A method for simultaneous estimation of inhomogeneous zeta potential and slip coefficient in microchannels

In the microchannels made of hydrophobic materials, the fluid velocity is determined by the zeta potential and velocity slip, both of which may be inhomogeneous due to the adsorption of protein to the channel wall. The inhomogeneity of zeta potential and slip coefficient sometimes causes recirculating flows which in turn affect the transport and mixing of solutes through the microchannels. In the present investigation we devise a method for the simultaneous estimation of inhomogeneous zeta potential and inhomogeneous slip coefficient using velocity measurements. A conjugate gradient method supplemented by the adjoint variable method is adopted in the solution of the relevant inverse problem to reduce the computational burden. The present method is found to estimate the inhomogeneous zeta potential and the slip coefficient simultaneously even with noisy velocity measurements. This method is expected to contribute to the optimal design and robust operation of various microfluidic devices, where the flow patterns and the volumetric flow rates are critically influenced by the profiles of inhomogeneous zeta potential and inhomogeneous slip coefficient.     

The zeta potential of cyclo-olefin polymer microchannels and its effects on insulative (electrodeless) dielectrophoresis particle trapping devices

While cyclo-olefin polymer microchannels have the potential to improve both the optical detection sensitivity and the chemical resistance of polymer microanalytical systems, their surface properties are to date not thoroughly characterized. These surface properties dictate, among other things, electrokinetic effects when electric fields are present. Here, we report the measurement of the zeta potential of cyclo-olefin polymers (injection-molded and hot-embossed Zeonor 1060R and 1020R) microchannels as a function of pH, counter-ion concentration, storage conditions, and chemical treatment in aqueous solutions both with and without EOF-suppressing additives. In contrast with previous reports, significant surface charge is measured, consistent with titration of charged sites with pK(a) = 4.8. Storage in air, acetonitrile, or aqueous solutions has relatively minor effects. While the source of the surface charge is unclear, chemical functionalization has shown that carboxylic acid groups are not present at the surface, consistent with the chemical structure of Zeonor. EOF-suppressing additives (hydroxypropylmethylcellulose) and conditioning in perchloric acid allow the surface charge to be suppressed. We demonstrate dielectrophoretic particle trapping devices in Zeonor 1060R substrates that show reduced trapping voltage thresholds as compared to previous implementations in glass.     

Active mixing inside microchannels utilizing dynamic variation of gradient zeta potentials

This study presents a new active micromixer with high mixing efficiency achieved by means of a gradient distribution of the surface zeta potential controlled by changing the frequency of voltage applied on shielding electrodes. Gradient surface zeta potential is generated by applying a high voltage to inclined buried shielding electrodes. While alternating the frequency of driving voltage, the zeta potential could be changed accordingly, thus providing a significant mixing effect inside microchannels. A theoretical model is proposed to predict the distribution of zeta potential. The results from this model are critically compared with the well-developed three-capacitor model. Additionally, two time-factor scales, the charge time of capacitor and mixing length flow time, are used to predict the optimum frequency. The prediction of optimum frequency, 0.5 Hz, is consistent with experimental results. Moreover, a five-pair inclined shielding electrode with a frequency of 0.5 Hz leads to a significant improvement in the mixing performance of the active micromixer. Numerical results indicate that a localized flow circulation is generated when the control voltage is applied to the inclined shielding electrodes. Furthermore, the streamlines are experimentally observed by using fluorescent beads. The shape of this circulation is dependent on the distribution of gradient zeta potential, which is determined by the arrangement of electrodes. The effects of the number of electrode pairs and the layout of shielding electrodes on the mixing performance of micromixer are also explored both numerically and experimentally. It is revealed that five-pair inclined electrodes at 0.5 Hz provide the highest mixing efficiency. Finally, a reaction between N-benzoyl-L-arginine-p-nitroanilide and trypsin enzyme is performed to verify the capability of micromixers. The experimental results reveal that the reaction can achieve a higher performance indicating a higher mixing efficiency. The active micromixers could be used in microfluidic systems for improving the mixing efficiency and thus enhancing the bioreaction.     

Dispersing phase viscometry and zeta potential in alumina dispersions

The stabilities of alumina dispersions were studied as a function of poly- and low molecular weight electrolyte concentration, using viscometry of the dispersing phase, and zeta potential measurements. The relation of polyelectrolyte adsorption to polymer concentration (at different low molecular weight electrolyte concentrations) was found to depend upon the dimensions of the polymer (which were a priori known to decrease with increasing poly- and low molecular weight electrolyte concentration). The occurrence of flocculation and bridging in the destabilization mechanism of the alumina dispersions was also characterized.     

Effect of zeta potential on retention and drainage of secondary fibres

The experiments have been carried out in the laboratory to investigate the interactions among negatively charged fibres, fines and positively charged polymer particles on two different papermaking slurries of old corrugated containers (OCC) pulp and mixed office waste (MOW) paper pulp. The effects of different wet-end chemicals such as polyacrylamide, polyamide amine and polyethylene amine on fibre surface chemistry have been followed by electrokinetic measurements (zeta potential) of papermaking stock to control the retention and drainage and see how well they perform in the wet-end of the paper machine. The flocculation behaviour has been investigated by different procedures: dewatering test and measuring the first pass retention. Treatment of pulps with chemical additives resulted in substantial improvement of drainage with increase in first pass retention. For OCC pulp, the best results were obtained with 0.5% polyacrylamide improving the drainage by 82% with corresponding first pass retention of 87.5% against 81.8% for the control pulp. However, for MOW pulp, the best results were obtained with 0.3% polyethylene amine improving the drainage by 63% with corresponding first pass retention of 86.0% against 74.2% for the control pulp. The studies on zeta potential of secondary fibres revealed the dependence of first pass retention and drainage on zeta potential of pulps. As the zeta potential approaches zero, the conditions approach optimum for first pass retention and drainage. This way, zeta potential control can lead to environmental protection and competitiveness by using secondary fibres more extensively.     

Determination of the Navier slip coefficient of microchannels exploiting the streaming potential

For most microchannels made of hydrophobic materials such as polymers, velocity slip occurs at the wall, affecting volumetric flow rate of electroosmotic flow Q(eof) and streaming potential (??(str)/?z). Since most techniques exploit Q(eof) or (??(str)/?z) to determine the zeta potential, ζ, it is very difficult to measure ζ of hydrophobic walls, if the slip coefficient b is not found a priori. Until now, Q(eof) and (??(str)/?z) are known to depend on ζ and b in a same functional form, which makes it impossible to estimate ζ or b separately using measurements of Q(eof) and (??(str)/?z). However, exploiting the analytic formula for Q(eof) and (??(str)/?z) derived in the present work, it is found that the effect of ζ and that of b on Q(eof) and (??(str)/?z) can be separated from each other by varying the bulk ionic concentration. Thus, the slip coefficient as well as the zeta potential of hydrophobic microchannels can be found with reasonable accuracy by means of a nonlinear curve fitting method using measured data of Q(eof) and (??(str)/?z) at various bulk ionic concentrations. The present method allows an accurate estimation of slip coefficient of hydrophobic microchannels, which is quite simple and cheap compared with methods employing microparticle velocimetry.     

Effects of heavy metals and oxalate on the zeta potential of magnetite

Zeta potential is a function of surface coverage by charged species at a given pH, and it is theoretically determined by the activity of the species in solution. The zeta potentials of particles occurring in soils, such as clay and iron oxide minerals, directly affect the efficiency of the electrokinetic soil remediation. In this study, zeta potential of natural magnetite was studied by conducting electrophoretic mobility measurements in single and binary solution systems. It was shown that adsorption of charged species of Co(2+), Ni(2+), Cu(2+), Zn(2+), Pb(2+), and Cd(2+) and precipitation of their hydroxides at the mineral surface are dominant processes in the charging of the surface in high alkaline suspensions. Taking Pb(2+) as an example, three different mechanisms were proposed for its effect on the surface charge: if pH<5, competitive adsorption with H(3)O(+); if 56, precipitation of heavy metal hydroxides prevails. Oxalate anion changed the associated surface charge by neutralizing surface positive charges by complexing with iron at the surface, and ultimately reversed the surface to a negative zeta potential. Therefore the adsorption ability of heavy metal ions ultimately changed in the presence of oxalate ion. The changes in the zeta potentials of the magnetite suspensions with solution pH before and after adsorption were utilized to estimate the adsorption ability of heavy metal ions. The mechanisms for heavy metals and oxalate adsorption on magnetite were discussed in the view of the experimental results and published data.     

Temperature and pressure effects on zeta potential values of reservoir minerals

An experimental study of the effect of temperature and pressure on zeta potential of typical reservoir minerals, including quartz, kaolinite, and calcite, is presented. Experiments included the design and construction of an electrophoretic cell for zeta potential measurements at variable pressure and temperature. Electrolyte concentration was varied in the range from 0.0001 to 0.1 M in the pH range from 2 to 9. For all the minerals it is found that the zeta potential decreases with temperature at a rate characteristic of each mineral; values are around -2.3 mV/degrees C for quartz, -0.96 mV/degrees C for kaolinite, and -2.1 mV/degrees C for calcite for pressure values less than 45 psi. The effect of pressure is found to depend on the mineral nature and pH of the electrolytic solution. In the case of quartz, a systematic increase in the value of the zeta potential with pressure is observed, whereas a decreasing trend is measured for the kaolinite. In the case of calcite, a decreasing trend is observed for pressures up to 45 psi, whereas the experimental data suggest an increasing trend for higher pressure values.     

Collaborative studies of zeta-potential measurements and electrophoretic measurements using reference sample

Simultaneous measurements of zeta-potential for two standard latex suspensions were carried out so as to assess the reliability of each of these measurement techniques and find means for their improvement. Furthermore, syntheses of a reference particle dispersion stabilized sterically in an aqueous medium without any electrostatic effects and measurements of zeta-potential using the reference dispersion as a standard were performed under various experimental conditions. It became apparent that the dense adsorption layer of hydroxypropylcellulose (HPC), with a lower critical solution temperature (LCST), formed on latex particles with a low surface charge density at temperatures higher than the LCST, plays a role in completely shielding the electrostatic effect arising from the surface charge on the bare particles. Such reference particles with zero zeta-potential allow us to determine the electrophoretic mobility of unknown samples at the one-half depth in the electrophoretic cell by subtracting the mobility of the reference sample at the same level. Furthermore, the zeta-potential of the cell wall can be easily determined from the mobility of the reference sample, because the apparent velocity profile of the reference sample indicates the liquid flow velocity in the cell.     

Changes in zeta potential caused by a dc electric current for thin double layers

An asymptotic solution was obtained to describe one-dimensional, steady-state transport of a symmetric binary electrolyte normal to two large parallel electrodes, in the limit in which the Debye length is infinitesimal compared to the distance separating the two electrodes. Despite the nonzero ion flux, Boltzmann's equation continues to describe the relationship between either ion concentration and the electrostatic potential inside the diffuse part of the double layer, while local electroneutrality applies outside, even for current densities approaching the limiting value. In the absence of ion adsorption or dissociation reactions at the electrodes, the magnitude of any charge or zeta potential arising on the electrodes at zero current is determined by the equilibrium constant for the redox reactions which would exchange ionic charge carriers for electric charge carriers at the electrode surface. Nonzero current causes the ionic strength of the bulk to vary with position. This perturbs the Debye length of the diffuse cloud on either electrode: it is the local ionic strength just outside the cloud which determines the Debye length for that cloud. Nonzero current also changes the zeta potential. The dimensionless rate of change dζ/dJ was as large as 30.     

Determination of zeta potential and cationic demand in ECF and TCF bleached pulp from eucalyptus and flax. Influence of measuring conditions

The electrical nature of cellulose fibres is known to govern flocculation, retention and drainage mechanisms during the papermaking process. Zeta (ζ) potential provides useful information towards better control of wet-end chemistry in respects such as the dosaging of chemical aids. The purpose of this work was to study two electrokinetic properties (ζ potential and cationic demand) in ECF (elementary chlorine free) and TCF (totally chlorine free) bleached pulps from eucalyptus and flax, and examine the influence of pH and conductivity on measurements of such properties made with various methods based on the streaming potential, electrophoresis, polyelectrolyte titration and colloidal titration. Measurements of the electrokinetic properties made at high conductivities (C > 0.1 mS/cm) afforded no discrimination between pulp types in terms of electric charge. In fact, the conductivity used had a strong influence on measurements and shifted ζ potential to less negative values at high levels and to more negative values at low levels. The streaming potential technique proved to be more sensitive to changes in the properties (pH and conductivity) of the fibre suspension than did electrophoresis. Conductivity also influenced polyelectrolyte adsorption in the determination of cationic demand. The study also involved assessing the effect of low conductivities (0.01 < C < 0.1 mS/cm), which allowed fibre types and bleaching processes to be easily distinguished. Based on the results, accurately characterizing and identifying not only pulp types, but also the effects of mechanical, chemical and biochemical treatments on fibres, requires measuring the electrokinetic properties at a fixed pH and low conductivity.     

Interaction of Na2B12H11SH with liposomes: Influence on zeta potential and particle size

The interaction of Na₂B₁₂H₁₁SH (BSH) with liposomes has been studied. BSH is a compound used clinically in boron neutron capture therapy of glioblastoma, and is known to enter tumor cells. Liposomes were used as a model for studying the interaction of BSH with cell membranes. BSH led to changes in the zeta potential of liposomes consisting of DODAB (dioctadecyldimethylammonium bromide) alone or with DOPC (dioleylphosphatidylcholine) or DOPE (dioleylphosphatidylethanolamine). It also led to changes of the size of DODAB liposomes, with a maximum size at small zeta potentials. A firm binding of BSH with the head groups of the lipid must be assumed.     

Determination of the zeta potential of surfactants with mixed counterions using a video enhanced microscopy electrophoresis cell

The zeta potential of 2 mM solutions of didodecyldimethylammonium and ditetradecyldimethylammonium surfactants containing a mixture of acetate and bromide are reported. The measuremtns were made using new electrophoretic cells which can be used with a video enhanced microscope. This permits the simultaneous visualization and zeta potential characterization of vesicles and microtobule surfactant solutions. With increasing acetate molar fraction, the zeta potential increases. These results are interpreted in terms of the charge density at the shear plane of the aggregate interface.     

Influence of the three-dimensional heterogeneous roughness on electrokinetic transport in microchannels

Surface roughness has been considered as a passive means of enhancing species mixing in electroosmotic flow through microfluidic systems. It is highly desirable to understand the synergetic effect of three-dimensional (3D) roughness and surface heterogeneity on the electrokinetic flow through microchannels. In this study, we developed a three-dimensional finite-volume-based numerical model to simulate electroosmotic transport in a slit microchannel (formed between two parallel plates) with numerous heterogeneous prismatic roughness elements arranged symmetrically and asymmetrically on the microchannel walls. We consider that all 3D prismatic rough elements have the same surface charge or zeta potential, the substrate (the microchannel wall) surface has a different zeta potential. The results showed that the rough channel's geometry and the electroosmotic mobility ratio of the roughness elements' surface to that of the substrate, epsilon(mu), have a dramatic influence on the induced-pressure field, the electroosmotic flow patterns, and the electroosmotic flow rate in the heterogeneous rough microchannels. The associated sample-species transport presents a tidal-wave-like concentration field at the intersection between four neighboring rough elements under low epsilon(mu) values and has a concentration field similar to that of the smooth channels under high epsilon(mu) values.     

Dynamic interfacial effect of electroosmotic slip flow with a moving capillary front in hydrophobic circular microchannels

Miniaturization of chemical analysis using microfabrication is an emerging technology. The use of polymeric materials as opposed to conventional glass substrate is also a promising alternative. As most polymeric materials are hydrophobic relative to glass, we describe here the implication for the loading process of electroosmotic flow (EOF) when a three-phase (solid-liquid-vapor) contact line exists. The presence of these interfaces can result in a large Laplace pressure that resists EOF and hence hinders its flow performance. This effect depends on the phenomenological contact angle at the solid-liquid interface. In our model for EOF, we considered simultaneously the presence of an electric double layer, liquid slips via a weaker solid-liquid interaction and Laplace pressure across a liquid-vapor interface.     

On the surface modification of microchannels for microcapillary electrophoresis chips

This paper presents systematic investigation of the microchannel surface properties in microCE chips. Three popular materials for microCE chips, polydimethylsiloxane (PDMS), quartz, and glass, are used. The zeta potentials of these microchannels are calculated by measuring the EOF velocity to evaluate the surface properties after surface modification. The hydrophobic PDMS is usually plasma-treated for microCE applications. In this study, a new method using a high-throughput atmospheric plasma generator is adopted to treat the PDMS surface under atmospheric conditions. In this approach, the cost and time for surface treatment can be significantly reduced compared with the conventional vacuum plasma generator method. Experimental results indicate that new functional groups could be formed on the PDMS surface after treatment, resulting in a change in the surface property. The time-dependent surface property of the plasma-treated PDMS is then measured in terms of the zeta potential. Results show that the surface property will reach a stable condition after 1 h of plasma treatment. For glass CE chips, two new methods for changing the microchannel surface properties are developed. Instead of using complicated and time-consuming chemical silanization procedures for CE channel surface modification, two simple and reliable methods utilizing organic-based spin-on-glass and water-soluble acrylic resin are reported. The proposed method provides a fast batch process for controlling the surface properties of glass-based CE channels. The proposed methods are evaluated using PhiX-174 DNA maker separation. The experimental data show that the surface property is modified and separation efficiency greatly improved. In addition, the long-term stability of both coatings is verified in this study. The methods proposed in this study show potential as an excellent solution for glass-based microCE chip surface modification.     

Improvement of the spinning electrophorometer for zeta potential measurements

In this study, bubbles are held by centripetal force at the center of a rotating cylinder filled with an aqueous solution. Their velocities along the axe of rotation, after application of an electrophoretic force, are used for the calculation of the so-called electrokinetic potential. But this process necessitates the elimination of the electro-osmosis which occurs on the interior sides of the glass cylinder by superposing a concurrent force on the bubble. Efficiency of DEAE-Dextran reticulated with 1,4 Butanediol Diglycidyl Ether can be tested by the observation of a cloud of latex microspheres injected in the interior of the tube and allowed to move in respect with the application of an electric field. The experimental control of these velocity profiles proves the adequacy of the polymer for many cases such as surfactant solutions, presence of electrolytes, utilization with moderate pH.The dynamic interpretation of the electrophoretic motion of bubbles is possible by considering that small ones behave like rigid spheres moving in a rotating fluid. In the second part of this paper and in a previous publication, we have experimentally proved that the use of the theoretical expressions of the forces involved for rigid spheres is justified for small bubbles. So, the electrokinetic potential can be expressed versus the velocity, leading to possible interpretations of the adsorption on gas-water interfaces.     

Visualizing the transient electroosmotic flow and measuring the zeta potential of microchannels with a micro-PIV technique

We have demonstrated a transient micro particle image velocimetry (micro-PIV) technique to measure the temporal development of electroosmotic flow in microchannels. Synchronization of different trigger signals for the laser, the CCD camera, and the high-voltage switch makes this measurement possible with a conventional micro-PIV setup. Using the transient micro-PIV technique, we have further proposed a method on the basis of inertial decoupling between the particle electrophoretic motion and the fluid electroosmotic flow to determine the electrophoretic component in the particle velocity and the zeta potential of the channel wall. It is shown that using the measured zeta potentials, the theoretical predictions agree well with the transient response of the electroosmotic velocities measured in this work.     

On the electrophoretic mobility and zeta potential of montmorillonite in non-aqueous media

Experimental results on the electrophoretic velocity and mobility of Ca-montmorillonite in 2-propanol are reported. The variation of the electrophoretic velocity with the externally applied electric field and the particle size range, at constant volume fraction and temperature, is considered. Given the difficulties for determining the types and concentrations of ions present in these liquid media, two methods are discussed for the estimation of the double layer thickness and hence the product, necessary for the determination of the zeta potential () of the interface. Although both methods of calculation yield different values of, the results for the zeta potential are very similar in the regions of and characteristic of our systems. The application to the experimental data of three theoretical relations between electrophoretic mobility and zeta potential is also discussed.