Showing papers on "Microchannel published in 2003"

Evolution of Microchannel Flow Passages-Thermohydraulic Performance and Fabrication Technology

Abstract: This paper provides a roadmap of development in the thermal and fabrication aspects of microchannels as applied in microelectronics and other high heat-flux cooling applications. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 200 micrometers. The impetus for microchannel research was provided by the pioneering work of Tuckerman and Pease [1] at Stanford University in the early eighties. Since that time, this technology has received considerable attention in microelectronics and other major application areas, such as fuel cell systems and advanced heat sink designs. After reviewing the advancement in heat transfer technology from a historical perspective, the advantages of using microchannels in high heat flux cooling applications is discussed, and research done on various aspects of microchannel heat exchanger performance is reviewed. Single-phase performance for liquids is still expected to be describable by conventional equations; however, the gas flow may...

Modeling gas flow through microchannels and nanopores

Abstract: Microchannel based systems have emerged as a critical design trend in development of precise control and maneuvering of small devices. In microelectronics, space propulsion and biomedical areas, these systems are especially useful. Nanoscale pores are recently becoming of great interest due to their beneficial drag and heat transfer properties. However it is difficult to predict the flow performance of these microsystems and nanosystems numerically since the standard assumptions of using Navier–Stokes equations break down at micrometer scales, while the computational times of applicable molecular-dynamics codes become exorbitant. A two-dimensional finite-element based microscale flow model is developed to efficiently predict the overall flow characteristics up to the transition regime for reasonably high Knudsen number flow inside microchannels and nanopores. Presented two-dimensional numerical results for Poiseuille flow of a simple fluid through the microchannel are comparable to the numerical and exper...

Second-order slip laws in microchannels for helium and nitrogen

Abstract: We perform gas flow experiments in a shallow microchannel, 1.14±0.02 μm deep, 200 μm wide, etched in glass and covered by an atomically flat silicon wafer. The dimensions of the channel are accurately measured by using profilometry, optical microscopy and interferometric optical microscopy. Flow-rate and pressure drop measurements are performed for helium and nitrogen, in a range of averaged Knudsen numbers extending up to 0.8 for helium and 0.6 for nitrogen. This represents an extension, by a factor of 3 or so, of previous studies. We emphasize the importance of the averaged Knudsen number which is identified as the basic control parameter of the problem. From the measurements, we estimate the accommodation factor for helium to be equal to 0.91±0.03 and that for nitrogen equal to 0.87±0.06. We provide estimates for second-order effects, and compare them with theoretical expectations. We estimate the upper limit of the slip flow regime, in terms of the averaged Knudsen number, to be 0.3±0.1, for the two gases.

Liquid flow in microchannels: experimental observations and computational analyses of microfluidics effects

Abstract: Experimental observations of liquid microchannel flows are reviewed and results of computer experiments concerning channel entrance, wall slip, non-Newtonian fluid, surface roughness, viscous dissipation and turbulence effects on the friction factor are discussed. The experimental findings are classified into three groups. Group I emphasizes 'flow instabilities' and group II points out 'viscosity changes' as the causes of deviations from the conventional flow theory for macrochannels. Group III caters to studies that did not detect any measurable differences between micro- and macroscale fluid flow behaviors. Based on numerical friction factor analyses, the entrance effect should be taken into account for any microfluidic system. It is a function of channel length, aspect ratio and the Reynolds number. Non-Newtonian fluid flow effects are expected to be important for polymeric liquids and particle suspension flows. The wall slip effect is negligible for liquid flows in microconduits. Significant surface roughness effects are a function of the Darcy number, the Reynolds number and cross-sectional configurations. For relatively low Reynolds numbers, Re < 2000, onset to turbulence has to be considered important because of possible geometric non-uniformities, e.g., a contraction and/or bend at the inlet to the microchannel. Channel-size effect on viscous dissipation turns out to be important for conduits with Dh < 100 µm.

Joule heating and heat transfer in poly(dimethylsiloxane) microfluidic systems

Abstract: Joule heating is a significant problem in electrokinetically driven microfluidic chips, particularly polymeric systems where low thermal conductivities amplify the difficulty in rejecting this internally generated heat. In this work, a combined experimental (using a microscale thermometry technique) and numerical (using a 3D "whole-chip" finite element model) approach is used to examine Joule heating and heat transfer at a microchannel intersection in poly(dimethylsiloxane)(PDMS), and hybrid PDMS/Glass microfluidic systems. In general the numerical predictions and the experimental results agree quite well (typically within +/- 3 degree C), both showing dramatic temperature gradients at the intersection. At high potential field strengths a nearly five fold increase in the maximum buffer temperature was observed in the PDMS/PDMS chips over the PDMS/Glass systems. The detailed numerical analysis revealed that the vast majority of steady state heat rejection is through lower substrate of the chip, which was significantly impeded in the former case by the lower thermal conductivity PDMS substrate. The observed higher buffer temperature also lead to a number of significant secondary effects including a near doubling of the volume flow rate. Simple guidelines are proposed for improving polymeric chip design and thereby extend the capabilities of these microfluidic systems.

Microchannel wall coatings for protein separations by capillary and chip electrophoresis.

Abstract: The necessity for microchannel wall coatings in capillary and chip-based electrophoretic analysis of biomolecules is well understood. The regulation or elimination of EOF and the prevention of analyte adsorption is essential for the rapid, efficient separation of proteins and DNA within microchannels. Microchannel wall coatings and other wall modifications are especially critical for protein separations, both in fused-silica capillaries, and in glass or polymeric microfluidic devices. In this review, we present a discussion of recent advances in microchannel wall coatings of three major classes – covalently linked polymeric coatings, physically adsorbed polymeric coatings, and small molecule additives. We also briefly review modifications useful for polymeric microfluidic devices. Within each category of wall coatings, we discuss those used to eliminate EOF, to tune EOF, to prevent analyte adsorption, or to perform multiple functions. The knowledgeable application of the most promising recent developments in this area will allow for the separation of complex protein mixtures and for the development of novel microchannel wall modifications.

suspended Microchannel Resonators for Biomolecular Detection

Abstract: We present a resonant mass sensor for specific biomolecular detection in a subnanoliter fluid volume. The sensing principle is based on measuring shifts in resonance frequency of a suspended microfluidic channel upon accumulation of molecules on the inside walls of the device. Confining the fluid to the inside of a hollow cantilever enables direct integration with conventional microfluidic systems, significantly increases sensitivity by eliminating high damping and viscous drag, and allows the resonator to be actuated by electrostatic forces. Fluid density measurements reveal a mass resolution of 10−17 g/μm2 in a 4 mHz–4 Hz bandwidth. To demonstrate biomolecular detection, we present real-time measurements of the specific binding between avidin and biotinylated bovine serum albumin. Based on these measurements, we expect that changes in surface mass loading on the order of 10−19 g/μm2 can be detected in an optimized system.

Control of the cross-sectional shape of a hollow microchannel embedded in photostructurable glass by use of a femtosecond laser

Abstract: Theoretical and experimental investigations have been made of the three-dimensional microchannel fabrication of photostructurable glass by use of a femtosecond (fs) laser. Generally, a microchannel fabricated inside glass by the scanning focal spot of a fs laser perpendicular to the direction of laser propagation assumes an elliptical shape with a cross section of large aspect ratio. We demonstrate that one can greatly reduce the aspect ratio merely by inserting a slit, which is oriented parallel to the laser’s scanning direction, before the focusing lens. Computer simulations show that a more symmetrical pattern is obtained in the vicinity of the focal point with the help of such a slit, owing essentially to a diffraction effect.

Evolution of microchannel flow passages – Thermohydraulic performance and fabrication technology

Abstract: This paper provides a roadmap of development in the thermal and fabrication aspects of microchannels as applied in the microelectronics and other high heat-flux cooling applications. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 200 micrometers. The impetus for microchannel research was provided by the pioneering work of Tuckerman and Pease [1] at Stanford University in the early eighties. Since that time, this technology has received considerable attention in microelectronics and other major application areas, such as fuel cell systems and advanced heat sink designs. After reviewing the advancement in heat transfer technology from a historical perspective, advantages of using microchannels in high heat flux cooling applications is discussed, and research done on various aspects of microchannel heat exchanger performance is reviewed. Singlephase performance for liquids is expected to be still describable by the conventional equations; however the gas flow may be influenced by the rarefaction effects. Two-phase flow is another topic that is still under active research. The evolution of research into microchannel heat sinks has paralleled the advancements made in microfabrication technology. The earliest microchannels were built using anisotropic wet chemical etching techniques based on alkali solutions. While this method has been exploited successfully, it does impose certain restrictions on silicon wafer type and geometry. Recently, anisotropic dry etching processes have been developed that circumvent these restrictions. In addition, dry etching methods can be significantly faster and, from a manufacturing standpoint, create fewer contamination and waste treatment problems. Advances in fabrication technology will continue to fuel improvements in microchannel heat sink performance and cost for the foreseeable future. Some fabrication areas that may spur advances include new materials, high aspect ratio patterning techniques other than dry etching, active fluid flow elements, and micromolding NOMENCLATURE

Efficient spatial-temporal chaotic mixing in microchannels

Abstract: A chaotic micro mixer with multiple side channels is designed and investigated, in which fluid can be stirred by pumps through the side channels. By stretching and folding fluid in the main and side channels, chaotic mixing can be achieved. A simple mathematic model is derived to understand the movement of particles in the microchannel. Spatial trajectories of fluid particles are projected to Poincare sections by mapping. The route from the quasi-period to chaos is revealed to be destruction of KAM curves and shrinkage of the quasi-periodic areas. Lyapunov exponents (LE) are used as the mixing index and the criteria to evaluate the chaotic behavior of the system. We found that LE is closely related to the amplitude and frequency of stirring and can be used to optimize our design and operation. From the relationship of LE and striation thickness, the minimal mixing length required can be estimated, which is much shorter than that needed in passive mixer design.

Electrokinetic microchannel battery by means of electrokinetic and microfluidic phenomena

Abstract: Pressure-driven flow in a microchannel induces a streaming current due to the presence of an electrical double layer in the interface between the electrolyte solution and channel wall. As the streaming current is of the order of a nano-amphere and is additive, we propose here a method to develop an electrokinetic battery consisting of an array of microchannels that converts the hydrostatic pressure of a liquid into electrical work. We have given oscillating analytical solutions by means of an electrical circuit analysis to model the multi-microchannel battery. Using superposition of the appropriate Fourier series, the derived analytical solutions are useful to predict the current when there is more general time-dependent flow through a microchannel array. To illustrate the idea, we have studied steady-state pressure-driven flow in micropore porous glass filter and compared the results with those predicted from our model. From a 30 cm hydrostatic pressure drop, an external current of 1–2 µA was obtained by means of water passing through the micropore porous glass filter. A larger current can be obtained by simply using a solution with higher salt concentration. This results in a new and potentially useful method of energy conversion by means of an array of microchannels.

Reduced pumping power and wall temperature in microchannel heat sinks with fractal-like branching channel networks

Abstract: Comparisons are made of the maximum channel wall temperature along, and total pressure drop across, a heat sink with a fractal-like branching channel network with those in a heat sink having a straight channel array. The total channel lengths are identical between the heat sinks, as are the applied heat fluxes. The hydraulic diameter of the straight channel array is equal to that of the terminal branch of the branching channel network. The number of branches per level, number of branching levels, and channel dimensions in the fractal-like network remain fixed. Minor losses are neglected and both hydrodynamic and thermal boundary layers are assumed to reinitiate following each channel bifurcation in the branching flow network. With identical total convective surface areas for both configurations and maintaining a heat sink surface area equal to that of the convective surface area, the fractal-like channel network yielded a 60% lower pressure drop for the same total flow rate and a 30°C lower wall temperatu...

DNA dynamics in a microchannel.

Abstract: An extended Brownian dynamics simulation method is used to characterize the dynamics of long DNA molecules flowing in microchannels. The relaxation time increases due to confinement in agreement with scaling predictions. During flow the molecules migrate toward the channel center line, and thereby segregate according to molecular weight. Capturing these effects requires the detailed incorporation of solvent flow in the simulation method, demonstrating the importance of hydrodynamic effects in the dynamics of confined macromolecules.

Evaluation of a Three-Dimensional Micromixer in a Surface-Based Biosensor†

Abstract: The performance of many biosensors is limited by the rate at which a soluble analyte solution is transported to a complementary receptor immobilized on the sensor transducer. In these cases, transport limitations due to the lack of sample mixing in the sensor can impede analyte transport to the immobilized receptors and thereby reduce the sensor performance. In this study, we apply methods that have been successfully used to mix bulk fluid streams in a microfluidic device, to stir the analyte solution as it passes over receptors bound to the surface of an optical biosensor. A three-dimensional serpentine microchannel, which has been shown to provide chaotic mixing in the sample flow, was interfaced with a surface plasmon resonance biosensor. The binding kinetics of soluble rabbit IgG to protein A, immobilized on one of the microchannel walls, was measured both in the serpentine device and in a straight channel. A comparison of the initial rates of analyte detection in these two devices shows that the mixi...

Design and simulation of the micromixer with chaotic advection in twisted microchannels.

Abstract: Chaotic mixers with twisted microchannels were designed and simulated numerically in the present study. The phenomenon whereby a simple Eulerian velocity field may generate a chaotic response in the distribution of a Lagrangian marker is termed chaotic advection. Dynamic system theory indicates that chaotic particle motion can occur when a velocity field is either two-dimensional and time-dependent, or three-dimensional. In the present study, micromixers with three-dimensional structures of the twisted microchannel were designed in order to induce chaotic mixing. In addition to the basic T-mixer, three types of micromixers with inclined, oblique and wavelike microchannels were investigated. In the design of each twisted microchannel, the angle of the channels' bottoms alternates in each subsection. When the fluids enter the twisted microchannels, the flow sways around the varying structures within the microchannels. The designs of the twisted microchannels provide a third degree of freedom to the flow field in the microchannel. Therefore, chaotic regimes that lead to chaotic mixing may arise. The numerical results indicate that mixing occurs in the main channel and progressively larger mixing lengths are required as the Peclet number increased. The swaying of the flow in the twisted microchannel causes chaotic advection. Among the four micromixer designs, the micromixer with the inclined channel most improved mixing. Furthermore, using the inclined mixer with six subsections yielded optimum performance, decreasing the mixing length by up to 31% from that of the basic T-mixer.

Interwoven manifolds for pressure drop reduction in microchannel heat exchangers

Abstract: A microchannel heat exchanger coupled to a heat source and configured for cooling the heat source comprising a first set of fingers for providing fluid at a first temperature to a heat exchange region, wherein fluid in the heat exchange region flows toward a second set of fingers and exits the heat exchanger at a second temperature, wherein each finger is spaced apart from an adjacent finger by an appropriate dimension to minimize pressure drop in the heat exchanger and arranged in parallel. The microchannel heat exchanger includes an interface layer having the heat exchange region. Preferably, a manifold layer includes the first set of fingers and the second set of fingers configured within to cool hot spots in the heat source. Alternatively, the interface layer includes the first set and second set of fingers configured along the heat exchange region.

Surface-chemistry technology for microfluidics

Abstract: A new technology to pattern surface charges, either negatively or positively, using a standard photolithography process is introduced. A positively charged poly(allylamine hydrochloride) (PAH) layer is coated onto a negatively charged silicon oxide surface by electrostatic self-assembly (ESA). Combined with photolithography in a lift-off-based process, several different surface charge patterns were successfully produced. Due to definition of the pattern by photolithography, no limitations in the pattern geometry exist. Any surface charge pattern can be created to enable fine control of fluid motion in microfluidic devices. Physical properties of this PAH layer were characterized. The generation of a bi-directional shear flow was demonstrated by using alternating longitudinal surface charge pattern with a single driving force, i.e. an externally applied electric field inside a microchannel.

Chemicofunctional Membrane for Integrated Chemical Processes on a Microchip

Abstract: Here we report a design and synthesis of a chemically functional polymer membrane by an interfacial polycondensation reaction and multilayer flow inside a microchannel. Single and parallel dual-membrane structures are successfully prepared by using organic/aqueous two-layer flow and organic/aqueous/organic three-layer flow inside the microchannel followed by an interfacial polycondensation reaction. By using the inner-channel membrane, permeation of ammonia species through the inner-channel membrane is successfully achieved. Furthermore, horseradish peroxidase is immobilized on one side of the membrane surface to integrate the chemical transform function onto the inner-channel membrane. Here substrate permeation through the membrane and subsequent chemical transformation at the membrane surface are realized. The polymer membrane prepared inside the microchannel has an important role in ensuring stable contact of different phases such as gas/liquid or liquid/liquid and the permeation of chemical species th...

Microfluidic system including a bubble valve for regulating fluid flow through a microchannel

Abstract: A microfluidic system includes a bubble valve for regulating fluid flow through a microchannel. The bubble valve includes a fluid meniscus interfacing the microchannel interior and an actuator for deflecting the membrane into the microchannel interior to regulate fluid flow. The actuator generates a gas bubble in a liquid in the microchannel when a sufficient pressure is generated on the membrane.

Analysis of Alternating Current Electroosmotic Flows in a Rectangular Microchannel

Abstract: Electroosmotic flow is widely used as a primary method of species transport in microfluidic devices. The recent introduction of several alternating current (ac) based microfluidic applications has led to enhanced interest in time periodic electroosmotic flows. In this work, an analytical solution, via a Green's function formulation, is developed for ac electroosmotic flow through a rectangular microchannel for the case of a sinusoidal applied electric field. The response of the flow field to excitation by more complex waveforms is also investigated using numerical simulations. It is shown that the steady time periodic (after the effects of the initial impulse are dissipated) velocity profile is characterized by the ratio of the period of oscillation to the time scale for viscous diffusion, by the surface ζ-potential distribution, and by the channel aspect ratio. Impulsively started flows are also shown to exhibit interesting transient behavior resulting in a net positive velocity at the channel midpoint d...

Catalysts, in microchannel apparatus, and reactions using same

Abstract: The present invention provides new microreactor systems, catalysts, and chemical processes. Methods of making novel catalysts and reaction apparatus are also described.

Multi-Stream Microchannel Device

Abstract: The invention is a process and device for exchanging heat energy between three or more streams in a microchannel heat exchanger which can be integrated with a microchannel reactor to form an integrated microchannel processing unit. The invention enables the combining of a plurality of integrated microchannel devices to provide the benefits of large-scale operation. In particular, the microchannel heat exchanger of the present invention enables flexible heat transfer between multiple streams and total heat transfer rates of about 1 Watt or more per core unit volume expressed as W/cc.

Enzymatic degradation of p-chlorophenol in a two-phase flow microchannel system

Abstract: Enzymatic degradation of p-chlorophenol was carried out in a two-phase flow in a microchannel (100 microm width, 25 microm depth) fabricated on a glass plate (70 mm x 38 mm). This is the first report on the enzymatic reaction in a two-phase flow on a microfluidic device. The surface of the microchannel was partially modified with octadecylsilane groups to be hydrophobic, thus allowing clear phase separation at the end-junction of the microchannel. The enzyme (laccase), which is surface active, was solubilized in a succinic aqueous buffer and the substrate (p-chlorophenol) was in isooctane. The degradation of p-chlorophenol occurred mainly at the aqueous-organic interface in the microchannel. We investigated the effects of flow velocity and microchannel shape on the enzymatic degradation of p-chlorophenol. Assuming that diffusion of the substrate (p-chlorophenol) is the rate-limiting step in the enzymatic degradation of p-chlorophenol in the microchannel, we proposed a simple theoretical model for the degradation in the microchannel. The calculated degradation values agreed well with the experimental data.

Chemical and physical processes for integrated temperature control in microfluidic devices

Abstract: Microfluidic devices are a promising new tool for studying and optimizing (bio)chemical reactions and analyses. Many (bio)chemical reactions require accurate temperature control, such as for example thermocycling for PCR. Here, a new integrated temperature control system for microfluidic devices is presented, using chemical and physical processes to locally regulate temperature. In demonstration experiments, the evaporation of acetone was used as an endothermic process to cool a microchannel. Additionally, heating of a microchannel was achieved by dissolution of concentrated sulfuric acid in water as an exothermic process. Localization of the contact area of two flows in a microfluidic channel allows control of the position and the magnitude of the thermal effect.

Visualization of convective mixing in microchannel by fluorescence imaging

Abstract: A novel measurement technique using fluorescent dye in combination with microresolution particle image velocimetry (micro-PIV) has been devised to investigate convective mixing in microspace. Tris(bipyridine)ruthenium(II), whose fluorescent intensity when excited by ultraviolet light is strongly temperature dependent, was applied to the bottom surface of a cover glass, that served as the upper boundary surface of a flow channel. This set-up thus realized a two-dimensional temperature measurement of the microflow channel. A spatial resolution of 5 µm × 5 µm and a temperature resolution of 0.26 K were achieved by using a cooled CCD camera and a 10× objective lens of a microscope. Pure water at differing temperatures was injected into opposite inlets of a T-shaped microchannel bound by cover glass and PDMS. The mixing process in the junction area was visualized by the present temperature and the micro-PIV techniques. The convective heat flux was calculated from measurement of velocity and temperature and compared to the heat conduction. It is found that the heat flux due to conduction was larger than that due to convection, thus it is noted that heat conduction may be an important factor in the design process of microfluidic devices.

Microchannel apparatus, methods of making microchannel apparatus, and processes of conducting unit operations

Abstract: Novel methods of making laminated, microchannel devices are described. Examples include: assembly from thin strips rather than sheets; and hot isostatic pressing (HIPing) to form devices with a hermetically sealed wall. Laminated microchannel articles having novel features are also described. The invention includes processes conducted using any of the articles described.