exploited. Cell or particle clusters can be produced at the edges or centers of bipolar electrodes (BPEs) with controlled organization, by designing electrodes with arbitrary shapes. This platform is further extended to flexibly organize biological units with different properties via the ICEO or dielectrophoresis (DEP) based assembly. Heterotypic cell clusters can be produced at the bipolar electrode (BPE) by adjusting the applied frequencies and the sample injection sequence. This hybrid approach by integrating ICEO flow vortexes and DEP properties of the cells, allows the wireless generation of high‐throughput 3D clusters or spheroids with size adjustability and biocompatibility, which shows great potential in tissue engineering, drug discovery, and tumor research.

Effect of elasticity on the induced charge electro-osmotic mixing of viscoelastic fluids in a micromixer with a conductive cylinder

Mixing of reagents in microfluidics is critical, and currently, the focus is on Newtonian fluids, but these reagents are often viscoelastic fluids. In this study, a micromixer containing a conductive cylinder is proposed based on the principle of induced charge electro-osmosis (ICEO). The Oldroyd-B constitutive model was chosen to characterize the flow properties of viscoelastic fluids, and the Poisson–Boltzmann model was used to describe the ion distribution in the electrolyte. The impact of the elasticity number (El) of viscoelastic fluids on the mixing efficiency, velocity, and vortex in the micromixer was studied. The results show that the mixing efficiency is only 55.41% when El = 0 (Newtonian fluid), and the mixing efficiency reaches 99.08% when El = 50. As El increases from 0 to 50, the cross-sectional average velocity at 50 μm from the micromixer exit decreases from 160 to 26.1 μm/s. Furthermore, the vortices around the conductive plate generated by the ICEO phenomenon begin to fluctuate at El = 7.

Fluidic switch using an elastic actuator due to induced charge electro-osmosis combined with a natural convection pump

An integrated fluid circuit consisting of a pump and a fluidic switch is a basic unit in a microfluidic circuit. Here, we propose a fluidic switch using an elastic actuator due to an induced charge electro-osmosis (ICEO) combined with a natural convection (NC) pump and experimentally demonstrate the switching behavior between the two branch fluidic channels in a flow state generated by the NC pump. In particular, we experimentally demonstrate that the NC pump can generate a net flow of ∼0.2 mm s−1 in the circular channel having branch channels, and the ICEO fluidic switch can change the flow state between the branch channels within ∼3 s. Our prototype system should contribute to innovative microfluidic systems in the future.

Induced-charge electroosmosis flow of viscoelastic fluids under different voltage arrangements

Efficient mixing of chemical analysis reagents with laboratory samples at a microscale is a key issue in numerous biomedical and chemical analyses but hardly to implement due to the limited of the low diffusivity in laminar flow. Induced-charge electroosmosis flow, as an innovative mixing method, has been proved to be effective and simple in rapid mixing attributes to its mechanism of vortex generation. This work aims to propose a new strategy for chaotic induced-charge electroosmosis flow based on different voltage arrangements to improve the mixing of viscoelastic fluids. The Phan–Thien–Tanner constitutive model is applied to characterize the flow behavior of viscoelastic fluid in a microfluidic preparation mixer. The direct numerical simulation method is used to solve the fully coupled Navier–Stokes and Poisson–Nernst–Planck equations for a polarizable cylinder in a two-dimensional cavity filled with electrolyte solution. The impact of Weissenberg number (Wi), Debye parameter, voltage strength on the velocity, net charge density, and potential profiles is investigated. The simulation results indicate that a greater Wi leads to the decrease in the maximum velocity, and a large voltage strength can heighten the net charge density and potential, thus improve the peak velocity. Moreover, the classical theoretical prediction that the maximum velocity is proportional to the square of the applied voltage has been authenticated.

Experimental investigation of inflow-outflow asymmetry in induced-charge electro-osmosis

Induced-charge electro-osmosis (ICEO) is a research hotspot in bioengineering and analytical chemistry. Inflow-outflow asymmetry of ICEO was reported in the existing literatures, but systematic study on this phenomenon is insufficient. In this experimental study, we found that in strong electric fields, not only the velocity magnitude but also the vortex positions of ICEO are asymmetrical along the inflow and outflow directions because of the pronounced non-uniform surface electrokinetic transport. On the inflow and outflow directions, the amplitudes of velocities are unequal, ICEO maximum velocity positions change depending on the electric field intensity and sodium chloride (NaCl) concentration. Additionally, the distances between vortex centers are different. At NaCl solution concentration of 0.001 mol·L–1, the outflow velocity almost vanishes. The asymmetry rises with the increasing electric field intensity. The new discoveries can direct the application of microscale devices.

Steric effects in induced-charge electro-osmosis for strong electric fields

Steric effects in induced-charge electro-osmosis for strong electric fields

Induced charge electro-osmotic mixing performance of viscoelastic fluids in microchannels with an electrically conductive plate

Micromixers have important applications in lab-on-a-chip, biomanufacturing, and chemical engineering. In this study, a micromixer with a conductive barrier plate based on the induced charge electro-osmosis is proposed. The Oldroyd-B constitutive model was chosen to characterize the flow characteristics of viscoelastic fluids, and the Poisson–Boltzmann model was used to characterize the electrokinetic properties. The effects of the installation of the conductive plate, the concentration of the polymer, and the shape of the conductive plate on the mixing were studied based on the finite volume method. The mixing efficiency of the viscoelastic fluids is 78.3% when a non-conductive plate is placed in the micromixer. However, placing a conductive plate increases the mixing efficiency to 89.8%. As the polymer concentration increases, the mixing efficiency increases, which is attributed to the elastic instability. As the curvature of the conductive plate increases from 0° to 360°, the mixing efficiency of the Newtonian fluid increases by 2.82%, while that of the polyacrylamide solutions at concentrations of 100 and 250 ppm increases by 5.31% and 1.97%, respectively.

Developing an Active Microfluidic Pump and Mixer Driven by AC Field-Effect-Mediated Induced-Charge Electro-Osmosis of Metal–Dielectric Janus Micropillars: Physical Perspective and Simulation Analysis

We propose herein a novel microfluidic approach for the simultaneous active pumping and mixing of analytes in a straight microchannel via the AC field-effect control of induced-charge electro-osmosis (ICEO) around metal–dielectric solid Janus cylinders of inherent inhomogeneous electrical polarizability immersed in an electrolyte solution. We coin the term “Janus AC flow field-effect transistor (Janus AC-FFET)” to describe this interesting physical phenomenon. The proposed technique utilizes a simple device geometry, in which one or a series of Janus microcylinders are arranged in parallel along the centerline of the channel’s bottom surface, embedding a pair of 3D sidewall driving electrodes. By combining symmetry breaking in both surface polarizability and the AC powering scheme, it is possible, on demand, to adjust the degree of asymmetry of the ICEO flow profile in two orthogonal directions, which includes the horizontal pump and transversal rotating motion. A comprehensive mathematical model was developed under the Debye–Hückel limit to elucidate the physical mechanism underlying the field-effect-reconfigurable diffuse-charge dynamics on both the dielectric and the metal-phase surfaces of the Janus micropillar. For innovation in applied science, an advanced microdevice design integrating an array of discrete Janus cylinders subjected to two oppositely polarized gate terminals is recommended for constructing an active microfluidic pump and mixer, even without external moving parts. Supported by a simulation analysis, our physical demonstration of Janus AC-FFET provides a brand-new approach to muti-directional electro-convective manipulation in modern microfluidic systems.

Experimental Demonstration of Vector Separator Using Induced Charge Electro-Osmosis

Separation is essential for biomedical applications. Here, we report an integrated fluidic circuit consisting of a natural convection pump and a vector separator using induced charge electro-osmosis (ICEO). In particular, we demonstrated that the particle orbit can be controlled by changing ICEO flow velocities around plural carbon poles. Our prototype system should be useful since it does not require mechanical parts.

Slow development of induced-charge electro-osmosis in cetyltrimethylammonium bromide solutions

Induced-charge electro-osmosis (ICEO) is an important phenomenon in microfluidics and has been widely studied. Existing studies, both theoretical and experimental, have shown that ICEO is fully developed in milliseconds after an electric field is applied. In this study, we experimentally found that ICEO of cetyltrimethylammonium bromide (CTAB) solutions takes several seconds to be fully developed after the electric field is applied, which is significantly different from the existing literature. To reveal the underlying mechanism of this slow development, we propose a phenomenological model for ICEO of CTAB solutions by incorporating a dynamic viscosity formula. The ICEO model successfully explains the slow development and attributes it to the thixotropy of CTAB solutions. In addition, we also found that ICEO velocity varies irregularly as CTAB concentration increases, which is due to the variations of viscosity and electrical conductivity of CTAB solutions. The ICEO velocity of CTAB solutions reduces as the electric field frequency increases, which is different from the existing literature. The findings may enrich the understanding of ICEO and promote the application in complex fluids such as biological fluids.

Induced-charge electroosmosis for rapid mixing of reactive precipitation systems to obtain small and uniform particles

The control of product quality in precipitation systems is often difficult due to high supersaturation levels and fast kinetics. Rapid and uniform mixing of fluids is required to minimize supersaturation gradients before the onset of nucleation. This study proposes an active mixing platform utilizing induced-charge electroosmosis (ICEO) to better control crystal quality attributes such as the size distribution. The mixing performance of the ICEO-based micromixer is characterized through experiments as a function of the solution properties and the electric field parameters such as phase angle, frequency, and voltage. Optimal mixing is achieved at lower conductivities when the phase difference is 180°. Furthermore, computational fluid dynamics simulations demonstrate that uniform supersaturation is achieved at shorter mixing lengths of the device along with a narrower residence time distribution in the presence of ICEO. Additionally, the performance of the ICEO-based crystallizer is demonstrated for the reactive precipitation of silver chloride particles and carbamazepine-succinic acid cocrystals. The application of ICEO leads to the production of smaller silver chloride particles with a narrower size distribution and an improved yield for cocrystallization compared to cases without ICEO. The ICEO-based micromixer characterized in this work is generally attractive for the continuous production of particles through reactive precipitation with stringent requirements on product uniformity and yield.

Thermal gradient and elastic dependence of induced charge electro-osmosis in viscoelastic fluids

Induced charge electro-osmosis has notable implementation possibilities in thermal management and efficient electrokinetic micropumps. We present the coupled numerical implementation around a polarized cylinder subject to an external electric field with the influence of different polymer elasticity and thermal gradients. The azimuthal velocity, flow types, kinetic energy, elastic energy, ion transport behavior, and heat transfer capability are investigated in detail. The results show that the inflow and outflow rates approximately overlap for a typical small voltage limit ϕ < 0.1. The Rayleigh number (Ra) significantly influences the elastic energy accumulation and evolution time to the final steady state. The thermal buoyancy forces are not sufficient to create typical thermogravitational convection with passive heat transfer when Ra < 1.3 × 10−3, resulting in heat diffusion and electro-osmosis velocity dominating the temperature distribution. The Nusselt number (Nu) plot with a weak viscoelastic effect implies an asymptotic Nu=0.44+2.65Ra0.35 relation. Relevant results open possibilities for enhanced mixing and heat transfer in microdevices, providing insight into barriers to the non-Newtonian nature of electrokinetic dynamics.

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

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

The weaving of the carbon fiber using induced-charge electro-osmosis with DC–AC alternating switching

Controlling the self-organizing structure is important for the use of self-organizing materials in a variety of applications. Here, we report the weaving method of the carbon fiber using induced-charge electro-osmosis (ICEO) with DC–AC alternating switching treatment. Specifically, we report that the spread low-density carbon fibers having a dendritic structure grow by applying a DC voltage, while the spread fibers shrink irreversibly with contacting and entangling by applying an AC voltage. Consequently, a weaving structure of carbon fibers can be achieved. Although the micro-scale weaving technology for polymers has been developed mainly in the field of tissue engineering, the weaving technology for conductive carbon fibers that can be used as a cilium-like ICEO actuator in a microfluidic circuit has not been explored so far; thus, we have first demonstrated that the woven structures of the conductive carbon microfibers can be made by the electrokinetic phenomena using a DC–AC alternating electric field. Moreover, through the visualization of the flow fields during the DC–AC weaving process, we clarified the mechanism of the DC–AC weaving phenomenon related to the dielectrophoresis and ICEO phenomena. Our technology should open a different design strategy to improve the structure of self-organizing materials that can be used in microfluidic circuits.

Modeling electrokinetic flows with the discrete ion stochastic continuum overdamped solvent algorithm.

In this article we develop an algorithm for the efficient simulation of electrolytes in the presence of physical boundaries. In previous work the discrete ion stochastic continuum overdamped solvent (DISCOS) algorithm was derived for triply periodic domains, and was validated through ion-ion pair correlation functions and Debye-Hückel-Onsager theory for conductivity, including the Wien effect for strong electric fields. In extending this approach to include an accurate treatment of physical boundaries we must address several important issues. First, the modifications to the spreading and interpolation operators necessary to incorporate interactions of the ions with the boundary are described. Next we discuss the modifications to the electrostatic solver to handle the influence of charges near either a fixed potential or dielectric boundary. An additional short-ranged potential is also introduced to represent interaction of the ions with a solid wall. Finally, the dry diffusion term is modified to account for the reduced mobility of ions near a boundary, which introduces an additional stochastic drift correction. Several validation tests are presented confirming the correct equilibrium distribution of ions in a channel. Additionally, the methodology is demonstrated using electro-osmosis and induced-charge electro-osmosis, with comparison made to theory and other numerical methods. Notably, the DISCOS approach achieves greater accuracy than a continuum electrostatic simulation method. We also examine the effect of under-resolving hydrodynamic effects using a "dry diffusion" approach, and find that considerable computational speedup can be achieved with a negligible impact on accuracy.

Mixing performance of the induced charge electro-osmosis micromixer with conductive chamber edges for viscoelastic fluid

Enhancing the micro-channel flow mixing is always a difficult problem. In this study, a micromixer based on induced charge electro-osmosis is proposed. A T-shaped micromixer, which has a chamber with conductive surfaces in the channel, is chosen. Due to the electro-osmotic effect of the induced charge, the induced potential is generated on the conductive surface. The Oldroyd-B constitutive model is chosen to characterize the flow characteristics of polyacrylamide solution, and an open-source solver named rheoTool based on the finite-volume method is used. The effect of the chamber shape, the chamber size, the conductive edge numbers in the chamber, and the applied electric intensity on the mixing efficiency are investigated. The results show that the micromixer with conductive edges in the chamber has better mixing effect because of the vortices. At the same time, compared with other shapes, the micromixer with diamond chamber has the best mixing effect, and the mixing efficiency reaches 79.51%. In addition, the mixing efficiency of one conductive edge in the diamond chamber is 4.39% higher than that of the two conductive edges chamber. It is found that increasing the chamber size will improve the mixing efficiency, and the mixing efficiency increased by 12.76% with the increase in chamber size. On the other hand, when increasing the electric field intensity from 100 to 200 V/cm, the mixing efficiency will decrease.

Asymmetrical motion of electrically actuated cilium using induced charge electro-osmosis with asymmetrical joint structure

Asymmetrical motion is essential for a cilium to produce a net flow in a low-Reynolds number regime. Here, we report that electrically actuated cilium having an asymmetrical joint structure shows an asymmetrical motion due to induced charge electro-osmosis (ICEO). In particular, we measured asymmetrical sweep area due to the forward and recovery strokes of our ICEO cilium and evaluated pumping performance. Our findings should be applied to innovative biomedical microfluidic systems that contribute to maintaining people’s health.

Induced-charge electroosmosis, polarization, electrorotation, and traveling-wave electrophoresis of horn toroidal particles

Induced-charge electroosmosis, polarization, electrorotation, and traveling-wave electrophoresis of horn toroidal particles