Control Of Electroosmotic Flow Research Articles

Investigation of induced electroosmotic flow in small-scale capillary electrophoresis devices: Strategies for control and reversal.

Electroosmotic flow (EOF) is the bulk flow of solution in a capillary or microchannel induced by an applied electric potential. For capillary and microchip electrophoresis, the EOF enables analysis of both cations and anions in one separation and can be varied to modify separation speed and resolution. The EOF arises from an electrical double layer at the capillary wall and is normally controlled through the pH and ionic strength of the background buffer or with the use of additives. Understanding and controlling the electrical double layer is therefore critical for maintaining acceptable repeatability during method development. Surprisingly, in fused silica capillaries at low pH, studies observe an EOF even though the capillary surface should be neutralized. Previous work has suggested the presence of an "induced electroosmotic flow" from radial electric fields generated across the capillary wall due to the separation voltage and grounded components external to the capillary. Using thin-wall (15µm) fused silica separation capillaries to facilitate the study of radial fields, we show that the EOF mobility depends on both the separation voltage and the location of external grounds. This is consistent with the induced EOF model, in which radial electric fields embed positive charges at the capillary walls to create an electrical double layer. The magnitude of the effect is characterized and shown to have long-range influences that are difficult to completely null by moving grounded components away from the separation capillary. Instead, active EOF control using externally applied potentials or a passive approach using a negative separation voltage are discussed as two possible methods for controlling the induced EOF. Both methods can reverse the EOF and improve the resolution and peak efficiency in amino acid separations.

Generalizing electroosmotic-flow predictions over charge-modulated periodic topographies: tuneable far-field effects

The classical Helmholtz–Smoluchowski (HS) model of electroosmosis holds for homogeneously charged interfaces in contact with a fluid layer bearing an equal and opposite net charge. However, inhomogeneities in the surface charge and topography are inevitable, either as practical materials and fabrication artefacts, or at times as deliberately introduced modulations for flow control. In an effort to arrive at an analytically tractable theoretical framework for addressing the underlying electro-mechanical coupling, here, we generalize the traditional HS theory to an extent where both the surface charge and topographies may bear arbitrary and independent periodic forms. Using a spectral-asymptotic approach, we further arrive at closed-form expressions for describing the resulting electroosmotic pumping for topographic features with small characteristic amplitude to pattern period ratio, as relevant for most practical scenarios. We subsequently execute full-scale numerical simulations without any restrictions on the surface charge and topography variations to assess the efficacy of the theoretical framework. The corresponding test beds include distinctive signature patterns – for example, a square-wave surface charge distribution on trapezoidal pit topographies. Our results reveal that the charge–topography interplay induces an anisotropic flow drift, deviating from the classical HS paradigm. This, in turn, provides new quantitative insights into highly selective electroosmotic flow control via judicious design of the charge and topographical patterns, resulting in controllable accentuation, attenuation, nullification, deflection and even complete reversal of the flow. Our analysis further establishes a provision of estimating the zeta potentials of naturally ‘contaminated’ surfaces, as well as explaining the electrophoresis of large inhomogeneous particles; a paradigm that remained to be explored thus far.

A bioinspired approach for the modulation of electroosmotic flow and protein-surface interactions in capillary electrophoresis using silylated amino-amides blocks and covalent grafting.

We explore a bioinspired approach to design tailored functionalized capillary electrophoresis (CE) surfaces based on covalent grafting for biomolecules analysis. First, the approach aims to overcome well-known common obstacles in CE protein analysis affecting considerably the CE performance (asymmetry, resolution, and repeatability) such as the unspecific adsorption on fused silica surface and the lack of control of electroosmotic flow (EOF). Then, our approach, which relies on new amino-amide mimic hybrid precursors synthesized by silylation of amino-amides (Si-AA) derivatives with 3-isocyanatopropyltriethoxysilane, aims to recapitulate the diversity of protein-protein interactions (π-π stacking, ionic, Van der Waals…) found in physiological condition (bioinspired approach) to improve the performance of CE protein analysis (electrochromatography). As a proof of concept, these silylated Si-AA (tyrosinamide silylation, serinamide silylation, argininamide silylation, leucinamide silylation, and isoglutamine silylation acid) have been covalently grafted in physiological conditions in different amount on bare fused silica capillary giving rise to a biomimetic coating and allowing both the modulation of EOF and protein-surface interactions. The analytical performances of amino-amide functionalized capillaries were assessed using lysozyme, cytochrome C and ribonuclease A and compared to traditional capillary coatings poly(ethylene oxide), poly(diallyldimethylammonium chloride), and sodium poly(styrenesulfonate). EOF, protein adsorption rate, protein retention factor k, and selectivity were determined for each coating. All results obtained showed this approach allowed to modulate the EOF, reduce unspecific adsorption, and generate specific interactions with proteins by varying the nature and the amount of Si-AA in the functionalization mixture.

Chemical vapour deposition in narrow capillaries: Electro-osmotic flow control in capillary electrophoresis

BackgroundIn capillary electrophoresis (CE), the inner surface of fused-silica capillaries is commonly covalently modified with liquid silanes to control electroosmotic flow (EOF). This liquid phase deposition (LPD) approach is challenging for long and narrow-diameter capillaries (≥1 m, ≤25 μm ID) inhibiting commercial production. Here, we use chemical vapour deposition (CVD) to covalently modify capillaries with different silanes. Using a home-built CVD device, capillaries were modified with neutral (3-glycidyloxypropyl) trimethoxysilane (GPTMS), the weak base (3-aminopropyl) trimethoxysilane (APTMS), the weak acid 3-mercaptopropyltrimethoxysilane (MPTMS) and the neutral hydrophobic trichloro(1H,1H,2H,2H-perfluorooctyl) silane (PFOCTS). Gas-phase modification of GPTMS with acid and ammonia allowed further modification of the surface prior to molecular layer deposition (MLD) of poly(p-phenylene terephthalamide) (PPTA) using the self-limiting sequential reaction between terephthalaldehyde (TA) and p-phenylenediamine (PD) vapours. ResultsCapillaries coated with GPTMS by CVD showed a greater reduction in EOF at all pH values than the conventional LPD. APTMS showed a reduction of the EOF at pH 9, with EOF reversal observed below pH 6. MPTMS provided a slightly lower EOF than an unmodified capillary at high pH, and a slightly higher EOF at lower pH. PFOCTS provided the most consistent EOF as a function of pH. The deposition of successive layers of PPTA resulted in increased surface coverage of the polymer and a greater reduction in EOF at pH higher than 5. The stability of a 10 μm ID GPTMS coated capillary was tested at pH 8.8 in a 200 mM CHES/Tris BGE for the separation of inorganic anions. Over 1.5 months of continuous operation (≈4130 runs), the reproducibility of the apparent mobilities for chloride, nitrite, nitrate and sulfate were 2.43%, 2.56%, 2.63% and 3.05%, respectively. The intra-day and inter-day column-to-column reproducibility and batch-to-batch reproducibility for all the coated capillaries ranged between 0.34% and 3.95%. SignificanceThe study demonstrates the superior performance of CVD coating for suppressing the EOF compared to LPD allowing the easy modification of long lengths of narrow capillary. The variation in silane, and the ability of MLD to modify and control the surface chemistry, provides a simple and facile method for surface modification. The stability of these coatings will allow long-term capillary electrophoresis monitoring of water chemistry, such as for monitoring fertiliser run-off in natural waters.

Design and simulation of a novel MEMS based microfluidic circulating tumor cell (CTC) detection system for a lab on a chip device

This paper proposes a novel microfluidic system for a lab on a chip device which includes three major systems: a microchannel, micromixer and a droplet generator. The novel system proposes electroosmotic fluid flow control combining droplet generation and immunocapturing based CTC separation. ANSYS Fluent is used to optimize the fluid flow parameters, droplet size and justify the mixing capability of the micromixer. COMSOL Multiphysics simulations justify the integration of the electroosmotic fluid flow control for precise generation of droplets and optimization of dimensional parameters of microchannels, followed by a fabrication method for the microfluidic system.

Electroosmotic Flow in Microchannel with Black Silicon Nanostructures.

Although electroosmotic flow (EOF) has been applied to drive fluid flow in microfluidic chips, some of the phenomena associated with it can adversely affect the performance of certain applications such as electrophoresis and ion preconcentration. To minimize the undesirable effects, EOF can be suppressed by polymer coatings or introduction of nanostructures. In this work, we presented a novel technique that employs the Dry Etching, Electroplating and Molding (DEEMO) process along with reactive ion etching (RIE), to fabricate microchannel with black silicon nanostructures (prolate hemispheroid-like structures). The effect of black silicon nanostructures on EOF was examined experimentally by current monitoring method, and numerically by finite element simulations. The experimental results showed that the EOF velocity was reduced by 13 ± 7%, which is reasonably close to the simulation results that predict a reduction of approximately 8%. EOF reduction is caused by the distortion of local electric field at the nanostructured surface. Numerical simulations show that the EOF velocity decreases with increasing nanostructure height or decreasing diameter. This reveals the potential of tuning the etching process parameters to generate nanostructures for better EOF suppression. The outcome of this investigation enhances the fundamental understanding of EOF behavior, with implications on the precise EOF control in devices utilizing nanostructured surfaces for chemical and biological analyses.

Separation of Small DNAs by Gel Electrophoresis in a Fused Silica Capillary Coated with a Negatively Charged Copolymer

Active development of compact analytical instruments suitable for point-of-care testing (POCT) requires optimization of existing methods. To aid the development of capillary gel electrophoresis instruments for POCT, we attempted to separate polymerase chain reaction products (small DNAs) using a short, fused silica capillary coated with an acrylamide (AM)/acrylic acid (AA) copolymer (poly(AM-co-AA)). To realize the high capability of this capillary to separate small DNAs, the magnitude of electroosmotic flow (EOF) was controlled by varying the content of negatively charged AA in the copolymer, which significantly affected the separation ability. At an AA content ≥3.75 mol %, sample DNAs could not be injected into the copolymer-coated capillary owing to strong EOF, whereas a 100 bp DNA ladder sample was successfully separated at an AA content of ≤3.5 mol %, showing that even slight AA content variations impact DNA flow. EOF values measured using a neutral coumarin 334 solution suddenly decreased at an AA content of 3.5 mol % relative to those at an AA content of ≥3.75 mol %. Theoretical plate values revealed that an AA content of 2.75 mol % was optimal for separating ladder DNAs with sizes ≥600 bp. Hence, EOF control achieved by varying the amount of negatively charged AA in the poly(AM-co-AA) coating can promote further development of short capillaries for POCT applications.

Effect of nanostructures orientation on electroosmotic flow in a microfluidic channel

Electroosmotic flow (EOF) is an electric-field-induced fluid flow that has numerous micro-/nanofluidic applications, ranging from pumping to chemical and biomedical analyses. Nanoscale networks/structures are often integrated in microchannels for a broad range of applications, such as electrophoretic separation of biomolecules, high reaction efficiency catalytic microreactors, and enhancement of heat transfer and sensing. Their introduction has been known to reduce EOF. Hitherto, a proper study on the effect of nanostructures orientation on EOF in a microfluidic channel is yet to be carried out. In this investigation, we present a novel fabrication method for nanostructure designs that possess maximum orientation difference, i.e. parallel versus perpendicular indented nanolines, to examine the effect of nanostructures orientation on EOF. It consists of four phases: fabrication of silicon master, creation of mold insert via electroplating, injection molding with cyclic olefin copolymer, and thermal bonding and integration of practical inlet/outlet ports. The effect of nanostructures orientation on EOF was studied experimentally by current monitoring method. The experimental results show that nanolines which are perpendicular to the microchannel reduce the EOF velocity significantly (approximately 20%). This flow velocity reduction is due to the distortion of local electric field by the perpendicular nanolines at the nanostructured surface as demonstrated by finite element simulation. In contrast, nanolines which are parallel to the microchannel have no effect on EOF, as it can be deduced that the parallel nanolines do not distort the local electric field. The outcomes of this investigation contribute to the precise control of EOF in lab-on-chip devices, and fundamental understanding of EOF in devices which utilize nanostructured surfaces for chemical and biological analyses.

A simple, openable and electroosmotic flow-free PMMA chip for electrophoretic titration of moving reaction boundary

Poor control of electroosmotic flow (EOF) often occurs in chip electrophoretic titration (ET) of moving reaction boundary due to both absence of PMMA channel coating and coexistence of acid-base. Also, chip ET still suffers from low reproducibility, instability, disposability and high cost as well. To address these issues, a reusable ET chip (60mm×60mm×5.0mm) was designed via an upper PMMA sheet, a middle silicon membrane and a lower sheet. In the upper sheet, enlarged channel (500μm×200μm×12mm) was fabricated and linked to the anodic and cathodic wells. The upper sheet, middle membrane and lower sheet could be easily assembled together in 20s as an ET chip of supporting 1.4kg tension force, indicating good mechanical stability. Regeneration and reassembling of chip could be finished in 50s. EOF could be simply suppressed by mass chemical cross-linked polyacrylamide gel or physical agarose gel in the enlarged channel. The EOF-free chip could be well used for: (i) acid-base ET; (ii) EOF monitoring; (iii) pKa determination; (iv) protein content analysis. The EOF-free chip showed the following merits: (i) 98% success rate in contrast to less than 30% in normal channels (80μm×20μm×5.0mm); (ii) more than 30 times reuse, indicating reusability and low cost; (iii) less than 10% of relative standard deviation, suggesting fair stability; and (iv) good recovery and accuracy. The developed chip makes great advancement towards real use of chip in acid- base titration, EOF monitoring, pKa determination and protein content analysis.

Wall embedded electrodes to modify electroosmotic flow in silica nanoslits.

Electroosmotic flow in a silica slit channel with nonuniform surface charge density is investigated. In nanoconfinement, the electrical double layer occupies a non-negligible fraction of the system. Therefore, modifying the charge density on specific locations on the channel wall surface allows effective manipulation of the electroosmotic flow rates. In the present study, extensive (160 ns) nonequilibrium molecular dynamics simulations are conducted to investigate the ability of controlling the electroosmotic flow control in a nanoslit by patterning the surface potential. The mechanism to modify the surface charge consists of a set of charged electrodes embedded within one of the channel walls. The presence of the embedded electrodes results in the redistribution of ions in the electrolyte solution and in the alteration of the electroosmotic flow throughout the nanochannel. Indeed, the results reveal significant changes in the electroosmotic driving force and velocity profiles including local flow reversal. This study provides physical insight into the direct manipulation of the electrokinetic flow in nanoslits.

Electro-osmosis of electrorheological fluids

Electrorheological fluids are suspensions that are characterized by a strong functional dependence of the constitutive behavior of the fluids on the electric field. In this work, we consider electro-osmosis of an electrorheological fluid through a channel where a transverse, nonuniform electric field is spontaneously induced due to the presence of an electric double layer that is manifested due to surface charge density at the channel wall. We reveal a nonlinear interplay between the applied electric field, the induced electric field, and the observed flow profiles, which is fundamentally distinctive from other types of nonlinear electrokinetic effects that have been extensively discussed in the literature, in a sense that here an interaction between the applied electric field, the induced electric field, and the dependence of the rheology on the resultant electric field happens to be the focal source of nonlinearity in the observed phenomena. We analyze the electro-osmotic flow control through the exploitation of a combined nonlinear interplay of the driving electrokinetic forces and the resistive viscous interactions, which gives rise to distinctive flow regimes as compared to those realized in cases of either Newtonian fluids or non-Newtonian fluids having electric-field-independent flow rheology.

Field Effect Modulation of Surface Charge Property and Electroosmotic Flow in a Nanochannel: Stern Layer Effect

Active control of surface charge property and electroosmotic flow (EOF) in a silica-based nanochannel using a field effect transistor (FET) is analyzed for the first time taking the Stern layer effect into account. Approximations for surface charge property and EOF have been derived and validated by comparing their predictions with experimental data available in the literature. We show that, in addition to the background solution properties such as its pH and salt concentration, the field effect control of the zeta potential of the nanochannel wall and the EOF velocity depends highly on the surface capacitance of the Stern layer, stemming from the attraction of immobile counterions within that layer. The Stern layer effect becomes significant when the background salt concentration, solution pH, and/or the applied gate potential are relatively high. Results gathered provide valuable information for designing relevant gated nanofluidic devices for regulating ion, fluid flow, and biomolecule transport.

Relaxation characteristics of a compliant microfluidic channel under electroosmotic flow

We investigate the effect of electroosmotic flow on the temporal response of an initially deformed microfluidic channel wall as it relaxes to its undeformed state. Our results reveal that the electrokinetic effects significantly alter not just the quantitative response of the relaxation dynamics but also the qualitative nature of the relaxation profiles. Furthermore, the electrokinetics brings about a preferential asymmetry to the nature of the dynamical evolution of the relaxation characteristics. Overall, we unveil a rich interplay between the pertinent physico-chemical interactions, the compliance of the deformable channel walls and the geometry of the fluidic confinement, bearing non-trivial consequences towards optimal control of electroosmotic flow in narrow channels with deformable confining boundaries.

Electroosmotic Flow Control in Microfluidic Chips Using a Self-Assembled Monolayer as the Insulator of a Flow Field-Effect Transistor

A novel concept for electroosmotic flow (EOF) control in a microfluidic chip is presented by using a self-assembled monolayer as the insulator of a flow field-effect transistor. Bidirectional EOF control with mobility values of 3.4 × 10(-4) and -3.1 × 10(-4) cm(2)/V s can be attained, corresponding to the applied gate voltage at -0.8 and 0.8 V, respectively, without the addition of buffer additives. A relatively high control factor (approximately 400 × 10(-6) cm(2)/V(2) s) can be obtained. The method presented in this study offers a simple strategy to control the EOF.

Field Effect Control of Surface Charge Property and Electroosmotic Flow in Nanofluidics

The surface charge property of nanofluidic devices plays an essential role in electrokinetic transport of ions, fluids, and particles in them. The nanofluidic field effect transistor (FET), referring to a nanochannel embedded with an electrically controllable gate electrode, provides a simple way to rapidly regulate its surface charge property, which in turn controls the electrokinetic transport phenomena within the nanochannel. In this study, approximate analytical expressions are derived for the first time to estimate the surface-charge property and electroosmotic flow (EOF) in charge-regulated nanochannels tuned by the nanofluidic FET and are validated by comparing their predictions to the existing experimental data available from the literature. The control of the surface charge property as well as the EOF by the nanofluidic FET depends on the pH and ionic concentration of the aqueous solution.

Physisorbed surface coatings for poly(dimethylsiloxane) and quartz microfluidic devices

Surface modifications of microfluidic devices are of essential importance for successful bioanalytical applications. Here, we investigate three different coatings for quartz and poly(dimethylsiloxane) (PDMS) surfaces. We employed a triblock copolymer with trade name F(108), poly(L-lysine)-g-poly(ethylene glycol) (PLL-PEG), as well as the hybrid coating n-dodecyl-β-D-maltoside and methyl cellulose (DDM/MC). The impact of these coatings was characterized by measuring the electroosmotic flow (EOF), contact angle, and prevention of protein adsorption. Furthermore, we investigated the influence of static coatings, i.e., the incubation with the coating agent prior to measurements, and dynamic coatings, where the coating agent was present during the measurement. We found that all coatings on PDMS as well as quartz reduced EOF, increased reproducibility of EOF, reduced protein adsorption, and improved the wettability of the surfaces. Among the coating strategies tested, the dynamic coatings with DDM/MC and F(108) demonstrated maximal reduction of EOF and protein adsorption and simultaneously best long-term stability concerning EOF. For PLL-PEG, a reversal in the EOF direction was observed. Interestingly, the static surface coating strategy with F(108) proved to be as effective to prevent protein adsorption as dynamic coating with this block copolymer. These findings will allow optimized parameter choices for coating strategies on PDMS and quartz microfluidic devices in which control of EOF and reduced biofouling are indispensable.

Passively operated microfluidic device for stimulation and secretion sampling of single pancreatic islets.

A passively operated polydimethylsiloxane (PDMS) microfluidic device was designed for sampling of hormone secretions from eight individual murine pancreatic islets in parallel. Flow control was achieved using a single hand-held syringe and by exploiting inherent fluidic resistances of the microchannels (R(sampling) = 700 ± 20 kPa s mm(-3) at 37 °C). Basal (3 mM) or stimulatory (11 mM) glucose levels were applied to islets, with stimulation timing (t(stim)) minimized to 15 ± 2 s using modified reservoirs. Using enzyme-linked immunosorbent assays (ELISA) for postsampling analyses, we measured statistically equal levels of 1 h insulin secretion (1.26 ± 0.26 and 6.55 ± 1.00 pg islet(-1) min(-1), basal and stimulated; 62 islets) compared to standard, bulk sampling methods (1.01 ± 0.224 and 6.04 ± 1.53 pg islet(-1) min(-1), basal and stimulated; 200 islets). Importantly, the microfluidic platform revealed novel information on single-islet variability. Islet volume measurements with confocal reflectance microscopy revealed that insulin secretion had only limited correlation to islet volume, suggesting a more significant role for cellular architecture and paracrine signaling within the tissue. Compared to other methods using syringe pumps or electroosmotic flow control, this approach provides significant advantages in ease-of-use and device disposability, easing the burden on nonexperts.

Electroosmotic flow in a nanofluidic channel coated with neutral polymers

We use molecular dynamics simulations to investigate the control of electroosmotic flow by grafting polymers onto two parallel channel walls. The effects of the grafting density and the electric field strength on electroosmotic flow velocity, counterion distribution and conformational characteristics of grafted chains have been studied in detail for athermal, good, and poor solvent cases. The simulation results indicate that in the range of grafting densities investigated, increasing the grafting density induces a different change tendency of electroosmotic flow velocity for three different solvent qualities. These tendencies are demonstrated to be related to counterion distribution, polymer coverage, and interactions between monomers and solvent particles. It is found that counterions tend to move toward the interface between polymer layer and solvent as the grafting density increases. Especially in the poor solvent case, most of the counterions gather near the interface at high grafting densities. A similar behavior is also observed when enhancing the electric field strength at a fixed grafting density.

Microchip electrochromatography: the latest developments and applications

This review summarizes recent developments and applications of microchip electrochromatography (microCEC) mainly in the past five years between 2005 and 2009 with a focus on column technologies. In addition, some new improvements in the chip design and fabrication, sample preconcentration, electroosmotic flow control as well as mechanisms that govern electrochromatographic separation are described and reviewed. The features and limitations of several practical aspects of their applications are highlighted.

Flow field effect transistors with polarisable interface for EOF tunable microfluidic separation devices

A method is proposed to control the zeta potential in microchannels using electrically polarisable interfaces in direct contact with the electrolyte. The approach is based on the use of conducting layers exhibiting minimal electrochemical reactions with aqueous electrolytes but a large potential window (typically from -2 V to +2 V) enabling tuning their zeta potential without detrimental faradic reactions. SiC, Al and CN(x) interfaces were deposited on glass surfaces and then integrated into glass-PDMS-glass devices. The effect of the zeta potential control was monitored by measuring the electro-osmotic flow using a microfluidic Wheatstone Bridge. The experimental results are in good agreement with theoretical predictions based on a one dimensional modeling. The electro-osmotic flow control obtained at high pH values suggests that it should be possible to use such devices as Polarisable Interface Flow-Field Effect Transistors (PI-FFETs) to overcome the difficulties met with conventional metal-isolator-electrolyte systems (MIE-FFETs) for electrokinetic separation applications.