Characterizing the mainly incompressible and laminar flows of aqueous electrolyte solutions through channels with an overall dimension of O (1–100 μm) is of interest in a variety of microfluidics applications. Solid surfaces such as the channel wall become (usually negatively) charged due to direct ionization or dissociation of surface groups, where the charge is typically characterized by the wall zeta-potential ζ w . The surface in turn attracts mobile counterions from the fluid to form a (usually positively) charged screening, or electric double, layer (EDL). An external electric field can therefore be used to “pump” fluids through microfluidic Labs-on-a-Chip (LOC) by driving the charged fluid in the EDL. The resulting electroosmotic flow (EOF) is uniform outside the EDL, which has a thickness less than 50 nm in most cases. This uniform flow results in a more favorable scaling of the volume flowrate with channel diameter for microchannels, and also has less convective dispersion than shear flows. Electroosmotic flow is, however, very sensitive to changes in ζ w . Various studies have shown, for example, that adding multivalent counterions to a monovalent electrolyte solution can greatly change ζ w through both electrostatic and chemical interactions, even leading to “charge inversion” where the zeta-potential changes its sign. Evanescent-wave particle velocimetry, which tracks the motion of colloidal fluorescent tracer particles illuminated by evanescent waves within ∼400 nm of the wall, was therefore used to study the flow of various aqueous monovalent electrolyte solutions with small amounts of divalent cations such as Mg ++ driven by an electric field through channels with a minimum dimension of ∼30 μm. The technique measures both the velocity components parallel to the wall and the steady-state distribution of these near-wall tracers. In these experiments, the tracers are convected parallel to the wall by both the EOF and directly by the applied electric field via electrophoresis because the surfaces of the particles also become negatively charged when suspended in the electrolyte solution. The electrophoretic contribution to the measured particle velocity was determined by measuring the particle zeta-potential with light scattering, and subtracted from the particle velocity to determine the actual EOF velocity.

Non-uniform heat flux generated by microchips can create “hot spots” in localized areas on the microchip surface. This research presents an improved design of an active cooling electro-osmotic flow (EOF) based micro-pump for hot spots thermal management. The design of the micro-pump was simpler and more practical for the application compared to designs presented in literature. Most micro-channel heat sink devices presented in literature were silicon based. Though silicon has better thermal conductivity when compared to polymers used in micro-devices fabrication, processes of silicon fabrication are complicated and time consuming. Also, most micro-channel fabrication processes use silicon etching which leads to rough walls within the micro-channel. An improved design, which uses a combination of silicon and Polydimethylsiloxane (PDMS), is being developed and tested. The main idea of this design is to utilize the favorable thermal properties of silicon while achieving both smoother charged surfaces and ease of fabrication of PDMS material. The EOF micro-pump was tested for four cooling fluid namely, DI water, distilled water, borax buffer, and Al 2 O 3 nano-particles suspended in water solution. A maximum flow rate of 31.2 μL/min was achieved using distilled water at 500 V of EOF voltage. Such micro-pump with this flow rate range can be implemented in a closed loop heat rejection system for hot spot thermal management. Moreover, it can be used in Lap-on-chip and uTAS application for sample transport.

In this study, electrokinetic transport in a micro-nanofluidic system is numerically investigated by solving the transient Poisson, Nernst-Planck, and Navier-Stokes equations simultaneously. The system considered is a nanochannel connected with two microchannels at its ends. Under various applied electric potential biases, the effect of concentration polarization on the fluid flow, induced pressure and electric current is examined. By comparing with the Donnan equilibrium condition and electroosmotic flow in microscale dimension, electric body force due to non-zero charge density is the mechanism for producing vortex flow and inducing positive pressure gradient in the anodic side of the system. The diffusive boundary layer thickness is reduced due to the stirring of the generated vortex flow and results in the over-limiting current when the applied electric potential bias is high.

The flow and electric field are simulated numerically with finite volume method first, then large number of nanoparticles and microparticles are injected into the microchannel separately, and these particles are traced with the Lagrangian method. It has been found that particle deposition efficiency in the bend usually decreases first then increases with particle size increasing, and there usually exists a minimum value and it corresponds to the particle diameter of about 3μm, which means that kind of particle can transport longer distance. The electric field doesn’t affect that specified value. This conclusion is helpful to the optimization of the design of microchips.

Electrokinetic transport in nanofluidics has attracted many attentions in recent years due to its potential applications in biomedical analysis and energy conversion systems. The biggest challenge to model the electrokinetic transport using atomistic simulations is the extensive cost in calculation of the long-range electrostatic force between ions. In this work, we develop an efficient molecular approach to simulate electrokinetic transport in nano-scale. The long-range Coulombic interactions are treated using the Particle-Particle Particle-Mesh (P 3 M) scheme and the Poisson equation for electric potential is solved in physical space using an iterative multi-grid technique. We implement this approach to systematically investigate two examples: the electroosmotic flow in random rough channels and the electrowetting on dielectric (EWOD). Both cases pose important engineering applications as a mechanism for transporting small amount of liquid in micro and nano devices.

To detect and identify biomolecules at its infectious dose, all sensors require a relatively long time — days or even weeks — for reliable results. This delay occurs because the collection of analyte molecules onto the sensing elements depends solely on the diffusion process of those agents to a sensor. This work seeks to enhance the transport process, and thereby significantly improve a sensor’s detection capability. This work investigates an in-situ, label-free nanoparticle (e.g. protein) preconcentrator based on a new AC electrothermal effect (ACET) that can operate even in a highly conductive solution for which current methods such as dielectrophoresis, electrophresis or electroosmosis won’t function effectively. The proposed AC electrokinetic preconcentrator is expected to significantly increase the nanoparticle counts, realizing sub-femto molar detection in real time.

This paper presents theoretical and experimental investigations on valveless microfluidic switch using the coupled effect of hydrodynamics and electroosmosis. In the experiment, two sheath streams (aqueous NaCl and glycerol) and the sample stream (silicon oil) are introduced by syringe pumps to flow side by side in a straight rectangular microchannel. External electric fields are applied on the two sheath streams. Under the constant inlet volumetric flowrates, the sample stream is delivered to the desired outlet ports using electroosmotic effect. The liquid fractions of sheath streams are measured using fluorescence imaging technique. The results indicate that under suitable cooperation of electric fields, the sample stream can be delivered to the desired outlet ports.

The multiphysiochemical transport in electroosmosis of dilute electrolyte solutions (<1mM) through microporous media with granular random structures has been modeled in this work by our numerical framework consisting of three steps. First, the three-dimensional microstructures of porous media are reproduced by a random generation-growth method. Then the effects of chemical adsorption and electrical dissociation at the solid-liquid interfaces are considered to determine the electrical boundary conditions, which vary with the ionic concentration, the pH, and the temperature. Finally the nonlinear governing equations for the electrokinetic transport are solved by a highly efficient lattice Poisson-Boltzmann algorithm. The simulation results indicate that the electroosmotic permeability through the granular microporous media increases monotonically with the porosity, the ionic concentration, the pH and the environmental temperature. When the surface electric potential is higher than 50 mV, the permeability increases with the electric potential exponentially. The electroosmotic permeability increases with the pH exponentially, but with the temperature linearly. The present modeling results may improve our understanding of hydrodynamic and electrokinetic transport in geophysical systems, and help guide the design of porous electrodes in micro energy systems.

This study demonstrated electroosmotic pumping with high flow rate per unit area at a rather low applied voltage through an alumina nano-porous membrane driven by Platinum mesh electrodes. The electrode was placed perpendicular to, and had direct contact with nano-channel inlet to reduce the electric voltage drop in the reservoir. The complete set of the Poisson-Nernst-Planck equations for electrical potential and ionic concentration, coupled with the Navier-Stokes equation were solved for the purpose of a full understanding of the ionic transport and flow characteristics of EOF in nano-fluidics capillaries. The measured flowrate versus electrolyte (KCl) concentration reveals that the flowrate is usually high in low concentration (10 −5 M∼10 −7 M) regime in which a maximum value also occurs. In addition, a remarkable surge of flow rate is observed when the concentration surpasses below 10 −4 M. The maximum flowrate achieved from this study is 0.09 mL min −2 V −1 cm −2 and the energy transfer efficiency is 0.43% at an operation voltage of 20V. The flowrates investigated in this study are comparable to other existing results whereas the corresponding operation voltage used this study is about one to two order lower than most existing results. Numerical results exhibit correct trends for nano flows involving strong overlap of electrical double layers. Comparisons of numerical and experimental results were made and discussed.

The governing equations of electroosmotic flow, including the Navier-Stokes (N-S) equations, Laplace equation and Poisson-Boltzmann equation, are set up in a straight microchannel. The meshless method is employed as a discrete scheme for the solution domain. The semi-implicit multistep (SIMS) method is used to solve the Navier-Stokes equations. The simulation results demonstrated that different patterns of the zeta potential over the channel surface could induce different flow profiles for the vortex. The rotational direction of the vortex is determined by the electroosmotic driving force.

Small triangle channels are frequently encountered in microelectromechanical systems (MEMS). In this paper, the electro-osmotic flows with a constant wall temperature in triangle microchannels are studied. We numerically solved the Poisson equation for electric potential, the Navier-Stokes equation, and the energy equation for the mixed electroosmotic/pressure driven flows with varying physical properties in triangle microchannels using the Galerkin method. The effects of the pressure gradient, the length ratio, κD h , and Joule heating on mass flux of electro-osmotic flow are discussed, respectively. The numerical results show the mass-flux increases with the increase of imposed positive pressure gradient. A larger Joule number leads to a larger mass flux of electrolytic solution under positive pressure gradient. It is also found that the mass flux increases with increasing the length ratio, κD h , at a given pressure gradient. The increase in Ju number induces a larger increase in the mass flux at a larger κD h .

To study the effect of geometry on electroosmotic flow in micro channels, we fabricated PDMS-glass microchannels of different designs, which have patterned channels with abrupt contraction of different sizes. Using fluorescent imaging technology, we demonstrated the effect of geometry on the instability of DC driven electroosmotic flow in microfluidic channels. For certain geometry and conductivity of the electrolyte solution (Sodium Bicarbonate), there is a threshold voltage for electroosmotic instability, exhibiting itself as “ripple”. Generally, the factors which affect the threshold voltage include channel width, channel geometry, and electrolyte conductivity. Narrower channel resulted in higher onset voltage. As conductivity of the electrolyte increases, the threshold voltage tends to increase. Early transition to unstable electroosmotic flow in microfluidic channels was observed under relatively low Re.

The bulk motion of an aqueous solution induced by the application of DC electric field is studied numerically. The physical model consists of a micro-cavity with two completely polarizable cylindrical electrodes. The electric double layer (EDL) model coupled with Navier-Stokes equations governing the electroosmotic flow has been described. The Nernst-Planck model uses two extra equations for the prediction of ion concentration. We employed IB (immersed boundary) technique for the implementation of boundary conditions and semi-implicit fractional-step method for solving the governing equations. A new method is described for implementing concentration boundary conditions on the electrodes. The bench mark problems, driven cavity flow and flow over a cylnder were used for the validation of our present code. The numerical results are compared with the analytical results obtained using Gouy-Chapman-Stern model for the one dimensional case. For the two dimensional case the flow field and the ionic concentration distributions obtained shows that the electoosmotic effect is predominant in the thin region around the electrode.

Understanding electroosmotic flow in microchannels is of both fundamental and practical significance for the design and optimization of various microfluidic devices to control fluid motion. Electroosmotic flows in microfluidic systems are restricted to the low Reynolds number regime, and mixing in these systems becomes problematic due to negligible inertial effects. To enhance the species mixing effect, the current study presents a numerical investigation of steady-state electroosmotic flow mixing in smooth microchannels, channels patterned with surface blocks, channels patterned with heterogeneous surfaces, as well as pulsating electroosmotic flow. The lattice Boltzmann equations, which recover the nonlinear Poisson-Boltzmann equation, the Navier-Stokes equation including the external force term, and the diffusion equation, were solved to obtain the electric potential distribution in the electrolyte, the velocity field, and the species concentration distribution, respectively. The simulation results confirm that wall blocks, heterogeneous surfaces, and electroosmotic pulsating flow can all change the flow pattern and enhance mixing in microfluidic systems. In addition, it is shown that pulsating flow provides the most promising method for enhancing the mixing efficiency.

This paper presents theoretical and experimental investigations of the pressure-driven two-liquid flow in microchannels with the electroosmosis effect. For a fully developed, steady state, laminar flow of two liquids combined the pressure gradient, electroosmosis and surface charges at the liquid-liquid interface, we have derived analytical solutions that relate the velocity profiles and flow rates to the liquid holdup, the aspect ratio of the microchannel, the viscosity ratio of the two liquids and the externally applied electric field. It was shown that adjusting the externally applied electric field could control the fluid interface position precisely. The prediction from the proposed model compares very well with measured data.