Finite element simulation of pinched pressure-driven flow injection in microchannels

A pinched pressure-driven flow injection on a microchip is numerically simulated in order to optimize the relative values of the operational parameters. The geometry studied is a two-dimensional rectangular channel featuring a cross-junction with a large depth-over-width ratio. The hydrodynamic and convection-diffusion equations are solved for the two steps of the process: first, the sample solution is pinched into the transversal channel (injection channel), and then it is injected into the longitudinal one (separation channel), where the time evolution of the concentration is analyzed for different types of the detectors. Electroosmotic flow calculations have also been performed and have shown a good agreement with literature. The results for pressure-driven flow point out that the shape of the detection signal is strongly dependent on the velocity in the separation channel and on the position of the detection probes. The so-called double-humped peak, caused by the parabolic flow profile at high driving flow rate is analyzed. A tight pinch greatly decreases the amount of injected sample and, consequently, the signal sensitivity without increasing its quality. A proper pullback of the sample during the separation process can decrease the tailing due to the sample leakage from the injection channel. Although a high sample pullback causes a considerable decrease in the signal sensitivity, it also greatly enhances the peak resolution. Finally, it is shown that a wider injection channel with high sample pullback ensures an improved signal sensitivity with good resolution.     

Hydrodynamic dispersion due to combined pressure-driven and electroosmotic flow through microchannels with a thin double layer

The hydrodynamic dispersion of a nonadsorbed and nonelectrolyte solute is considered for the case of a flow driven through a straight microchannel by pressure and electric potential differences. The analysis is conducted using a thin double layer approximation developed in the previous paper (Zholkovskij, E. K.; Masliyah, J. H.; Czarnecki, J. Anal. Chem. 2003, 75, 901-909). On the basis of this approach, an expression is derived to address the dispersion coefficient for arbitrary electrokinetic potential, electrolyte type, and cross-section geometry. In the derived expression, the influence of cross-section geometry manifests itself through the channel hydrodynamic radius and through three dimensionless geometrical factors. The procedure for obtaining the geometrical factors is presented for an arbitrary cross-section geometry. The geometrical factors are evaluated for several examples of cross section: (i) unbounded parallel planes; (ii) circle; (iii) annulus; (iv) ellipse; (v) rectangle. The dependency of the dispersion coefficient on different parameters is discussed. It is shown that the dependencies are substantially affected by the cross-section geometry, electrolyte type, and electrokinetic potential.     

New strategies for separations through reactions

Separations through reactions can provide reliable and economically viable alternatives to established methods of separation, particularly for close boiling substances. New strategies in ‘Dissociation Extraction’ and ‘Dissociation Extractive Crystallization’ for separation of close boiling acidic/basic mixtures have been highlighted. Separations with aqueous solutions of hydrotrope and aqueous micellar solutions have been brought out. Separations by membranes with facilitated transport is potentially attractive. This paper is dedicated to Dr L K Doraiswamy on his sixtieth birthday.     

Deconvolution microscopy for flow visualization in microchannels

Quantitative visualization of microflows is often needed to evaluate the efficiency of fluid mixing, study flow properties, investigate unusual flow behavior, and verify computational fluid dynamic simulations. In this work, we explore the technique of coupling a conventional optical microscope with a computational deconvolution algorithm to produce images of three-dimensional flows in plastic microfluidic channels. The approach, called deconvolution microscopy, is achieved by (1) optically sectioning the flow in the microchannel by collecting a series of fluorescence images at different focal planes along the optical axis and (2) removing the out-of-focus fluorescence signal by a deconvolution method to reconstruct the corrected three-dimensional concentration image. We compare three different classes of deconvolution algorithms for a uniform concentration test case and then demonstrate how deconvolution microscopy is useful for flow visualization and analysis of mixing in microfluidic channels. In particular, we employ the method to confirm the presence of twisting flows in a microchannel containing microfabricated ridges.     

Dispersion reduction in pressure-driven flow through microetched channels

Fluid is often moved about microetched channels in lab-on-a-chip applications using electrokinetic flows (electrophoresis or electroosmosis) rather than pressure-driven flows because the latter result in large Taylor dispersion. However, small pressure gradients may arise unintentionally in such systems due to a mismatch in electroosmotic flow rates or hydrostatic pressure differentials along the microetched channel. Under laminar flow conditions, Doshi et al. (Chem. Eng. Sci. 1978, 33, 795-804) have shown that for a channel with rectangular cross-section of width W and depth d, longitudinal diffusivities can attain values as large as approximately 8 K0 for small values of the aspect ratio d/W, where K0 is the value of the longitudinal diffusivity obtained by ignoring all variations across the channel. Microchannels in lab-on-a-chip geometries are often not rectangular in cross-section. Isotropic etching techniques, for example, lead to channels with quarter-circular ends. In this paper we examine the effect of this geometry on the magnitude of longitudinal dispersivity for pressure-driven flows and also investigate modifications to this design which may minimize such dispersion. Optimal channel profiles are shown to lead to dispersivities approaching K0, the theoretical minimum, for small values of d/W.     

On-chip high-pressure picoliter injector for pressure-driven flow through porous media

A high-pressure (> 3 MPa) on-chip injector has been developed for microchip applications including HPLC. The mechanical injector is implemented using in situ photopolymerization of fluorinated acrylates inside wet-etched silica microchips. The injector allows reproducible injections as small as 180 pL with < 250 ms duration. The injector operated robustly over 60 days and over 1000 injections. The injector is unique among polymer-based valves as it functions in aqueous, acetonitrile, and mixed buffers at high pressures without detectable leakage.     

Electroosmotic flow in irregular shape microchannels

The electrokinetic flow through star-point family microchannels is studied. Mathematical solutions using a semi-analytical approach are employed to model the steady, fully developed electroosmotic flow in the closed-end star point family microchannels. The model can be extended to other irregular shape channels. The effects of sharp corners, electrolyte concentration and packed capillary radius, R₀ are analyzed. Comparisons of the flow characteristics and induced pressure gradient inside and outside of the packed capillaries are presented.     

压力驱动膜分离过程的操作模式及其优化

压力驱动膜过程主要包括微滤、超滤、纳滤和反渗透4种，操作时可采用浓缩和渗滤两种模式．典型操作过程由预浓缩、恒容渗滤和后浓缩3个阶段组成，对其进行优化，以使总过程时间最短和渗滤操作时渗滤溶剂的消耗量最小．     

Imaging of electroosmotic flow in plastic microchannels

We have characterized electroosmotic flow in plastic microchannels using video imaging of caged fluorescent dye after it has been uncaged with a laser pulse. We studied flow in microchannels composed of a single material, poly(methyl methacrylate) (acrylic) or poly(dimethylsiloxane) (PDMS), as well as in hybrid microchannels composed of both materials. Plastic microchannels used in this study were fabricated by imprinting or molding using a micromachined silicon template as the stamping tool. We examined the dispersion of the uncaged dye in the plastic microchannels and compared it with results obtained in a fused-silica capillary. For PDMS microchannels, it was possible to achieve dispersion similar to that found in fused silica. For the acrylic and hybrid microchannels, we found increased dispersion due to the nonuniformity of surface charge density at the walls of the channels. In all cases, however, electroosmotic flow resulted in significantly less sample dispersion than pressure-driven flow at a similar velocity.     

Parameters influencing pulsed flow mixing in microchannels

The rapid mixing of reagents is a crucial step for on-chip chemical and biological analysis. It has been recently suggested that microfluidic mixing can be greatly enhanced by simply using time pulsing of the incoming flow rates of the two fluids to be mixed (Glasgow, I.; Aubry, N. Lab Chip 2003, 3, 114-120). This paper elaborates on the latter technique, showing through computational fluid dynamics how the mixing efficiency strongly depends on certain dimensionless parameters of the system, while remaining nearly insensitive to others. In particular, it is demonstrated that higher Strouhal numbers (ratio of flow characteristic time scale to the pulsing time period) and pulse volume ratios (ratio of the volume of fluid pulsed to the volume of inlet/outlet intersection) lead to better mixing. This paper also presents a physical device capable of mixing two reagents using pulsing, which shows improved mixing with greater values of the Strouhal number.     

Surface modification method of microchannels for gas-liquid two-phase flow in microchips

A capillarity restricted modification method for microchannel surfaces was developed for gas--liquid microchemical operations in microchips. In this method, a microstructure combining shallow and deep microchannels and the principle of capillarity were utilized for chemical modification of a restricted area of a microchannel. A hydrophobic--hydrophilic patterning in microchannels was prepared as an example for guiding gas and liquid flows along the respective microchannels. Validity of the patterning was confirmed by measuring aqueous flow leak pressure from the hydrophilic microchannel to the hydrophobic one. The leak pressure of 7.7-1.1 kPa agreed well with that predicted theoretically from the Young-Laplace equation for the microchannel depth of 8.6-39 microm. In an experiment to demonstrate usefulness and effectiveness of the method, an air bubble was first introduced into the hydrophilic microchannel and purged from the hydrophobic-hydrophilic patterned microchannels. Next, the patterning structure was applied to remove dissolved oxygen by contacting the aqueous flow with a nitrogen flow. The concentration of dissolved oxygen decreased with contact time, and its time course agreed well with numerical simulation. These demonstrations showed that the proposed patterning method can be used in general microfluidic gas-liquid operations.     

Features of water microdrop dispersion flow in microchannels

A remarkable property of inverse water-hydrocarbon emulsions, which is manifested during their flow in microchannels, has been discovered and called dynamic blocking. According to this phenomenon, the flow of emulsion through a microchannel ceases with the time despite the presence of a continuously applied pressure gradient. Experiments show that this effect can be observed for the flow of emulsions with different compositions and rheological properties. The effect is manifested at rather significant pressure gradients, although it is accompanied by a partial degradation of dispersions. A physical mechanism is proposed to explain the dynamic blocking of water-hydrocarbon emulsions in microchannels, which is based on the notion of an interaction between nanodimensional surface shells consisting of surfactant molecules that surround water microdrops.     

Two-fluid pressure-driven channel flow with wall deposition and ageing effects

Two-fluid, stratified pressure-driven channel flow is studied in the limit of small viscosity ratios. Cases are considered in which the core fluid undergoes phase separation that results in the ‘precipitation’ of a distinct phase and the formation of a wall layer; these situations are common in the oil industry where ‘fouling’ deposits are formed during the flow. The thickness of this layer increases as a result of continual deposition through Stefan-like fluxes, which are related to the phase behaviour of the core fluid through a chemical equilibria model that treats the fluid as a bi-component mixture. The deposit also undergoes an ‘ageing’ process whereby its viscosity increases due to the build-up of internal structure; the latter is modelled here via a Coussot-type relation. Lubrication theory is used in the wall layer and an integral balance in the core fluid wherein inertial effects are important. By choosing appropriate semi-parabolic velocity and temperatures closures for the laminar flow in the channel core, and a closure relation for the wall layer rheology, evolution equations are derived that describe the flow dynamics. In the presence of ageing but absence of deposition, it is demonstrated how the time-varying deposit rheology alters the wave dynamics; for certain parameter ranges, these effects can give rise to the formation of steep waves and what appears to be finite-time ‘blow-up’. With both ageing and deposition, the spatio-temporal evolution of the deposit is shown together with the increase in the deposition rate with increasing temperature difference between the wall and the inlet.     

Sample dispersion for segmented flow in microchannels with rectangular cross section

Hydrodynamic dispersion in microchannels can be significantly reduced by segmentation with a second immiscible phase. We address the effect of microchannel cross section on the dispersion of analytes in a segmented gas-liquid flow of alternating bubbles and liquid segments. Channels of square or nearly square cross section are considered. A significant fraction of the liquid surrounds the bubbles and wets the channel walls in the form of films or menisci. This stagnant fraction of the liquid remains when gas and liquid segments flow by, and it is connected to the liquid within the liquid segments by diffusion only and it effectively increases dispersion. We design and fabricate a microchip with integrated analyte injection and detection to investigate the effects of the influence of the stagnant liquid in segmented flow through square microchannels on the analyte bandwidth. The measured data and a corresponding model confirm the experimental trends and suggest operating conditions at which the unwanted effect of dispersion in segmented microchannel flow is minimized. Dispersion is least when the liquid flow rate is greater than the gas flow rate, and the optimum ratio of the two flow rates slightly increases with increasing bubble velocity.     

Dispersion reduction in open-channel liquid electrochromatographic columns via pressure-driven back flow

Application of electrokinetic forces to drive the mobile phase diminishes analyte dispersion in open-channel liquid chromatographic columns due to minimization of shear in the flow field. However, the retentive layer coating the inner walls of such devices slows down the average convective velocity of solute molecules in its vicinity, inherently causing dispersion of analyte bands. In this article, we explore the possibility of reducing such dispersion in electrochromatographic columns by imposing a pressure-driven back flow in the system. Analysis shows that although such a strategy introduces shear in the flow field, the overall dispersion in the mobile phase is reduced. This occurs as the streamline velocity in such a system is greater near the channel walls than that in the center of the conduit, thereby allowing fluid dispersion to counteract wall retention effects. For an optimally chosen magnitude of the back flow, hydrodynamic dispersion of any target species in the mobile phase may be shown to diminish by a factor of 3 and 10/3 in a circular tube and a parallel-plate geometry, respectively. A similar reduction in slug dispersion is also realized in rectangular conduits for all aspect ratios. In trapezoidal geometries with large wedge angles or isotropically etched profiles, this reduction factor may attain values of 10 or greater.     

Simulations of pressure-driven flows through channels and pipes with unified flow solver

The aim of this paper is to demonstrate the benefits of direct methods of solving kinetic equations and adaptive kinetic-fluid solvers for vacuum science and technology. We consider pressure-driven flows through short channels for a wide range of gas rarefaction degrees. Our Unified Flow Solver combines Adaptive Mesh Refinement (AMR) with automatic selection of kinetic and fluid solvers in different parts of computational domain. The discrete velocity method is used for direct numerical solution of Boltzmann and model kinetic equations. The advantages of adaptive hybrid method are demonstrated for compressible flows at large pressure gradients. For small pressure drops, direct solutions of the Boltzmann equation provide accurate solutions of rarefied flows not achievable by the traditional direct simulation Monte Carlo methods.     

Electroosmotic flow in composite microchannels and implications in microcapillary electrophoresis systems

The electroosmotic flow in laminated excimer laser-ablated microchannels has been studied as a function of the depth of the rectangular channels, and particular emphasis has been given to the difference in the zeta-potentials between the lamination layer and the ablated substrate. Experimental electroosmotic flow follows the tendency predicted by a recently published model. The zeta-potentials of lamination and ablated surfaces were determined for poly(ethylene terephthalate) and poly(carbonate) substrates by fitting the experimental data with a numerical implementation of this model. In the experimentally investigated range of channel cross sections, a linear fit to the data gives a good approximation of the zeta-potentials for both materials. Moreover, a flow injection analysis of fluorescein dye has been performed to show the severe loss in numbers of theoretical plates, caused by Taylor dispersion, when such microchannels, dedicated to microcapillary electrophoresis, are used.     

High-speed separations of DNA sequencing reactions by capillary electrophoresis

Fluorescently labeled DNA fragments generated in enzymatic sequencing reactions are rapidly separated by capillary gel electrophoresis and detected at attomole levels within the gel-filled capillary. The application of this technology to automated DNA sequence analysis may permit the development of a second generation automated sequencer capable of efficient and cost-effective sequence analysis on the genomic scale.