Planar Microchannel Research Articles

Highly dilute gas flows through a non-isothermal planar microchannel

This paper reports theoretical and numerical investigations on free molecular gas flows through microchannels. Both diffusely and specularly reflective channel surfaces are considered. Gas kinetic methods are adopted to develop the analytical solutions for surface and flowfield properties. The crucial steps include constructing the velocity distribution functions (VDFs) for points at the plate surfaces and inside flowfield and then completing the integration over the related velocity phases. For diffusely reflective surfaces, the VDFs are related to the densities and temperatures at the two exits and the plate, respectively. For surfaces with specular reflections, the VDFs at the plate surface and inside the flowfield are identical and are independent of the surface temperature ratio and the geometric aspect ratio. Based on the VDFs and velocity phases, surface property coefficients (e.g., Cp, Cf, and Cq) and flowfield properties (e.g., density, velocity components, and temperature) are obtained. For the diffusely reflective surface scenario, the mass flow rate can be approximated and the results include four non-dimensional parameters: the aspect ratio, the density ratio, and two temperature ratios. For the specularly reflective surface scenario, the surface and flowfield properties are uniform everywhere, and the channel aspect ratio and plate temperatures do not have any influence. Particle simulations with the direct simulation Monte Carlo method are performed, and essentially identical results validate the theoretical work. This work is heuristic and can be used to investigate less rarefied microchannel gaseous flows, for example, aid experimental measurement design for thermal transpiration flows.

Effects of Polymer Entanglement in Sodium Hyaluronate Solution on Elastic Instability in Abrupt Contraction-Expansion Micro Channels

In this study, the effects of polymer flexibility and entanglement on elastic instability were investigated by observing sodium hyaluronate (Hyaluronic Acid Sodium salt, Na-HA) solution in planar abrupt contraction-expansion microchannels. The rigidity of Na-HA in a solution is affected by ion concentration in the solution. Therefore, we prepared Na-HA water solution and Na-HA PBS solution with concentrations from 0.15 wt% to 0.45 wt%. The rheological properties were measured and analyzed to detect the Na-HA overlap and entanglement concentrations. The flow regimes of the Na-HA solutions in several planar abrupt contraction-expansion channels were characterized in terms of rheological properties, polymer flexibility and polymer entanglement.

Modelling double emulsion formation in planar flow-focusing microchannels

Double emulsion formation in a hierarchical flow-focusing channel is systematically investigated using a free energy ternary lattice Boltzmann model. A three dimensional formation regime diagram is constructed based on the capillary numbers of the inner ($Ca_i$), middle ($Ca_m$) and outer ($Ca_o$) phase fluids. The results show that the formation diagram can be classified into periodic two-step region, periodic one-step region, and non-periodic region. By varying $Ca_i$ and $Ca_m$ in the two-step formation region, different morphologies are obtained, including the regular double emulsions, decussate regimes with one or two alternate empty droplets, and structures with multiple inner droplets contained in the continuous middle phase thread. Bidisperse behaviors are also frequently encountered in the two-step formation region. In the periodic one-step formation region, scaling laws are proposed for the double emulsion size and for the size ratio between the inner droplet and the overall double emulsion. Furthermore, we show that the interfacial tension ratio can greatly change the morphologies of the obtained emulsion droplets, and the channel geometry plays an important role in changing the formation regimes and the double emulsion sizes. In particular, narrowing the side inlets or the distance between the two side inlets promotes the conversion from the two-step formation regime to the one-step formation regime.

Effects of flexibility and entanglement of sodium hyaluronate in solutions on the entry flow in micro abrupt contraction-expansion channels

In this study, the effects of polymer flexibility and entanglement on elastic instability were investigated by observing sodium hyaluronate (hyaluronic acid sodium salt, Na-HA) solution in planar abrupt contraction-expansion microchannels. As the rigidity of Na-HA depends on the ionic strength of a solvent, Na-HA was dissolved in water and phosphate buffered saline with concentrations from 0.15 wt. % to 0.45 wt. %. The rheological properties were measured and analyzed to detect the Na-HA overlap and entanglement concentrations. The flow regimes of the Na-HA solutions in several planar abrupt contraction-expansion channels were characterized in the Reynolds number and Weissenberg number space. The effects of the solvent, solution concentration, and channel geometry on the elastic corner vortex growth curve and flow regimes characterized by the Weissenberg number were analyzed. It was found that the entanglement of Na-HA in the solution is a more dominant factor affecting the flow regimes than the solution relaxation time and polymer rigidity.

Performance and implementation of centrifugal serpentine micromixers with non-rectangular cross-section

The fluid flow and mixing characteristics of planar serpentine microchannels consisting of repeating S-shaped units was characterized both computationally and experimentally for Reynolds numbers Re < 100. In particular, it was found that using non-rectangular cross-sections with changing orientation for the geometrical units greatly enhances their mixing performance relative to standard rectangular profile serpentine channels. The optimization of the cross-section enables efficient mixing of two-fluid components at Reynolds numbers as low as 20, making these designs practical for chemical reaction engineering and biological assays platforms. A computational analysis of the cross-sectional flow structures present in this type of channels indicates that their enhanced performance is correlated with the formation of secondary transversal vortexes. In the optimal designs, their presence increases the mass transfer in the microchannel by both stretching the interface between the fluid components to be mixed, as well as by promoting chaotic advection.

Brinkman‐Forchheimer slip flow subject to exponential space and thermal‐dependent heat source in a microchannel utilizing SWCNT and MWCNT nanoliquids

AbstractThis communication examines the impact of carbon nanotubes (single‐wall carbon nanotubes [SWCNT] and multi‐wall carbon nanotubes [MWCNT]) on magnetohydrodynamic Brinkman and Forchheimer flow in a planar microchannel with multiple slips. Flow through a porous medium is modeled via Brinkman and Forchheimer theory. The impacts of thermal‐dependent heat source (THS) and exponential space‐dependent heat source (ESHS) are deployed. Aspects of Joule and viscous dissipations are also retained. The dimensionless equations are solved using the Runge‐Kutta‐Fehlberg joint with shooting methodology. The significance of various nondimensional parameters on the flow distributions as well as skin‐friction and Nusselt number is illustrated and analyzed. Closed form solution of momentum quantity is developed for a particular case. Obtained numerical results are in perfect agreement with analytical results. Further, the results of SWCNT and MWCNT are compared.

Nut and Bolt Microfluidics with Helical Minichannel for Counting CD4+ T-Cells.

In this study, we developed the prototype of an optical imaging-based point-of-care (POC) device for monitoring human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) progression that can detect CD4+ T-lymphocytes in human blood. The proposed portable cell-counting system, Helios CD4 Analyzer (Helios), can acquire sample images and analyze the cells automatically using a simple fluorescence imaging module and sample cartridge with a three-dimensional (3D) helical minichannel. The helical minichannel formed on the cylindrical surface enables the sample cartridge to hold a cell suspension present in a fixed sample volume for absolute counting of the cells. With a given total channel length, the helical minichannel-based sample cartridge is smaller than the conventional sample cartridge with a planar microchannel. The implemented nut and bolt mechanism allows the scanning of a relatively large volume of the sample along the helical minichannel by just rotating the cylindrical chamber coupled with a single DC motor rather than using a two-axis motorized translation stage, which considerably simplifies the associated electromechanical parts. It has distinct advantages over the existing devices because of its small size and simple scanning mechanism. We optimized various imaging parameters to enhance the fluorescence detection efficiency of the prototype. Performance evaluations using human blood samples demonstrated good agreement for low CD4 count between the Helios and the PIMATM, one of the most widely used POC CD4+ analyzers.

Combustion in wavy micro-channels for thermo-photovoltaic applications – Part I: Effects of wavy wall geometry, wall temperature profile and reaction mechanism

The output power of a Micro-Thermo-Photovoltaic (MTPV) system strongly depends on the micro-combustor geometry and the micro-flame stability. In this context, numerous numerical and experimental works have been conducted and focused on geometries limited to circular and planar micro-channels (MCs) with and without sudden expansion. Therefore, the present work proposes a novel MC configuration with wavy walls. The use of wavy walls increases the MC surface and consequently the heat transfer characteristics of this system could be improved. In this study, two-dimensional numerical simulations on CH4–air premixed flames in the novel configuration have been carried out. The wavy MCs are heated with a wall temperature gradient in the streamwise direction. The investigations have been carried out with a MC height H = 600 μm, a mixture velocity U = 1 m/s and the equivalence ratio Φ = 1. Moreover, the effects of various parameters on the flame behavior are reported: the wavy wall geometries, the maximum values of the temperature profile and the kinetic mechanisms of CH4 combustion. The results are original, since the effect of the wavy wall geometries are not available in the literature. In addition, the effects of the other parameters are not well documented in the literature, especially the CH4 combustion mechanisms. It is found that the flame location is sensible to the wavy wall geometry. As the number of waves increases, the flame moves towards the MC exit. The maximum value of the temperature gradient has an opposite effect. As the maximum temperature increases, the flame moves towards the MC inlet. The results show also that the flame location, shape and temperature are strongly affected by the different reaction mechanisms.

Cross-stream migration of a Brownian droplet in a polymer solution under Poiseuille flow.

The migration of a Brownian fluid droplet in a parallel-plate microchannel was investigated using dissipative particle dynamics computer simulations. In a Newtonian solvent, the droplet migrated toward the channel walls due to inertial effects at the studied flow conditions, in agreement with theoretical predictions and recent simulations. However, the droplet focused onto the channel centerline when polymer chains were added to the solvent. Focusing was typically enhanced for longer polymers and higher polymer concentrations with a nontrivial flow-rate dependence due to droplet and polymer deformability. Brownian motion caused the droplet position to fluctuate with a distribution that primarily depended on the balance between inertial lift forces pushing the droplet outward and elastic forces from the polymers driving it inward. The droplet shape was controlled by the local shear rate, and so its average shape depended on the droplet distribution.

Modeling of x-ray fluorescence full field imaging using planar square pore micro-channel plate optics.

X-ray fluorescence imaging using perfect planar square pore micro-channel plate x-ray optics (MPO) is investigated through the modeling of the MPO point spread function (PSF). A semi-continuous model based on the use of a simplified two parameters reflectivity curve is developed including, in particular, three kinds of contributions. A validation of this model is carried out by calculating variations of several PSF characteristics with the MPO and fluorescence imaging parameters and comparing the results with ray-tracing simulations. A good agreement is found in a large range of x-ray energies; however, it is shown that for the lower values of the working distance a discrete model should be used to take into account the periodic nature of the PSF. Ray-tracing simulated images of extended monochromatic sources are interpreted in light of both the semi-continuous and discrete models. Finally, solutions are proposed to improve the imaging properties of the MPOs.

Brinkman-Forchheimer flow of SWCNT and MWCNT magneto-nanoliquids in a microchannel with multiple slips and Joule heating aspects

Purpose The microfluidics has a wide range of applications, such as micro heat exchanger, micropumps, micromixers, cooling systems for microelectronic devices, fuel cells and microturbines. However, the enhancement of thermal energy is one of the challenges in these applications. Therefore, the purpose of this paper is to enhance heat transfer in a microchannel flow by utilizing carbon nanotubes (CNTs). MHD Brinkman-Forchheimer flow in a planar microchannel with multiple slips is considered. Aspects of viscous and Joule heating are also deployed. The consequences are presented in two different carbon nanofluids. Design/methodology/approach The governing equations are modeled with the help of conservation equations of flow and energy under the steady-state situation. The governing equations are non-dimensionalized through dimensionless variables. The dimensionless expressions are treated via Runge-Kutta-Fehlberg-based shooting scheme. Pertinent results of velocity, skin friction coefficient, temperature and Nusselt number for assorted values of physical parameters are comprehensively discussed. Also, a closed-form solution is obtained for momentum equation for a particular case. Numerical results agree perfectly with the analytical results. Findings It is established that multiple slip effect is favorable for velocity and temperature fields. The velocity field of multi-walled carbon nanotubes (MWCNTs) nanofluid is lower than single-walled carbon nanotubes (SWCNTs)-nanofluid, while thermal field, Nusselt number and drag force are higher in the case of MWCNT-nanofluid than SWCNT-nanofluid. The impact of nanotubes (SWCNTs and MWCNTs) is constructive for thermal boundary layer growth. Practical implications This study may provide useful information to improve the thermal management of microelectromechanical systems. Originality/value The effects of CNTs in microchannel flow by utilizing viscous dissipation and Joule heating are first time investigated. The results for SWCNTs and MWCNTs have been compared.

Approximate Solution for Electroosmotic Flow of Power-Law Fluids in a Planar Microchannel with Asymmetric Electrochemical Boundary Conditions.

Electroosmotic flow (EOF) is widely used in microfluidic systems and chemical analysis. It is driven by an electric force inside microchannel with highly charged boundary conditions. In practical applications, electrochemical boundary conditions are often inhomogeneous because different materials as walls are commonly utilized in routine fabrication methods. In the present study, we focus on the analytic solutions of the EOF generated in a planar microchannel with asymmetric electrochemical boundary conditions for non-Newtonian fluids. The velocity profile and flow rate are approximated by employing the power-law model of fluids in the Cauchy momentum equation. The hydrodynamic features of the EOF under asymmetric zeta potentials are scrutinized as a function of the fluid behavior index of the power-law fluid, thickness of Debye length, and zeta potential ratios between planes. The approximate solutions of the power-law model are comparable to the numerically obtained solutions when the Debye length is small and the fluid behavior index is close to unity. This study provides insights into the electrical control of non-Newtonian fluids, such as biological materials of blood, saliva, and DNA solution, in lab-on-a-chip devices.

Three-dimensional simulation of droplet dynamics in planar contraction microchannel

In droplet-based microfluidic systems, microchannel design plays a primary role in transport and manipulation of liquid droplets. The objective of this paper is to investigate dynamics of a droplet in planar contraction microchannel via three-dimensional numerical simulation and theoretical analysis. In particular, this study characterizes three regimes of the droplet dynamics, namely, trap, squeeze and breakup, depending on capillary number (Ca) and contraction ratio (C). In addition, theoretical models have been proposed to describe transitions from one to another regime as a function of Ca and C. For the transition from trap to squeeze, the critical capillary number (CaIc) was found to follow CaIc = a(CM−1), whereas the critical capillary number (CaIIc) of transition from squeeze to breakup corresponds to CaIIc = c1C−1. Furthermore, details of the droplet dynamics along downstream of the contraction have been explored as well to depict deformation, retraction and/or breakup of the droplet. The present results would be useful guidelines in designing contraction microfluidic channel for precise control and manipulation of droplets.

Parallel comprehensive two-dimensional gas chromatography

We introduce an information rich analytical approach called parallel comprehensive two-dimensional gas chromatography (2GC×2GC). This parallel chromatography approach splits injected samples into two independent two-dimensional column ensembles and provides two GC×GC separations by using contra-directional thermal modulation. The first-dimension (1D) and second-dimension (2D) columns are connected using planar three-port microchannel devices, which are supplied with supplementary flow via two pressure controller modules. Precise carrier gas flow control at the junction of the 1D and 2D columns permits independent control of flow conditions in each separation column. The 2GC×2GC approach provides two entirely independent GC×GC separations for each injection. Analysis of hop (Humulus lupulus L.) essential oils is used to demonstrate the capability of the approach. The analytical performance of each GC×GC separation in the 2GC×2GC experiment is comparable to individual GC×GC separation with matching column configurations. The peak capacity of 2GC×2GC is about 2 times than that of single GC×GC system. The dual 2D chromatograms produced by this single detector system provide complementary separations and additional identification information by harnessing different selectivity provided by the four separation columns.

Mixing in planar spiral microchannel

The mixing characteristics in spiral microhannels are studied experimentally and numerically in this work. Effect of cross section aspect ratio (α=0.3–1.2) on mixing characteristic is studied both qualitatively and quantitatively using top as well as side view details. The study is carried out for a wide range of Reynolds number (1–468) and Dean number (0.1–75). The pathline behavior and species fraction are tracked to understand the effect of Reynolds number and aspect ratio on mixing. It is observed that spiral microchannel with higher aspect ratio results in better mixing as compared to smaller aspect ratio microchannel. An optimal Reynolds number at which the mixing efficiency is maximized is observed from the experimental data. It is observed that the fluid is trapped within the vortex core at higher Reynolds number, leading to a reduction in mixing efficiency and appearance of a maxima. Frequent twisting of pathlines is further identified to affect mixing and leads to a dependence of mixing on aspect ratio. The analysis presented in this work is of significance in the design of spiral microchannel based microreactors.

Microfluidic approaches to the study of angiogenesis and the microcirculation.

Microfluidic systems have emerged as a new class of perfusable in vitro culture models that have helped advance and refine our understanding of microvascular function. Cutting-edge microfluidic models have successfully integrated principles from quantitative analysis of vascular function, in vitro flow chambers, microfabrication techniques, and 3D tissue scaffolds. Here, we review the evolution of microfluidic systems, namely their progression from 2D planar microchannel arrays to 3D microtissue analogs, and highlight their recent contributions in elucidating the role of biomolecular transport and fluid mechanical stimuli in controlling angiogenesis. Further advancement of microfluidic systems in recapitulating tissue-level phenomena in vitro, controlling important physiochemical and biological parameters, and integrating cellular and molecular analysis will help further enhance their application within the microcirculation research community.

In-situ shear-banding quantification of surfactant solutions in straight microfluidic channels

Surfactant molecules aggregate into various micellar morphologies, depending on temperature, concentration, formulation, and flow. Micellar solutions are known to undergo shear-banding when subjected to strong shear rates, as the fluid spontaneously divides itself into bands of high and low-shear rate, both under the same applied shear stress. This phenomenon occurs because of the complex structure of micellar solutions, which undergo phase transitions upon applied flow, changing the viscosity accordingly. Here, we study shear-banding of micellar solutions in one of the simplest microfluidic geometries, a straight planar channel with rectangular cross section. Four solutions with similar zero-shear viscosity and nonlinear rheological response, but different structures are compared to investigate the flow-structure relation and its impact on shear-banding. Micellar solutions are prepared by adding different amounts of the same organic salt, sodium salicylate, to surfactant molecules with different headgroups, i.e., cetyltrimethylammonium bromide and cetylpyridinium chloride. From spatially resolved microparticle image velocimetry and flow-induced birefringence measurements, the shear rate and shear stress profiles developed on the xy-plane of a planar microchannel are obtained from a series of volumetric flow rates. Based on these profiles, in-situ rheological parameters, such as the local viscosity, are calculated by applying the stress-optical rule. The local response in a microfluidic channel is compared to the bulk rheological response in a rotational rheometer and clear correlations are found especially for the stress plateau region, the fingerprint of shear-banding. Based on the local rheological characterization of these micellar solutions, the development and growth of shear-bands is observed and quantified. The role of salt concentration and surfactant headgroup on the resulting micellar morphology is discussed, as well as its impact on the development of shear-banding.

Numerical analysis of surface roughness effects on the Poiseuille flow caused by a small pressure drop

The rarefied gas flow caused by a pressure drop in a planar microchannel of finite length is analysed numerically to investigate the effect of surface roughness on thermal and hydrodynamic flow characteristics. The kinetic approach based on the nonlinear S-model of the Boltzmann kinetic equation coupled with the Maxwell’s diffuse gas-surface interaction model imposed at the wall is used. An implicit scheme for the solution of the S-model kinetic equation is applied and the algorithm has been optimised for the use of massive parallelization in the velocity space. A surface roughness is configured with series of triangular, semicircular, trapezoidal and rectangular obstructions. A competition between rarefaction and effects of roughness geometry is estimated by varying a rarefaction regime of flow from continuum up to free–molecular regime. The results indicate that the global flow performances in terms of the Poiseuille number, mass and thermal energy flow rates are affected by the wall roughness geometry when compared to the flow performances for a smooth channel. The triangular roughness produces the smallest impact on a gas flow, while the rectangular roughness the largest one. Rarefaction plays a significant role enhancing the effect of the surface roughness. In the continuum flow regime the roughness elements induce strong recirculation and flow separation regions. Moreover, the weakly rarefied flow strongly depends on the shape and the spacing of roughness elements. On the other hand, for the highly rarefied flow the global parameters are almost independent of the shape of elements. The increase in the spacing of roughness elements results in the reduction of the Poiseuille number and an enhancement of mass and thermal energy flow rates for all shapes of roughness elements.

Magnetogasdynamic natural convection in a long vertical microchannel

● Original study: There were no studies on microscale magnetogasdynamic natural convection. ● New finding—Velocity: Results show that a greater magnetic force would result in a flatter velocity profile. ● New finding—Thermal-flow characteristics: Results reveal that magnetic force results in decreases in thermal-flow characteristics. ● New finding—Enhanced magnetic effects: These effects could be further enhanced with higher by increasing gas rarefaction. ● Potential benefits of the present study: This study could benefit the designs of magnetically controllable microfluidic devices. Since the transport behavior of ionized gases at the microscale could be influenced by an applied magnetic field with ease, microscale magnetogasdynamics (MGD) promises to be particularly advantageous for magnetically controllable microfluidic devices. The purpose of this study is to investigate how magnetic force affects the MGD natural convection within a long asymmetrically heated vertical planar microchannel. The fully developed solutions of the thermal-flow fields and their characteristics are analytically derived on the basis of the first-order slip and jump boundary conditions and then presented for the thermophysical properties of ionized air at the standard reference state flowing through the microchannel with complete accommodation. The calculated results reveal that magnetic force plays a damping role in flow and results in decreases in flow rate, average flow drag, and average heat transfer rate. In addition, it is interesting that because the flow near the core is suppressed and the shear stress on the wall surface is reduced by the magnetic effects, a flatter velocity profile could be achieved by a greater magnetic force. These magnetic effects could be further magnified by increasing gas rarefaction or increasing cooler wall temperature.

Combustion stability and hetero-/homogeneous chemistry interactions for fuel-lean hydrogen/air mixtures in platinum-coated microchannels

The hetero-/homogeneous combustion and stability limits of fuel-lean hydrogen/air mixtures (equivalence ratio φ=0.40) were investigated numerically in a platinum-coated planar microchannel with a length of 10mm and a height of 1mm. A two-dimensional numerical model was used for both the gas and the solid, which included elementary heterogeneous and homogeneous reaction mechanisms, detailed transport, heat conduction in the solid, surface radiation heat transfer, and external losses via a heat transfer coefficient h. Pressures of 1 and 5bar and solid thermal conductivities ks = 1 and 16Wm-1K−1 were analyzed, while stability maps were constructed in terms of the critical extinction heat transfer coefficient hcr versus inlet velocity UIN (or mass throughput). For a given solid thermal conductivity, there existed a crossover mass throughput above (below) which the stability envelope was broader at 5bar (1bar). Simulations with a surface perfectly stirred reactor (SPSR) model qualitatively reproduced the crossover points, which originated from a shift in the pressure dependence of the catalytic reactivity of hydrogen. For the low solid thermal conductivity ks = 1 Wm-1K−1, a non-monotonic dependence of the stability limits on the mass throughput was shown, with local minima created below the crossover point. The stability limits of hydrogen were solely determined by catalytic chemistry, as it sustained combustion at temperatures down to 320–380K, at which gas-phase chemistry was frozen. Away from the critical extinction points, both catalytic and gas-phase reaction pathways were controlling. The diffusional imbalance of hydrogen, which led to catalytically-induced superadiabatic surface temperatures, and the suppression of the surface superadiabaticity by gaseous chemistry resulted in rich combustion phenomena, such as increasing peak wall temperatures with increasing heat transfer coefficients h. Critical extinction heat transfer coefficients for hydrogen were three to four orders of magnitude higher than those reported for methane and propane fuels in a similar channel geometry.