Theoretical prediction of fast 3D AC electro-osmotic pumps

AC electro-osmotic (ACEO) pumps in microfluidics currently involve planar electrode arrays, but recent work on the underlying phenomenon of induced-charge electro-osmosis (ICEO) suggests that three-dimensional (3D) geometries may be exploited to achieve faster flows. In this paper, we present some new design principles for periodic 3D ACEO pumps, such as the "fluid conveyor belt" of ICEO flow over a stepped electrode array. Numerical simulations of these designs (using the standard low-voltage model) predict flow rates almost twenty times faster than existing planar ACEO pumps, for the same applied voltage and minimum feature size. These pumps may enable new portable or implantable lab-on-a-chip devices, since rather fast (mm s(-1)), tuneable flows should be attainable with battery voltages (<10 V).     

Effects of 3D electrodes arrangement in a novel AC electroosmotic micropump: Numerical modeling and experimental validation

To date, a comprehensive systematic optimization framework, capable of accurately predicting an efficient electrode geometry, is not available. Here, different geometries, including 3D step electrodes, have been designed in order to fabricate AC electroosmosis micropumps. It is essential to optimize both geometrical parameters of electrode, such as width and height of steps on each base electrode and their location in one pair, the size of each base electrode (symmetric or asymmetric), the gap of electrode pairs, and nongeometrical parameters such as fluid flow in a channel and electrical characteristics (e.g., frequency and voltage). The governing equations comprising of electric domain and fluid domain have been coupled using finite element method. The developed model was employed to investigate the effect of electrode geometric parameters on electroosmotic slip velocity and its subsequent effect on pressure and flow rate. Numerical simulation indicates that the optimal performance can be achieved using a design with varying step height and displacement, at a given voltage (2.5 V) and frequency (1 kHz). Finally, in order to validate the numerical simulation, the optimal microchip was fabricated using a combination of photolithography, electroplating, and a polydimethylsiloxane microchannel. Our results indicate that our micropump is capable of generating a pressure, velocity, and flow rate of 74.2 Pa, 1.76 mm/s, and 14.8 µl/min, respectively. This result reveals that our proposed geometry outperforms the state-of-the-art micropumps previously reported in the literature by improving the fluid velocity by 32%, with 80% less electrodes per unit length, and whereas the channel length is ∼80% shorter.     

Porous glass electroosmotic pumps: design and experiments

An analytical model for electroosmotic flow rate, total pump current, and thermodynamic efficiency reported in a previous paper has been applied as a design guideline to fabricate porous-structure EO pumps. We have fabricated sintered-glass EO pumps that provide maximum flow rates and pressure capacities of 33 ml/min and 1.3 atm, respectively, at applied potential 100 V. These pumps are designed to be integrated with two-phase microchannel heat exchangers with load capacities of order 100 W and greater. Experiments were conducted with pumps of various geometries and using a relevant, practical range of working electrolyte ionic concentration. Characterization of the pumping performance are discussed in the terms of porosity, tortuosity, pore size, and the dependence of zeta potential on bulk ion density of the working solution. The effects of pressure and flow rate on pump current and thermodynamic efficiency are analyzed and compared to the model prediction. In particular, we explore the important tradeoff between increasing flow rate capacity and obtaining adequate thermodynamic efficiency. This research aims to demonstrate the performance of EOF pump systems and to investigate optimal and practical pump designs. We also present a gas recombination device that makes possible the implementation of this pumping technology into a closed-flow loop where electrolytic gases are converted into water and reclaimed by the system.     

Synchronization of ion exchangers by an oscillating electric field: theory

Can we physically manipulate functions of membrane proteins, such as ion exchangers, especially the active transporters? This is a fascinating question which has attracted many scientists. Recently, we developed a new technique that we call synchronization modulation with which we realized significant (many-folds) activation of Na/K pumps by a well-designed oscillating electric field. In this technique, we consider activation of the pump molecules as a dynamic entrainment procedure, where individual pumps are first entrained to run at the same pumping rate and phase as the oscillating electric field and then the two transports are electrically facilitated separately and alternately by gradually increasing the field oscillating frequency. The procedure consists of two steps: synchronization and modulation. In this paper, we discuss the underlying mechanism involved in the first step: synchronization of the pump molecules.     

Application of a high-pressure electro-osmotic pump using nanometer silica in capillary liquid chromatography

A novel designed electro-osmotic pump (EOP) with simple structure was assembled using three 20 cm x 530 microm i.d. fused-silica capillaries packed with 20 +/- 5 nm silica grains for capillary liquid chromatography. It was found that the pump could generate pressures over 20 MPa and several microL/min flow rate for most of the liquids being delivered with the applied voltage less than 10 kV. By increasing the pressure, decreasing the applied voltage and the electrical current, the thermodynamic efficiency was about 1-4%. A practical application of the EOP in a 20cm x 150 microm i.d. 3 microm C18 fused-silica analytical capillary column demonstrated the applicability of the pump.     

Low-voltage electroosmosis pump for stand-alone microfluidics devices

Two types of low-voltage electroosmosis pumps were developed using microfabrication technology for usage in handy or stand-alone applications of the micrototal analysis systems (micro-TAS) and the lab-on-a-chip. This was done by making a thin (< 1 microm) region in the flow path and by only applying voltages near this thin region using electrodes inserted into the flow path. The inserted electrodes must be free from bubble formation and be gas-tight in order to avoid pressure leakage. For these electrodes, Ag/AgCl or a gel salt bridge was used. For patterning the gel on the chip, a hydrophilic photopolymerization gel and a photolithographic technique were optimized for producing a gel with higher electric conductivity and higher mechanical strength. For high flow rate application, wide (33.2 mm) and thin (400 nm) pumping channels were compacted into a 1 mm x 6 mm area by folding. This pump achieves an 800 Pa static pressure and a flow of 415 nL/min at 10 V. For high-pressure application, a pump was designed with the thin and thick regions in series and positive and negative electrodes were inserted between them alternatively. This pump could increase the pumping pressure without increasing the supply voltage. A pump with 10-stage connections generated a pressure of 25 kPa at 10 V.     

Electro-osmotic pumping of sodium chloride solutions

Electro-osmotic pumping (EOP) theory and its characteristics (transport numbers, brine concentration, current density, current efficiency, electro-osmotic coefficients, etc.) of Selemion AMV and CMV ion-exchange membranes were studied. The brine concentration increased with increase in current density and feed water concentration. Current efficiency was nearly constant in a wide range of current densities and feed water concentrations. The water flow through the membranes also increased with increasing current density and feed water concentration. The increase in water flow increased the current efficiency significantly. Consequently, water flow through electrodialysis (ED) membranes had a positive effect on ED. Electro-osmotic coefficients decreased with increasing feed water concentration. Osmotic flow in EOP-ED decreased relative to the total flow with increasing current density while the electro-osmotic flow increased relative to the osmotic flow. Osmotic flow significantly contributes to the total water flow in EOP. Selemion AMV and CMV membranes performed well for salt concentration. A simple membrane potential measurement has been demonstrated to function reasonably satisfactorily to predict membrane performance for salt concentration.     

Flow inconsistency: The evil twin of column switching—Hardware aspects

Solvent flow, generated by HPLC pumps is consistent and accurate. This statement, while true for single column (one dimensional) liquid chromatography applications, may not apply to column switching applications. Connection of pumps and/or columns to one flow path may cause substantial pressure changes. Immediate post valve switch pressure differences between pumps can cause backflow where the mobile phase stored at higher pressure will temporary flow into the lower pressure area. A more common side effect of column switching is flow inconsistency during pump pressurization. For the duration of pump pressurization, liquid flow through the column will be smaller than expected since the HPLC column acts like a flow restrictor.     

Characterization of a microscale cyclical electrical field flow fractionation system

A microscale cyclical electrical field flow fractionation (CyElFFF) channel is characterized with regard to the effect of various operating parameters and comparison made to recent theoretical developments. Challenges associated with various operating conditions are reported along with some of the optimized operating parameters. The effect of retention wall choice, an offset voltage, relaxation steps, and flow rates, along with the basic operating parameters of voltage, frequency, and electrophoretic mobility are reported. Retention of polystyrene nanoparticle standards is accomplished and the first separations using this technique in a microscale system are also demonstrated. Relaxation steps and offset voltages are found to be effective in eliminating early peaks and in improving plate heights. Plate heights were also found to decrease with increasing flow rates, which is the opposite of the behavior seen in most existing chromatographic systems. The experimental results are compared to the analytical and empirical models of CyElFFF and found to be compatible. Suggestions are made for improving the separation and analysis methods used with CyElFFF.     

Micro-flow injection analysis system: on-chip sample preconcentration, injection and delivery using coupled monolithic electroosmotic pumps

A micro-fluidic chip, within which two monolithic electroosmotic pumps are utilised for sample preconcentration, injection and delivery is presented. The monolithic pumps were capable of producing stable and bubble free flow rates at applied voltages below 2 kV, with a current <10 microA. Electrokinetic (EK) sample injection, down to low nano-litre volumes, was quantitatively controlled through applied voltage and injection times, whilst the sample pump delivered a carrier solution to indirectly dispense the sample. A nano-flow sensor (NFS) was used to continuously monitor the flow rate stability of each pump, showing response times of <5-10 s for changes in applied voltage. A capacitively coupled contactless conductivity detector (C(4)D), as an off-chip on-capillary detector, was used to complete the micro-flow injection analysis (FIA) system. A monolithic electroosmotic pump (EOP), modified with an anionic surfactant, was used to demonstrate a novel approach to on-chip cation preconcentration and elution.     

Predictive chromatographic simulations for the optimization of recovery and aggregate clearance during the capture of monoclonal antibodies

Predictive chromatographic simulations were used to assess whether significant aggregate clearance, in addition to high step recovery and limited eluate pool volumes, can be achieved during protein A affinity chromatography capture steps. Such aggregates of the antibody monomer are commonly found in manufacturing processes. A lumped desorption-kinetic limiting model was used to describe the elution from the chromatography column, as batch isotherm measurements indicated no adsorption under elution conditions. In order to quantify the trade-off between step recovery and aggregate clearance, independent experiments were first performed to obtain the key kinetic parameters. These parameters were used in simulations to predict the behavior of bench-scale protein A column runs and identify robust operating windows within which good yields and significant aggregate clearance can be achieved. Two examples are described. For antibody A, a robust window of operation was identified. In this case, the optimal conditions were transferred to pilot-plant scale, and the resulting experimental data were shown to be in good agreement with model predictions. For antibody B, it was found that conditions resulting in high recovery and good aggregate clearance were not robust: at the optimal elution conditions, changes of ±0.1 units in pH or ±1 mS/cm in conductivity affected the results substantially.     

An integrated micropump and electrospray emitter system based on porous silica monoliths

The work presents the design of an integrated system consisting of a high-pressure electroosmotic (EO) micropump and a microporous monolithic emitter, which together generate a stable and robust electrospray. Both the micropump and electrospray emitter are fabricated using a sol-gel process. Upon application of an electric potential of sufficient amplitude (>2 kV), the pump delivers fluids with an electroosmotically induced high pressure (>1 atm). The same potential is also harnessed to electrostatically generate a stable electrospray at the porous emitter. Electrokinetic coupling between pump and spray produces spray features different from sprays pressurized by independent mechanical pumps. Four typical spray modes, each with different drop sizes and charge-to-mass ratios, are observed and have been characterized. Since the monolith is silica-based, this integrated device can be used for a variety of fluids, especially organic solvents, without the swelling and shrinking problems that are commonly encountered for polymer monoliths. The maximum pressure generated by a 100 microm id monolithic pump is 3 atm at an applied voltage of 5 kV. The flow rate can be adjusted in the range of 100 nL/min to 1 microL/min by changing the voltage. For a given applied voltage across the pump and emitter system, it is seen that there exists one unique flow rate for which flow balance is achieved between the delivery of liquid to the emitter by the pump and the liquid ejection from the emitter. Under such a condition, a stable Taylor cone is obtained. The principles that lead to these results are also discussed.     

An AC electroosmotic micropump for circular chromatographic applications

Flow rates of up to 50 microm s(-1) have been successfully achieved in a closed-loop channel using an AC electroosmotic pump. The AC electroosmotic pump is made of an interdigitated array of unequal width electrodes located at the bottom of a channel, with an AC voltage applied between the small and the large electrodes. The flow rate was found to increase linearly with the applied voltage and to decrease linearly with the applied frequency. The pump is expected to be suitable for circular chromatography for the following reasons: the driving forces are distributed over the channel length and the pumping direction is set by the direction of the interdigitated electrodes. Pumping in a closed-loop channel can be achieved by arranging the electrode pattern in a circle. In addition the inherent working principle of AC electroosmotic pumping enables the independent optimisation of the channel height or the flow velocity.     

Molecular dynamics simulations of multilayer polyelectrolyte films: effect of electrostatic and short-range interactions

The effect of the strength of electrostatic and short-range interactions on the multilayer assembly of oppositely charged polyelectrolytes at a charged substrate was studied by molecular dynamics simulations. The multilayer buildup was achieved through sequential adsorption of charged polymers in a layer-by-layer fashion from dilute polyelectrolyte solutions. The strong electrostatic attraction between oppositely charged polyelectrolytes at each deposition step is a driving force behind the multilayer growth. Our simulations have shown that a charge reversal after each deposition step is critical for steady multilayer growth and that there is a linear increase in polymer surface coverage after the first few deposition steps. Furthermore, there is substantial intermixing between chains adsorbed during different deposition steps. We show that the polymer surface coverage and multilayer structure are each strongly influenced by the strength of electrostatic and short-range interactions.     

Electrokinetic effect of the bottom surface of a vessel on pearl-chain formation of silica particles

We have tackled in situ electric conductance measurements under microscopic observations for alignments of silica particles that are induced by ionic polarization of the electrical double layer (EDL) around the particles. Using the in situ conductance measurements, we have presented evidence that electro-osmotic flow at a vessel bottom/water interface would be coupled with the ionic polarization in the EDL of spherical silica particles settling at the bottom (Langmuir 2007, 23, 8797). In this study, we followed this phenomenon further. We altered the zeta potential of a platform of a glass plate on which a pearl chain of silica particles was formed under an ac electric field to control the mobility of electro-osmotic flow at the macroscopic interface of the platform/water. As the magnitude of the zeta potential of the platform increased, the surface distance between neighboring particles in the pearl chains decreased and the in situ conductance totally increased due to the enhancement of the dipole moments induced by the ionic polarizations of the particles. These results could be explained by considering that the electro-osmotic contribution to the surface conduction around the particles would be coupled with that occurring at the platform/water interface.     

Polyethyleneimine-bonded phases in the separation of proteins by capillary electrophoresis

A hydrophilic, positively charged, durable coating has been developed for capillary electrophoresis of macromolecules. Polyethyleneimine is adsorbed to the inner wall of fused silica capillaries and the adsorbed coating cross-linked into a stable layer. Capillaries of polyethyleneimine-coated silica gave unique separations owing to the reversal of electro-osmotic flow caused by the positively charged coating. The resulting coating was stable from pH 2-12 and could be used over a wide pH range without substantial change in electro-osmotic flow. High-molecular-weight polymers were needed to give thick coatings which mask silanol groups on the wall. Proteins were resolved quickly and efficiently with good recovery using capillaries of 50 cm in length.     

A new method of evaluating the average electro-osmotic velocity in microchannels

A new and simple method to evaluate the average electro-osmotic flow velocity in microchannels is presented in this paper. In this method, the average electro-osmotic flow velocity is determined by using the slope of the measured current-time relationship during the electro-osmotic flow of one solution replacing another similar solution. The two solutions have the same electrolyte and a small difference in ionic concentration. Careful experiments were conducted to measure the electrical current change with time during such a displacing process under a constant applied electrical field. KCl and LaCl3 electrolyte solutions and 10-cm-long polyamide-coated silica capillary tubes of 100 and 200 microm in internal diameter were used in this study. The average velocities were determined by using the slope method. A numerical model was also developed to predict the average velocity of such an electro-osmotic flow. An excellent agreement in the average velocities between the slope method and the model predictions was found.     

Where does the planar-to-nonplanar turnover occur in small gold clusters?

Several levels of theory, including both Gaussian-based and plane wave density functional theory (DFT), second-order perturbation theory (MP2), and coupled cluster methods (CCSD(T)), are employed to study Au6 and Au8 clusters. All methods predict that the lowest energy isomer of Au6 is planar. For Au8, both DFT methods predict that the two lowest isomers are planar. In contrast, both MP2 and CCSD(T) predict the lowest Au8 isomers to be nonplanar.     

Fabrication of a novel cascade high-pressure electro-osmotic pump

A novel cascade electro-osmotic pump (EOP) has been fabricated by alternately connecting a cation monolithic column and anion monolithic column in series. In this manner, the change of electric polarity between each stage of the cascade EOP is easily achieved and the pressure output of the EOP could be greatly enhanced without increase of the applied voltage.