Generation of concentration gradient by controlled flow distribution and diffusive mixing in a microfluidic chip

We have developed a simple method to generate a concentration gradient in a microfluidic device. This method is based on the combination of controlled fluid distribution at each intersection of a microfluidic network by liquid pressure and subsequent diffusion between laminas in the downstream microchannel. A fluid dynamic model taking into account the diffusion coefficient was established to simulate the on-chip flow distribution and diffusion. Concentration gradients along a distance of a few hundred micrometers were generated in a series of microchannels. The gradients could be varied by carefully regulating the liquid pressure applied to the sample injection vials. The observed concentration gradients of fluorescent dyes generated on the microfluidic channel are consistent with the theoretically predicted results. The microfluidic design described in this study may provide a new tool for applications based on concentration gradients, including many biological and chemical analyses such as cellular reaction monitoring and drug screening.     

Numerical modeling of Joule heating-induced temperature gradient focusing in microfluidic channels

Temperature gradient focusing (TGF) is a recently developed technique for spatially focusing and separating ionic analytes in microchannels. The temperature gradient required for TGF can be generated either by an imposed temperature gradient or by Joule heating resulting from an applied electric field that also drives the flow. In this study, a comprehensive numerical model describing the Joule heating induced temperature development and TGF is developed. The model consists of a set of governing equations including the Poisson-Boltzmann equation, the Laplace equation, the Navier-Stokes equations, the energy equations and the mass transport equation. As the thermophysical and electrical properties including the liquid dielectric constant, viscosity, and electric conductivity are temperature-dependent, these governing equations are coupled, and therefore the coupled governing equations are solved numerically by using a CFD-based numerical method. The numerical simulations agree well with the experimental results, suggesting the valid mathematical model presented in this study.     

Effect of liquid slip in electrokinetic parallel-plate microchannel flow

Liquid slip at hydrophobic surfaces in microchannels has frequently been observed. We present here an analytical solution for oscillating flow in parallel-plate microchannels by combining the electrokinetic transport phenomena with Navier's slip condition. Our parametric results suggest that electrokinetic transport phenomena and liquid slip at channel walls are both important and should be considered simultaneously. Their significance depends on channel wall material, electrolyte concentration, and pH. For pressure-driven-flow, liquid slip counteracts the effect by the electrical double layer and induces a larger flow rate. A higher apparent viscosity would be predicted if slip is neglected. For electroosmotic flow, liquid slip alters the flow rate by about 20% for a thick electrical double layer. Our results provide design guidelines to precisely control time-dependent microflow in hydrophobic microfluidic microelectromechanical system devices.     

An analysis of induced pressure fields in electroosmotic flows through microchannels

Induced pressure gradients are found to cause band-broadening effects which are important to the performance of microfluidic devices, such as capillary electrophoresis and capillary chromatography. An improved understanding of the underlying mechanisms causing an induced pressure gradient in electroosmotic flows is presented. The analysis shows that the induced pressure distribution is the key to understanding the experimentally observed phenomena of leakage flows. A novel way of determining the static pressures at the inlet and outlet of microchannels is also presented that takes account of the pressure losses due to flow contraction and expansion. These commonly neglected pressure losses at the channel entrance and outlet are shown to be important in accurately describing the flow. The important parameters that define the effect of induced pressure on the flows are discussed, which may facilitate the design of improved microfluidic devices. The present model clearly identifies the mechanism behind the experimentally observed leakage flows, which is further confirmed by numerical simulations. Not only can the leakage flow occur from the electric-field-free side channel to the main channel, but also the fluid in the main channel can be attracted into the side channel by the induced pressure gradient.     

On the surface conductance, flow rate, and current continuities of microfluidics with nonuniform surface potentials

The characteristics of electrokinetic flow in a microchannel depend on both the nature of surface potentials, that is, whether it is uniform or nonuniform, and the electrical potential distribution along the channel. In this paper, the nonlinear Poisson-Boltzmann equation is used to model the electrical double layer and the lattice Boltzmann model coupled with the constraint of current continuity is used to simulate the microfluidic flow field in a rectangular microchannel with a step variation of surface potentials. This current continuity, including surface conduction, convection, and bulk conduction currents, has often been neglected in the literature for electroosmotic flow with nonuniform (heterogeneous) microchannels. Results show that step variation of ion distribution caused by step variation surface potential will influence significantly the electrical potential distribution along the channel and volumetric flow rate. For the system considered, we showed that the volumetric flow rate could have been overestimated by as much as 70% without consideration of the current continuity constraint.     

添加剂对LiBr溶液吸收蒸汽过程中的强化机理

The surface tensions of aqueous lithium bromide（LiBr）with additive（2-ethyl-1-hexanol and 1-Octanol）have been measured by using a Wihelmy plate method,and the enhancement effect of the additives on the absorption of steam into aqueous LiBr in a static pool has been studied by a real-time type laser holographic visualization method. The experimental results show that both of liquid additive and vapor additive can decrease the surface tensions of aqueous LiBr significantly,vapor additive not only can trigger the Marangoni convection at the absorption interface just like the liquid additive,but can bring about better enhancement effect on the absorption performance than that liquid additive can. The enhancement mechanism of additive on absorption has been concluded that both liquid additive and vapor additive can be adsorbed by aqueous LiBr at the liquid-vapor-interface from the liquid side and the vapor side respectively,which result in surface tension gradient,and then cause Marangoni convection at the interface which enhances the heat and mass transfer performance during the absorption process.     

Hydrodynamics and mass exchange in gas-liquid slug flow in microchannels

The present state of hydrodynamics and mass transfer studies in segmented gas-liquid flow in microchannels has been analyzed. It has been shown that such parameters as gas bubble velocity, gas hold-up, relative gas bubble length, pressure drop, mass transfer coefficients from gas bubbles to liquid slugs and to liquid film, as well as mass transfer coefficient from liquid to channel wall can be satisfactorily predicted. Nevertheless, some correlations were obtained under definite conditions and should be summarized. The purpose of further research is to develop reliable methods for calculation of mass transfer coefficients as functions of channel geometry, phase properties, and phase velocities in mini- and microchannels.     

High-throughput separation of DNA and proteins by three-dimensional geometry gel electrophoresis: feasibility studies

We report a novel separation method that is applicable to both DNA and protein samples, based on electrophoresis in a three-dimensional (3-D) geometry. In contrast to conventional electrophoresis, samples are applied in a two-dimensional, planar array to one of the surfaces of a 3-D geometry separation medium. Loading onto a plane results in a very high sample capacity. Sample migration and separation occur along the third spatial dimension, which is perpendicular to the loading plane. The key problem of electrophoresis in a 3-D geometry separation setup is that temperature gradients are caused by Joule's heat, affecting the electrical conductivity and viscosity of the separation medium. A means of achieving straight sample migration under these circumstances is to force heat flow through the separation medium parallel to the axis of sample migration. This can be done by dissipating the heat via the electrode sides of the gel and blocking any other heat transfer. The separation of DNA and proteins by this method has been tested using agarose gel electrophoresis, polyacrylamide gel electrophoresis, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Data were acquired off-line by conventional staining methods as well as on-line by detection of laser-induced fluorescence. We describe how to excise samples from the separation medium for preparative purposes. Possible unique applications of this 3-D geometry electrophoresis separation method are also discussed.     

Peak compression and resolution for electrophoretic separations in diverging microchannels

We report the results of experiments and simulations on electrokinetic flow in diverging microchannels (with cross-sectional area that increases with distance along the channel). Because of conservation of mass and charge, the velocity of an analyte in the channel decreases as the channel cross-section increases. Consequently, the leading edge of a band of sample moves more slowly than the trailing edge and the sample band is compressed. Sample peak widths, rather than increasing diffusively with time, can then be controlled by the geometry of the channel and can even be made to decrease with time. We consider the possibility of using this peak compression effect to improve the resolution of electrophoretic separations. Our results indicate that for typical separations that are dispersion limited, this peak compression effect is more than offset by the decreased distance between peaks, and the separation resolution in diverging channels is worse than that found for straight channels at the same applied voltage. For separations in very short channels or at very high field strengths, however, when the separation efficiency is injection limited, the peak compression effect is dominant and diverging channels can then be used to achieve improved separation resolution.     

Diffusion of liquid hydrogen in time-dependent MHD mixed convective flow

An innovative study on liquid hydrogen diffusion in time-dependent mixed convection flow is carried out in the presence of magnetic field effects. In fact, this is the first approach to analyze such flow problems, and also this is the first research paper to study the non-uniform heat sink/source and nonlinear chemical reaction in the presence of liquid hydrogen diffusion. Initially, the governing equations are reduced to dimensionless form by using non-similar transformations and are linearized by applying quasilinearization technique. Then, the finite difference approximation is utilized to discretize the resulting equations. The mixed convection is analyzed along with exponentially stretching surface through various graphs on profiles as well as gradients. The results display that the non-uniform heat source parameter increases the fluid velocity as well as temperature, and the magnetic parameter reduces the friction at the wall. Specifically, the skin friction coefficient decreases about 40% in the presence of magnetic field. The mass transfer rate increases for high-order chemical reaction and for destructive chemical reaction rate. The mass transfer rate is found to be high for the diffusion of liquid nitrogen than that for the diffusion of liquid hydrogen. In fact, the mass transfer rate increases about 22% for the diffusion of liquid nitrogen. This study can assist the design engineers who are working in pertain to the diffusion of liquid gases in mixed convection regimes. Also, the obtained data in the present study can be more useful for future investigations about time-dependent mixed convection nanofluid flow problems.

     

Water under temperature gradients: polarization effects and microscopic mechanisms of heat transfer

We report non-equilibrium molecular dynamics simulations (NEMD) of water under temperature gradients using a modified version of the central force model (MCFM). This model is very accurate in predicting the equation of state of water for a wide range of pressures and temperatures. We investigate the polarization response of water to thermal gradients, an effect that has been recently predicted using Non-Equilibrium Thermodynamics (NET) theory and computer simulations, as a function of the thermal gradient strength. We find that the polarization of the liquid varies linearly with the gradient strength, which indicates that the ratio of phenomenological coefficients regulating the coupling between the polarization response and the heat flux is independent of the gradient strength investigated. This notion supports the NET theoretical predictions. The coupling effect leading to the liquid polarization is fairly strong, leading to polarization fields of ~10(3-6) V m(-1) for gradients of ~10(5-8) K m(-1), hence confirming earlier estimates. Finally we employ our NEMD approach to investigate the microscopic mechanism of heat transfer in water. The image emerging from the computation and analysis of the internal energy fluxes is that the transfer of energy is dominated by intermolecular interactions. For the MCFM model, we find that the contribution from hydrogen and oxygen is different, with the hydrogen contribution being larger than that of oxygen.     

Measurement of the axial and radial temperature profiles of a chromatographic column. Influence of thermal insulation on column efficiency

The temperatures of the metal wall along a chromatographic column (longitudinal temperature gradients) and of the liquid phase across the outlet section of the column (radial temperature gradients) were measured at different flow rates with the same chromatographic column (250 mm x 4.6 mm). The column was packed with 5 microm C18-bonded silica particles. The measurements were carried out with surface and immersion thermocouples (all junction Type T, +/-0.1 K) that measure the local temperature. The column was either left in a still-air bath (ambient temperature, T(ext) = 295-296 K) or insulated in a packing foam to avoid air convection around its surface. The temperature profiles were measured at several values of the inlet pressure (approximately = 100, 200, 300 and 350 bar) and with two mobile phases, pure methanol and a 2.5:97.5 (v/v, %) methanol:water solution. The experimental results show that the longitudinal temperature gradients never exceeded 8 K for a pressure drop of 350 bars. In the presence of the insulating foam, the longitudinal temperature gradients become quasi-linear and the column temperature increases by +1 and +3 K with a water-rich (heat conductivity approximately = 0.6 W/m/K) and pure methanol (heat conductivity approximately = 0.2 W/m/K), respectively. The radial temperature gradients are maximum with methanol (+1.5 K at 290 bar inlet pressure) and minimum with water (+0.8 K at 290 bar), as predicted by the solution of the heat transfer balance in a chromatographic column. The profile remains parabolic all along the column. Combining the results of these measurements (determination of the boundary conditions on the wall, at column inlet and at column outlet) with calculations using a realistic model of heat dispersion in a porous medium, the temperature inside the column could be assessed for any radial and axial position.     

Large increase in the heat transfer through monolayers detected by beam deflection

We report the application of photothermal beam deflection for studying heat transfer across the liquid/ gas interface covered with surfactant monolayers. Laser-induced Marangoni convection was generated on the surface of an absorbing aqueous solution with and without a monolayer. The change in heat transfer to the air was monitored by the deflection of a probe beam propagating close to the solution surface. A large increase in the deflection as well as in the air temperature for monolayer-covered surfaces was observed. A film-balance study showed that the magnitude of deflection increases as a function of monolayer compression.     

Pulsating flow in a channel filled with a porous medium under local thermal non-equilibrium condition: an exact solution

Journal of Thermal Analysis and Calorimetry - The present work investigates analytically the problem of forced convection heat transfer of a pulsating flow, in a channel filled with a porous medium...     

Dielectrophoresis switching with vertical sidewall electrodes for microfluidic flow cytometry

A novel dielectrophoresis switching with vertical electrodes in the sidewall of microchannels for multiplexed switching of objects has been designed, fabricated and tested. With appropriate electrode design, lateral DEP force can be generated so that one can dynamically position particulates along the width of the channel. A set of interdigitated electrodes in the sidewall of the microchannels is used for the generation of non-uniform electrical fields to generate negative DEP forces that repel beads/cells from the sidewalls. A countering DEP force is generated from another set of electrodes patterned on the opposing sidewall. These lateral negative DEP forces can be adjusted by the voltage and frequency applied. By manipulating the coupled DEP forces, the particles flowing through the microchannel can be positioned at different equilibrium points along the width direction and continue to flow into different outlet channels. Experimental results for switching biological cells and polystyrene microbeads to multiple outlets (up to 5) have been achieved. This novel particle switching technique can be integrated with other particle detection components to enable microfluidic flow cytometry systems.     

Microfabrication of a tapered channel for isoelectric focusing with thermally generated pH gradient

A simple microfabrication technique for the preparation of a tapered microchannel for thermally generated pH gradient isoelectric focusing (IEF) has been demonstrated. The tapered channel was cut into a plastic sheet (thickness was 120 microm), and the channel was closed by sandwiching the plastic sheet between two glass microscope slides. The length of the microchannel was 5 cm. The width of the separation channel was 0.4 mm at the narrow end and 4 mm at the wide end. The channel was coated with polyacrylamide to prevent electroosmotic flow (EOF) during focusing. Two electrolyte vials were mounted on top of each end of the channel with the wide end of the channel connected to the cathodic vial and the narrow to the anodic vial. The feasibility of the thermally generated pH gradient in a tapered channel was demonstrated. Important parameters that determined the feasibility of using a thermally generated pH gradient in a tapered channel were analyzed. Parameters to be optimized were control of EOF and hydrodynamic flow, selection of power supply mode and prevention of local overheating and air bubble formation. Tris-HCl buffer, which has a high pK(a) dependence with temperature, was used both to dissolve proteins and as the electrolyte. The thermally generated pH gradient separation of proteins was tested by focusing dog, cat and human hemoglobins with a whole column detection capillary IEF (CIEF) system.     

On chip steady liquid-gas phase separation for flexible generation of dissolved gas concentration gradient

In this study, steady liquid-gas phase separation is realized by applying a hydrophobic small microchannel array (SMA) to bridge two large microchannels, one for liquid phase and one for gas phase. In this structure, a capillary pressure difference between that in the SMA and the larger channel results in a steady liquid-gas interface. The generated liquid-gas interface allows for fast gas dissolving speed. By coupling the liquid-gas interface with a one directional fluidic field, a steady dissolved gas concentration gradient (DgCG) is generated. The DgCG distribution is easily designable for linear or exponential modes, providing improved flexibility for gas participated processes on chip. To demonstrate its applicability, a CO(2) DgCG chip is fabricated and applied for screening CaCO(3) crystal growth conditions in the DgCG chip. Crystals with transitional structures are successfully fabricated, which is consistent with the CO(2) DgCG distribution.     

Retention time and peak width in the combined pH/organic modifier gradient high performance liquid chromatography

The purpose of this work was to test the applicability of the current theory to predict the peak retention time and the peak width in the combined pH/organic modifier gradient reversed phase high performance liquid chromatography (RP HPLC). A series of 38 isocratic measurements have been conducted for a wide range of pH and methanol contents for ketoprofen (weak acid) and papaverine (weak base). It served to find the model describing dependence of retention factor and the height equivalent of a theoretical plate (HETP) on pH and organic modifier content. The information gathered in the isocratic mode was used to simulate retention times and peak widths for 30 various methanol gradients, 25 pH gradients, and 3 combined pH/methanol gradients. The simulations were compared with the experimental data. We also proposed a simplified version of this model that was parameterized based on 12 initial organic modifier gradients carried out for different pHs and for the 20 min and 60 min gradient development times. The full and the simplified model described the experimental data very well. In conclusion, the proposed modeling approach allowed predicting analyte retention times and peak width for various pH and organic modifier changes. Its simplified version required only 12 initial experiments and seems to be very promising in the optimization RP HPLC separations for complex samples and for conditions providing peak compression.     

Monolithic stationary phases with a longitudinal gradient of porosity

The duration of the hypercrosslinking reaction has been used to control the extent of small pores formation in polymer‐based monolithic stationary phases. Segments of five columns hypercrosslinked for 30–360 min were coupled via zero‐volume unions to prepare columns with segmented porosity gradients. The steepness of the porosity gradient affected column efficiency, mass transfer resistance, and separation of both small‐molecule alkylbenzenes and high‐molar‐mass polystyrene standards. In addition, the segmented column with the steepest porosity gradient was prepared as a single column with a continuous porosity gradient. The steepness of porosity gradient in this type column was tuned. Compared to a completely hypercrosslinked column, the column with the shallower gradient produced comparable size‐exclusion separation of polystyrene standards but allowed higher column permeability. The completely hypercrosslinked column and the column with porosity gradient were successfully coupled in online two‐dimensional liquid chromatography of polymers.