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)).     

Transient analysis of electroosmotic flow in a slit microchannel

The transient aspects of electroosmotic flow in a slit microchannel are studied. Exact solutions for the electrical potential profile and the transient electroosmotic flow field are obtained by solving the complete Poisson-Boltzmann equation and the Navier-Stokes equation under an analytical approximation for the hyperbolic sine function. The characteristics of the transient electroosmotic flow are discussed under influences of the electric double layer and the geometric size of the microchannel.     

Frequency-dependent laminar electroosmotic flow in a closed-end rectangular microchannel

This article presents an analysis of the frequency- and time-dependent electroosmotic flow in a closed-end rectangular microchannel. An exact solution to the modified Navier-Stokes equation governing the ac electroosmotic flow field is obtained by using the Green's function formulation in combination with a complex variable approach. An analytical expression for the induced backpressure gradient is derived. With the Debye-Hückel approximation, the electrical double-layer potential distribution in the channel is obtained by analytically solving the linearized two-dimensional Poisson-Boltzmann equation. Since the counterparts of the flow rate and the electrical current are shown to be linearly proportional to the applied electric field and the pressure gradient, Onsager's principle of reciprocity is demonstrated for transient and ac electroosmotic flows. The time evolution of the electroosmotic flow and the effect of a frequency-dependent ac electric field on the oscillating electroosmotic flow in a closed-end rectangular microchannel are examined. Specifically, the induced pressure gradient is analyzed under effects of the channel dimension and the frequency of electric field. In addition, based on the Stokes second problem, the solution of the slip velocity approximation is presented for comparison with the results obtained from the analytical scheme developed in this study.     

Radial basis function interpolation supplemented lattice Boltzmann method for electroosmotic flows in microchannel

Large gradients of physical variables near the channel walls are characteristic of EOF. The previous numerical simulations of EOFs with the lattice Boltzmann method (LBM) utilize uniform lattice and are not efficient, especially when the electric double layer (EDL) thickness is significantly smaller than the channel height. The efficient LBM simulation of EOF in microchannel calls for a nonuniform mesh which is dense in the EDL region and sparse in the bulk region. In this article, we formulate a radial basis function (RBF)-based interpolation supplemented LBM (ISLBM) to solve the governing equations of EOF, that is, the Poisson, Nernst–Planck, and Navier–Stokes equations, in a nonuniform mesh system. Unlike the conventional ISLBM, the RBF-ISLBM determines the prestreaming distribution functions by using the local RBF-based interpolation over circular supporting regions and is particularly suitable for nonuniform meshes. The RBF-ISLBM is validated by the EOFs in infinitely long and finitely long microchannels. The results show that the RBF-ISLBM possesses excellent robustness and accuracy. Finally, we use the RBF-ISLBM to simulate the EOFs with the hitherto highest electrokinetic parameter, κa, defined by the ratio of channel height a to EDL thickness κ⁻¹, in LBM simulations of EOF.     

Analysis of electroosmotic flow of power-law fluids in a slit microchannel

Electroosmotic flow of power-law fluids in a slit channel is analyzed. The governing equations including the linearized Poisson-Boltzmann equation, the Cauchy momentum equation, and the continuity equation are solved to seek analytical expressions for the shear stress, dynamic viscosity, and velocity distribution. Specifically, exact solutions of the velocity distributions are explicitly found for several special values of the flow behavior index. Furthermore, with the implementation of an approximate scheme for the hyperbolic cosine function, approximate solutions of the velocity distributions are obtained. In addition, a generalized Smoluchowski velocity is introduced by taking into account contributions due to the finite thickness of the electric double layer and the flow behavior index of power-law fluids. Calculations are performed to examine the effects of kappaH, flow behavior index, double layer thickness, and applied electric field on the shear stress, dynamic viscosity, velocity distribution, and average velocity/flow rate of the electroosmotic flow of power-law fluids.     

Elastic instabilities in the electroosmotic flow of non-Newtonian fluids through T-shaped microchannels

Electroosmotic flow (EOF) has been widely used to transport fluids and samples in micro- and nanofluidic channels for lab-on-a-chip applications. This essentially surface-driven plug-like flow is, however, sensitive to both the fluid and wall properties, of which any inhomogeneity may draw disturbances to the flow and even instabilities. Existing studies on EOF instabilities have been focused primarily upon Newtonian fluids though many of the chemical and biological solutions are actually non-Newtonian. We carry out a systematic experimental investigation of the fluid rheological effects on the elastic instability in the EOF of phosphate buffer-based polymer solutions through T-shaped microchannels. We find that electro-elastic instabilities can be induced in shear thinning polyacrylamide (PAA) and xanthan gum (XG) solutions if the applied direct current voltage is above a threshold value. However, no instabilities are observed in Newtonian or weakly shear thinning viscoelastic fluids including polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), and hyaluronic acid (HA) solutions. We also perform a quantitative analysis of the wave parameters for the observed elasto-elastic instabilities.     

Analytical solutions for velocity, temperature and concentration distribution in electroosmotic microchannel flows of a non-Newtonian bio-fluid

In this paper, analytical solutions are derived, describing the transport characteristics of a non-Newtonian fluid flow in a rectangular microchannel, under the sole influence of electrokinetic forces. Apart from estimating the fully-developed velocity and temperature distributions, an explicit expression is derived for solutal concentration distribution within the microchannel. Finally, as an illustrative case study, the flow behaviour of a blood sample is analyzed, in which the flow parameters are modeled as functions of the hematocrit fraction in the sample. It is revealed that a higher hematocrit fraction may result in significant reductions in species concentration levels, on account of stronger dispersions in the velocity profiles, characterized by more significant viscous effects. It is also demonstrated that cases in which characteristic length scale of RBC suspensions turns out to be consequential relative to the microchannel dimensions, a significant augmentation in the electroosmotic transport may occur. Such observations can be of particular significance in the design of electroosmotically actuated bio-microfluidic systems as efficient solutal carriers.     

Adaptive Kalman filtering used to compensate for model errors in multicomponent methods

An adaptive Kalman filtering technique is developed and evaluated as a tool for compensation of model errors that commonly occur in multicomponent problems. Results presented demonstrate that the adaptive Kalman filter should be of considerable value for obtaining accurate concentrations and spectral information from multicomponent responses. The restrictions of the technique are that the model must be correct for a portion of the response, and the errors must be attributable to a single component. Concentration estimates are accurate for those components which are modeled correctly, but the concentration estimate obtained for the component not included in the model may not always be reliable. The technique is evaluated for multicomponent problems in spectroscopy and voltammetry.     

Quantitative model for predicting the electroosmotic flow in dual-pole nanochannels

Developing and assessing nanofluidic systems is time-consuming and costly owing to the method's novelty; hence, modeling is essential to determine the optimal areas for implementation and to grasp its workings. In this work, we examined the influence of dual-pole surface and nanopore configuration on ion transfer simultaneously. To achieve this, the two trumpet and cigarette configuration were coated with a dual-pole soft surface so that the negative charge could be positioned in the nanopore's small aperture. Subsequently, the Poisson–Nernst–Planck and Navier–Stokes equations were simultaneously solved under steady-state circumstances using varied values physicochemical properties for the soft surface and electrolyte. The pore's selectivity was $S_{\mathrm{Trumpet}}>S_{\mathrm{Cigarette}}$, and the rectification factor, on the other hand, was , when the overall concentration was very low. When the ion partitioning effect is taken into account, we clearly show that the rectifying variables for the cigarette configuration and the trumpet configuration can reach values of 45 and 49.2, when the charge density and mass concentration were 100 mol/m³ and 1 mM, respectively. By using dual-pole surfaces, the controllability of nanopores’ rectifying behavior may be modified to produce superior separation performance.     

Comparison of the Nernst-Planck model and the Poisson-Boltzmann model for electroosmotic flows in microchannels

In the analysis of electroosmotic flows, the internal electric potential is usually modeled by the Poisson-Boltzmann equation. The Poisson-Boltzmann equation is derived from the assumption of thermodynamic equilibrium where the ionic distributions are not affected by fluid flows. Although this is a reasonable assumption for steady electroosmotic flows through straight microchannels, there are some important cases where convective transport of ions has nontrivial effects. In these cases, it is necessary to adopt the Nernst-Planck equation instead of the Poisson-Boltzmann equation to model the internal electric field. In the present work, the predictions of the Nernst-Planck equation are compared with those of the Poisson-Boltzmann equation for electroosmotic flows in various microchannels where the convective transport of ions is not negligible.     

Studies of electroosmotic flow and the effects of protein adsorption in plasma‐polymerized microchannel surfaces

This paper presents a study of EOF properties of plasma‐polymerized microchannel surfaces and the effects of protein (fibrinogen and lysozyme) adsorption on the EOF behavior of the surface‐modified microchannels. Three plasma polymer surfaces, i.e. tetraglyme, acrylic acid and allylamine, are tested. Results indicate EOF suppression in all plasma‐coated channels compared with the uncoated glass microchannel surfaces. The EOF behaviors of the modified microchannels after exposure to protein solutions are also investigated and show that even low levels of protein adsorption can significantly influence EOF behavior, and in some cases, result in the reversal of flow. The results also highlight that EOF measurement can be used as a method for detecting the presence of proteins within microchannels at low surface coverage (<1 ng/cm² on glass). Critically, the results illustrate that the non‐fouling tetraglyme plasma polymer is able to sustain EOF. Comparison of the plasma‐polymerized surfaces with conventionally grafted polyelectrolyte surfaces demonstrates the stabilities of the plasma polymer films, enabling multiple EOF runs over 3 days without deterioration in performance. The results of this study clearly demonstrate that plasma polymers enable the surface chemistry of microfluidic devices to be tailored for specific applications. Critically, the deposition of the non‐fouling tetraglyme coating enables stable EOF to be induced in the presence of protein.     

Effect of viscoelasticity on the flow pattern and the volumetric flow rate in electroosmotic flows through a microchannel

Many lab-on-a-chip based microsystems process biofluids such as blood and DNA solutions. These fluids are viscoelastic and show extraordinary flow behaviors, not existing in Newtonian fluids. Adopting appropriate constitutive equations these exotic flow behaviors can be modeled and predicted reasonably using various numerical methods. In the present paper, we investigate viscoelastic electroosmotic flows through a rectangular straight microchannel with and without pressure gradient. It is shown that the volumetric flow rates of viscoelastic fluids are significantly different from those of Newtonian fluids under the same external electric field and pressure gradient. Moreover, when pressure gradient is imposed on the microchannel there appear appreciable secondary flows in the viscoelastic fluids, which is never possible for Newtonian laminar flows through straight microchannels. The retarded or enhanced volumetric flow rates and secondary flows affect dispersion of solutes in the microchannel nontrivially.     

An analytical approach to the effect of finite-sized end reservoirs on electroosmotic transport through narrow confinements

The presence of end reservoirs often alters the electrical potential distributions inside narrow fluidic confinements of electrokinetically actuated miniaturized devices to a significant extent. This paper examines the influence of finite size effect of the end reservoirs on the concerned potential distribution analytically, using the Schwarz-Christoffel conformal mapping. The effective electric field directly acting across the channel is accordingly represented by a calibration curve, which sums up the role of the key geometric parameters (reservoir-to-channel height and length ratios) on the potential distribution. The analytical model is further augmented to assess the flow characteristics through the channel. The results indicate that the alterations in the flow characteristics due to alterations in the effective electric field on account of the finite size effects of the end reservoirs can turn out to be significantly more prominent than the corresponding alterations due to the axial pressure gradients induced by the sudden contraction and expansion effects associated with the end reservoirs. The derived results can be further utilized to facilitate the practical design of miniaturized fluidic devices, using conveniently tractable analytical tools.     

Cationic polymer coatings for design of electroosmotic flow and control of DNA adsorption

A difficulty with the design and operation of an electrokinetically operated DNA hybridization microfluidic chip is the opposite direction of the electroosmotic flow and electrophoretic mobility of the oligonucleotides. This makes it difficult to simultaneously deliver targets and an appropriate hybridization buffer simultaneously to the probe sites. In this work we investigate the possibility of coating the inner walls of the microfluidic system with hexadimentrine bromide (polybrene, PB) and other cationic polymers in order to reverse the direction of electroosmotic flow so that it acts in the same direction as the electrophoretic transport of the oligonucleotides. The results indicated that the electroosmotic flow (EOF) in channels that were coated with the polymer could be reversed in 1× TBE buffer or 1× SSC buffer. Under these conditions, the DNA and EOF move in the same direction, and the flow can be used to deliver DNA to an area for selective hybridization within the channel. The effects of coating the surface of a nucleic acid microarray with polybrene were also studied to assess non-selective adsorption and stability. The polybrene coating significantly reduced the extent of non-selective adsorption of oligonucleotides in comparison to adsorption onto a glass surface, and the coating did not alter the extent of hybridization. The results suggest that use of the coating makes it possible to achieve semi-quantitative manipulation of nucleic acid oligomers for delivery to an integrated microarray or biosensor.     

A new design of double-layered microchannel heat sinks with wavy microchannels and porous-ribs

Journal of Thermal Analysis and Calorimetry - A new design of double-layered microchannel heat sinks, which combines wavy microchannels with porous vertical ribs, is proposed in this work. The flow...     

An investigation of electroosmotic flow and pressure pumped luminol chemiluminescence detection for cobalt analysis in a miniaturised total analytical system

The luminol chemiluminescence reaction has been investigated for the determination of cobalt in a planar glass micro reactor. Reagents were mobilised within an etched glass chip with 200 microm wide channels using either electrically driven flow with voltages below 200 V cm(-1) or negative pressure pumping. Surfactants were used to assist mobilisation of the luminol for electrically driven flow and this also increased the chemiluminescence intensity in that system. A calibration over seven orders of magnitude was achieved for cobalt(II) nitrate with the negative pressure pumping system, and the limit of detection at the 95% confidence level was 3 x 10(-11) mol l(-1). The limit of detection for the electrokinetic flow was 4 x 10(-11) mol l(-1) and the linear range was between 10(-10) to 10(-6) mol l(-1). The advantages and disadvantages of both mobilisation methods are discussed for this reaction.     

A method for DNA detection in a microchannel: fluid dynamics phenomena and optimization of microchannel structure

A simple DNA diagnosis method using microfluidics has been developed which requires simple and straightforward procedures such as injection of sample and probe DNA solutions. This method takes advantage of the highly accurate control of fluids in microchannels, and is superior to DNA microarray diagnosis methods due to its simplicity, highly quantitative determination, and high-sensitivity. The method is capable of detecting DNA hybridization for molecules as small as a 20 mer. This suggests the difference in microfluidic behavior between single strand DNA (ssDNA) and double stranded DNA (dsDNA). In this work, influence of both the inertial force exerted on DNA molecules and the diffusion of DNA molecules was investigated. Based on the determination of these parameters for both ssDNA and dsDNA by experiments, a numerical model describing the phenomena in the microchannel was designed. Computational simulation results using this model were in good agreement with previously reported experimental results. The simulation results showed that appropriate selection of the analysis point and the design of microchannel structure are important to bring out the diffusion and inertial force effects suitably and increase the sensitivity of the detection of DNA hybridization, that is, the analytical performance of the microfluidic DNA chip.     

Diffusion-controlled instabilities and stationary structures in a model for a ferroin-catalyzed Belousov-Zhabotinskii reaction

A model for the bromate-ferroin-bromomalonic acid reaction has been used to examine the conditions under which the homogeneous state loses stability as a result of Turing bifurcation. Stationary periodic structures with small or large amplitudes can exist. There is also a structure modification effect.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 440–449, July–August, 1988.I am indebted to A. M. Zhabotinskii and Yu. A. Kuznetsov for useful discussions, and also to I. I. Goryanin for providing access to his software suite for defining a curve in n-dimensional space.     

Heteroscedastic controlled calibration model applied to analytical chemistry

In chemical analyses performed by laboratories, one faces the problem of determining the concentration of a chemical element in a sample. In practice, one deals with the problem using the so‐called linear calibration model, which considers that the errors associated with the independent variables are negligible compared with the former variable. In this work, a new linear calibration model is proposed assuming that the independent variables are subject to heteroscedastic measurement errors. A simulation study is carried out in order to verify some properties of the estimators derived for the new model and it is also considered the usual calibration model to compare it with the new approach. Three applications are considered to verify the performance of the new approach. Copyright © 2010 John Wiley & Sons, Ltd.     

Maximum symmetrical split of molecular graphs. Application to organic synthesis design

Whereas the potential symmetry of a molecule may be a feature of importance in synthesis design, this one is often difficult to detect visually in the structural formula. In the present article, we describe an efficient algorithm for the perception of this molecular property. We have addressed this problem in terms of graph theory and defined it as the Maximum Symmetrical Split of a molecular graph. A solution is obtained by deleting in such a graph a minimum number of edges and vertices so that the resulting subgraph consists of exactly two isomorphic connected components that correspond to a pair of synthetically equivalent synthons. In view to reduce the search space of the problem, we have based our algorithm on CSP techniques. In this study, we have found that the maximum symmetrical split is an original kind of Constraint Satisfaction Problem. The algorithm has been implemented into the RESYN_Assistant system, and its performance has been tested on a set of varied molecules which were the targets of previously published synthetic studies. The results show that potential symmetry is perceived quickly and efficiently by the program. The graphical display of this perception information may help a chemist to design reflexive or highly convergent syntheses.