Cross-section Microchannels Research Articles

Three-Dimensional Numerical Study of Conjugate Heat Transfer in Diverging Microchannel

Increase in applications of varying cross sectional area microchannels in microdevices has provided the need to understand fluid flow and heat transfer through such flow passages. This study focuses on conjugate heat transfer study through a diverging microchannel. Three-dimensional numerical simulations are performed using commercially available package. Diverging microchannels with different geometrical configurations (i.e. varying angle: 1-8°, depth: 86-200 μm, solid-to- fluid thickness ratio: 1.5-4) are employed for this purpose. Simulations are carried out for varying mass flow rate (3.3 x 10 –5 -8.3 x 10 –5 kg/s) and heat flux (2.4-9.6 W/cm 2 ) conditions. Heat distribution along the flow direction is studied to understand the effect of wall conduction. Wall conduction number ( M ) varies from 0.006 to 0.024 for the range of parameters selected in the study. Wall conduction is observed to be a direct function of depth and solid-to-fluid thickness ratio, and varies inversely with angle of diverging microchannel. It is observed that the area variation and wall conduction contribute separately towards redistribution of the supplied heat flux. This leads to reduced temperature gradients in diverging microchannel. The results presented in this work will be useful for designing future microdevices involving heating or cooling

Characterization of enzymatic micromachining for construction of variable cross-section microchannel topologies.

The ability to harness enzymatic activity as an etchant to precisely machine biodegradable substrates introduces new possibilities for microfabrication. This flow-based etching is straightforward to implement, enabling patterning of microchannels with topologies that incorporate variable depth along the cross-sectional dimension. Additionally, unlike conventional small-molecule formulations, the macromolecular nature of enzymatic etchants enables features to be precisely positioned. Here, we introduce a kinetic model to characterize the enzymatic machining process and its localization by co-injection of a macromolecular inhibitor species. Our model captures the interaction between enzyme, inhibitor, and substrate under laminar flow, enabling rational prediction of etched microchannel profiles so that cross-sectional topologies incorporating complex lateral variations in depth can be constructed. We also apply this approach to achieve simultaneous widening of an entire network of microchannels produced in the biodegradable polymeric substrate poly(lactic acid), laying a foundation to construct systems incorporating a broad range of internal cross-sectional dimensions by manipulating the process conditions.

Kinetic Rate Determination via Electrophoresis along a Varying Cross-Section Microchannel.

High throughput, efficient, and readily adoptable analytical tools for the validation and selection of reliable antibody reagents would impact the life sciences, clinical chemistry, and clinical medicine. To directly quantify antibody-antigen association and dissociation rate constants, kon and koff, in a single experiment, we introduce a microfluidic free-standing kinetic polyacrylamide gel electrophoresis (fsKPAGE) assay. Here, an antibody is immobilized in zones along the length of a single freestanding polyacrylamide gel lane of varying cross-sectional width. Fluorescently labeled antigen is electrophoresed through each immobilized antibody zone, with local cross-sectional area determining the local electric field strength and, thus, the local interaction time between immobilized antibody and electromigrating antigen. Upon crossing, the interaction yields immobilized immunocomplex. The kon is quantified by assessing the amount of immunocomplex formed at each interaction time. To quantify koff, immobilized zones of fluorescently labeled immunocomplex are subjected to a buffer dilution and monitored over time. We determine kon and koff for prostate-specific antigen (PSA) and make a comparison to gold-standard values. The fsKPAGE assay determines kon and koff in a single experiment of less than 20 min, using 45 ng of often limited antibody material and standard laboratory equipment. We see the fsKPAGE assay as forming the basis for rapid, quantitative antibody-screening tools.

Investigation of rarefied gas flow in microchannels of non-uniform cross section

Study of rarefied gas flow in converging and diverging cross sections is crucial to the development of micro-nozzles and micro-thrusters. In other practical cases too, a microchannel may not always be straight and may include diverging and converging sections in the flow path. In this context, isothermal rarefied gas flow in microchannels of longitudinally varying cross section is studied experimentally in this work. The primary objective is to investigate the existence of Knudsen minimum in microchannels of varying cross sections. The effect of geometrical cross section and fluid properties on the Knudsen minimum are also investigated by performing experiments on three divergence angles (4°, 8°, and 12°) and three different gases (argon, nitrogen, and oxygen) to prove the robustness of the result. The Knudsen minimum, which is one of the characteristic features of rarefied flows, is experimentally observed for the first time in a microchannel of varying cross section. The position of the Knudsen minimum (at Kn ≈ 1) is seen to depend only weakly on the divergence angle and fluid properties.

Transient Analysis of Microchannel Heat Sink during Single Phase and Flow Boiling Conditions

In the present work, an experimental investigation has been made to analyze the performance of microchannel heat sink under transient operating conditions. The transient analysis has been made by estimating the response time for different input heat flux and coolant mass flow rate. Analysis has been made for rectangular cross-section microchannels fabricated on a copper block of size 25.7 × 12 × 10 mm3. Twelve (12) numbers of microchannels are fabricated in the copper block. The width and depth of individual channels are 400 μm and 750 μm respectively. Performance analysis has been made for both single phase and flow boiling conditions of the coolant flow using deionized water as coolant. Experiments have been performed for coolant mass flux (G) range of 90 - 250 kg/m2s and input heat flux (q) range of 20 - 300 kW/m2 respectively. It has been observed that at constant input heat flux, response time decreases with the increase in coolant mass flux during single phase cooling. However this trend is not strongly followed during the two-phase or flow boiling cooling condition.

Inertial focusing in non-rectangular cross-section microchannels and manipulation of accessible focusing positions.

Inertial focusing in microfluidic channels has been extensively studied experimentally and theoretically, which has led to various applications including microfluidic separation and enrichment of cells. Inertial lift forces are strongly dependent on the flow velocity profile and the channel cross-sectional shape. However, the channel cross-sections studied have been limited to circles and rectangles. We studied inertial focusing in non-rectangular cross-section channels to manipulate the flow profile and thus the inertial focusing of microparticles. The location and number of focusing positions are analyzed with varying cross-sectional shapes and Reynolds number. We found that the broken symmetry of non-equilateral triangular channels leads to the shifting of focusing positions with varying Reynolds number. Non-rectangular channels have unique mapping of the focusing positions and the corresponding basins of attraction. By connecting channels with different cross-sectional shapes, we were able to manipulate the accessible focusing positions and achieve focusing of microparticles to a single stream with ∼99% purity.

APPLICATION OF ELECTRICAL CAPACITANCE TOMOGRAPHY FOR DENSE CROSS-SECTIONAL PARTICLE MIGRATION IN A MICROCHANNEL

In microfluidic applications, in order to produce the high yield of desired product, the study of particle migration is very important to enhancement and increases the efficiency of positioning and sorting process. One of an effective and robust method for visualization imaging, passively positioning and sorting microparticles and cells without the assistance of sheath fluid is electrical capacitance. In this study, to study the behavior of particle migration, a fine particle concentration in a cross-sectional microchannel is determined for the high dense initial particle concentrations (ξ=10.0%) and small particle diameters (dp= 2.1 μm) by using a high speed multiplexer and 12 multi-layer electrical capacitance tomography (ECT) sensing to discuss the stream migration along five cross-sections. The polystyrene particles as solid phase and non-conductive deionized water as a liquid phase are non-uniformly injected into the inlets microchannel. From the electrical capacitance distribution, the tomography images that show the equilibrium particle migration is reconstructed by used the Tikhonov regularization method. It has been observed that the particle concentration at the wall vicinity area is increased as ξ and dp are increased while the particle concentration at the center area is decreased. It shows the particles are moved away from the center towards wall vicinity area and particles migrated towards the wall increased in the outlet area as the particles move along the cross-sectional microchannel. The experimental result is verified with the COMSOL simulation.

Bubble confinement and deformation during flow boiling in microchannel

The bubble dynamics during subcooled flow boiling were investigated in a single rectangular cross-section (0.5mm×1mm) microchannel. Degassed deionized water was used as the working fluid with the confined bubble behavior observed using a high-speed CCD camera. The microchannel wall confines the bubble growth in the bubble height direction when the bubble departure diameter was larger than the microchannel cross section, resulting in strong deformation of the bubble top interface even before the liquid–vapor interface touched the microchannel wall. The microchannel confinement effect works by exerting a wall confining force on the top interface of the growing bubble. The negative feedback between the velocity of the top interface and the wall confining force is the main reason for the fluctuations in the increasing maximum local void fraction and the bubble aspect ratio during the confined bubble growth period.

Spontaneous capillary flows in piecewise varying cross section microchannels

The study of the dynamics of capillary wetting has started in the 1920s with the studies of Lucas, Washburn, and Rideal. The LWR law states a square root dependency with time for the penetration distance. This property was shown to be valid for arbitrary, uniform cross section microchannels.However, the dynamics of capillary wetting in non-uniform cross section channels is still a subject of investigation.In this work an analytical model for piecewise varying cross section channels is developed. It is shown that the model compares favorably to experiments. Moreover the results are also in agreement with a former numerical approach.

Electroosmotic Flow in Hydrophobic Microchannels of General Cross Section

Adopting the Navier slip conditions, we analyze the fully developed electroosmotic flow in hydrophobic microducts of general cross section under the Debye–Hückel approximation. The method of analysis includes series solutions which their coefficients are obtained by applying the wall boundary conditions using the least-squares matching method. Although the procedure is general enough to be applied to almost any arbitrary cross section, eight microgeometries including trapezoidal, double-trapezoidal, isosceles triangular, rhombic, elliptical, semi-elliptical, rectangular, and isotropically etched profiles are selected for presentation. We find that the flow rate is a linear increasing function of the slip length with thinner electric double layers (EDLs) providing higher slip effects. We also discover that, unlike the no-slip conditions, there is not a limit for the electroosmotic velocity when EDL extent is reduced. In fact, utilizing an analysis valid for very thin EDLs, it is shown that the maximum electroosmotic velocity in the presence of surface hydrophobicity is by a factor of slip length to Debye length higher than the Helmholtz–Smoluchowski velocity. This approximate procedure also provides an expression for the flow rate which is almost exact when the ratio of the channel hydraulic diameter to the Debye length is equal to or higher than 50.

A generalization of the Lucas–Washburn–Rideal law to composite microchannels of arbitrary cross section

Capillary microfluidics or capillarics has been lately gaining importance in the biotechnological and biological domains. In these domains where biological and chemical targets are transported by fluids, it has been shown that capillary actuation of fluids does not require bulky pumps or syringes and produces microsystems with a low cost of fabrication, which are user-friendly, portable and compatible with telemedicine. Capillary systems for biotechnology can be confined or open; i.e., the fluid moves inside a closed channel or in a channel with a boundary with air. In this work, we propose a general expression for the determination of the velocities of spontaneous capillary flows in composite, confined microchannels of arbitrary shapes. This expression generalizes the conventional Lucas–Washburn model which is valid for cylindrical channels. It is shown that the use of an equivalent hydraulic diameter in the Lucas–Washburn–Rideal model introduces a bias when the shape of the channel cross section differs notably from a circle. The approach also shows that relatively large velocities—at the scale of microsystems—can be reached by capillary microflows, depending on the shape of the channel, and that transport distances can be important.

Clogging behavior of an elongated bubble in uniform and diverging microchannel

A numerical investigation has been performed to make a comparative study of clogging and detachment behavior of an elongated bubble during adiabatic flow through a uniform and diverging cross-sectional microchannel. The adiabatic gas–liquid two-phase flow has been numerically solved using volume of fluid approach in a finite volume code. Simulations have been made to investigate the effects of wall wettability, surface tension, and flow velocity on the bubble dynamics for both the configurations of microchannel. The net pressure drop across the bubble in the diverging channel has been calculated analytically and the results have been compared with numerical results. In both types of channels, the bubble shape goes through a transient stage followed by a steady and stable shape. The change in bubble shape during transient phase depends on the surface wettability, flow velocity, and the confinement of the channels. The bubble attains the symmetrical shape earlier in diverging channel. The formation of liquid film between the bubble and the wall plays an important role in the clogging behavior and movement of the bubble. Wall wettability has less impact on the pressure drop characteristics in the diverging channel.

Effects of surface roughness and electrokinetic heterogeneity on electroosmotic flow in microchannel

In this paper, a hybrid lattice-Boltzmann and finite-difference (LB-FD) model is applied to simulate the effects of three-dimensional surface roughness and electrokinetic heterogeneity on electroosmotic flow (EOF) in a microchannel. The lattice-Boltzmann (LB) method has been employed to obtain the flow field and a finite-difference (FD) method is used to solve the Poisson-Boltzmann (PB) equation for the electrostatic potential distribution. Numerical simulation of flow through a square cross-section microchannel with designed roughness is conducted and the results are critically analysed. The effects of surface heterogeneity on the electroosmotic transport are investigated for different roughness height, width, roughness interval spacing, and roughness surface potential. Numerical simulations reveal that the presence of surface roughness changes the nature of electroosmotic transport through the microchannel. It is found that the electroosmotic velocity decreases with the increase in roughness height and the velocity profile becomes asymmetric. For the same height of the roughness elements, the EOF velocity rises with the increase in roughness width. For the heterogeneously charged rough channel, the velocity profile shows a distinct deviation from the conventional plug-like flow pattern. The simulation results also indicate locally induced flow vortices which can be utilized to enhance the flow and mixing within the microchannel. The present study has important implications towards electrokinetic flow control in the microchannel, and can provide an efficient way to design a microfluidic system of practical interest.

Concurrent photopatterning of elastic modulus and structures in photosensitive silicone elastomers

Photosensitive elastomers are an interesting class of polymers combining within one material the mechanical properties of an elastomer with the features of a photoresist. To date, they have mainly been implemented in soft lithography or to locally modulate the elasticity of the elastomer. In this paper, we demonstrate concurrent mechanical and structural patterning of a film of photosensitive PDMS (P-PDMS) upon UV light exposure through a lithography mask. Absorption of UV light restricts cross-linking therefore softens the elastomer, but also induces distinct lateral material transfer and structural shrinking of the non-exposed elastomer. P-PDMS microchannels of sub-millimeter cross-section and millimeter length, and patterned Young’s moduli were reliably prepared with this single step process. This combination of properties presents a rapid prototyping approach for soft MEMS, microfluidics and stretchable electronics.

A comparative study of flow boiling heat transfer in three different configurations of microchannels

In this work, an experimental investigation has been made to compare the flow boiling characteristics of deionized water in three different configurations of microchannels. The investigated channel configurations are uniform cross-section, diverging cross-section and segmented finned microchannels. In each configuration, an array consisting of twelve numbers of microchannels with rectangular cross-section has been fabricated on a copper block with footprint area of 25.7×12.02mm2. Experiments have been conducted with subcooled liquid state at the entry with coolant mass and heat fluxes vary in the range 100–350kg/m2s and 10–350kW/m2, respectively. Depending upon the heat flux and coolant flow rate different regimes of two phase boiling have been observed. The comparison of three configurations has been made in terms of heat transfer coefficient, pressure drop characteristics and affinity towards backflow or flow reversal in the channels. Bubbles dynamics and their role in flow reversal and flow instability have been discussed for all three types of channels. For entire operating conditions, segmented finned channels demonstrate the highest heat transfer coefficient with negligible higher pressure drop compared to other two configurations of channels. The performance of diverging cross-section channels is better than the uniform cross-section channels. However, they underperform compared with segmented finned channels. At higher heat flux, bubble clogging and flow reversal problem is worst in uniform cross-section channels. The problem is partially solved in diverging cross-section channels. Segmented channels completely relieve the problem of bubble clogging allowing smooth and easy passage of growing bubbles. Moreover flow reversal is not observed in segmented channels for entire operating range of heat flux and coolant mass flux.

Numerical Simulation of Heat Transfer Enhancement in Periodic Converging-diverging Microchannel

Heat transfer performance and fluid flow in three-dimensional converging–diverging microchannel of circular cross-section are studied numerically. Three different types of microchannel, diameter 100μm and length 6.125mm each are used for simulation for the Reynolds number range of 50 to 1000. The effect of Reynolds Number, converging/diverging angle and converging-diverging cross-section length on pressure drop and heat transfer in proposed microchannels are investigated and discussed. The result indicated that the flow in the converging-diverging cross-section induces stronger recirculation and flow separation, which decreases with decreasing the converging and diverging angle and increases with increasing aspect ratio. However, the local and global heat transfer performances in the channels are improved by the converging-diverging cross-section at the expense of higher pressure drop compared to the straight channel with the same cross-section. Conversely, special attentions are given to analyze the thermal performance of microchannels through variation of thermal resistance with pumping power.

Performance enhancement of PV cells through micro-channel cooling

Efficiency of a PV cell is strongly dependent on its surface temperature. The current study is focused to achieve maximum efficiency of PV cells even in scorching temperatures in hot climates like Pakistan where the cell surface temperatures can even rise up to around 80 ℃. The study includes both the CFD and real time experimental investigations of a solar panel using micro channel cooling. Initially, CFD analysis is performed by developing a 3D model of a Mono-Crystalline cell with micro-channels to analyze cell surface temperature distribution at different irradiance and water flow rates. Afterwards, an experimental setup is developed for performance investigations under the real conditions of an open climate of a Pakistan's city, Taxila. Two 35W panels are manufactured for the experiments; one is based on the standard manufacturing procedure while other cell is developed with 4mm thick aluminum sheet having micro-channels of cross-section of 1mm by 1mm. The whole setup also includes different sensors for the measurement of solar irradiance, cell power, surface temperature and water flow rates. The experimental results show that PV cell surface temperature drop of around 15 ℃ is achieved with power increment of around 14% at maximum applied water flow rate of 3 LPM. Additionally, a good agreement is also found between CFD and experimental results. Therefore, that study clearly shows that a significant performance improvement of PV cells can be achieved through the proposed cell cooling technique.

Impact of Ultrasonic Field on Viscous Dissipation Effects of Polymer Melts in Micro Channel

For viscous dissipation effect of the polymer melts flowing in micro-scale channel on their flow performance. The micro-injection molding experimental system with temperature sensors and ultrasonic field are designed. The temperature rises at the outlet of the rectangular cross-section micro-channel caused by viscous dissipation at different process parameters and ultrasonic power changes are investigated, using polypropylene(PP) and high-density polyethylene(HDPE) two polymer melts by the single factor molding experiments. The results show that the viscous dissipation effect of the polymer melts flowing in micro-scale channels is enhanced with the increase of injection rate, while it can be weakened by the rise of inlet melts temperature and mold temperature and the outlet temperature rise of the two materials with ultrasonic vibration can be significantly higher than that without ultrasonic vibration. However, their temperature increases vary greatly for microscopic molecular structures of different materials and their thermo-physical properties. PP melt temperature increase applying ultrasonic vibration is 34.7% higher than that without ultrasonic vibration at the set injection speed, while it is 71.7% for HDPE. But the under a certain ultrasonic frequency and process parameter,increasing the ultrasonic power make PP melt temperature increase by 24.8%, while it is 83.6% for HDPE. It's obvious that the viscous dissipation effects of polymer melts flowing the micro channel can be enhanced by applying ultrasonic field.

Viscoelastic focusing of microparticles in circular cross-sectional microchannels

Particle focusing induced by viscoelasticity of fluids has attracted increasing interest in recent years. However, the regulation mechanisms of critical parameters affecting the particle focusing behaviors are still unclear. This paper systematically characterized the dynamics of particle migration in non-Newtonian fluid flows, and analyzed the effects of flow rate and channel length on particle focusing behaviors. Först, the lateral migration behaviors of particles suspended in Newtonian fluids (e.g., pure water and 22 wt% glycerol aqueous solution) are compared with those in non-Newtonian fluids (8 wt% polyvinylpyrrolidone aqueous solution). It is found that the particles suspended in non-Newtonian fluids would migrate towards the channel centerline and form a single-line particle array under the action of elastic force while the particles suspended in Newtonian fluids would migrate to form a famous Segré-Silberberg particle annular ring due to the effects of inertial lift forces. Second, the effects of particle size and driving flow rate on particle viscoelastic focusing are quantitatively analyzed. Results show that with increasing flow rate the focusing degree increases and finally stabilize at a certain value, and the large particles have better focusing quality than the small ones. Finally, the dynamic focusing process of particles along the channel length is investigated. A mathematical model of safe channel length for achieving particle focusing is derived and validated by experiments. It is found that the safe channel length for large particles is significantly shorter than that for small ones. The obtained results would improve the understanding of particle focusing processes and mechanisms, and help realize the flexible control of particle migration behaviors in non-Newtonian fluids.

Effect of aligned and offset roughness patterns on the fluid flow and heat transfer within microchannels consist of sinusoidal structured roughness

This research aims at numerical study of the effect of aligned and offset roughness patterns on the fluid flow and heat transfer phenomenon within rectangular cross section microchannels containing structured sinusoidal roughness. To do this, the pressure losses, heat transfer coefficients as well as thermal performance indexes in aligned pattern are compared with corresponding offset pattern at different roughness pitches, heights and microchannel heights. In this work, the upper and lower microchannel walls involve sinusoidal roughness and other walls are assumed to be smooth. Air and water are also chosen as the working fluids. To validate the current numerical model, comparisons are also made with previous experimental, theoretical and numerical results and reasonable agreements are observed. Results show that for the air, the effect of change in the channel parameters on the heat transfer coefficient is of little significance compared to the water and the two roughness arrangements represent a same behavior. It is found that, at high roughness height or low channel height, the offset arrangement provides lower pressure loss for the considered fluids and also lower heat transfer rate for water than the aligned pattern. It is shown that for both fluids, as the roughness pitch increases, the offset pattern provides lower pressure loss than the aligned pattern. Numerical results indicate that for water, the increase in roughness pitch leads to negligible variation in the heat transfer rate in aligned configuration while different behavior is observed in offset pattern. Results exhibit that both air and water represent a same thermal performance index and the offset effect could increase the thermal performance of microchannel with the increase in roughness pitch ratio. It is also revealed that, in both roughness patterns, the decrease in constricted relative roughness leads to the increase in the thermal performance index and high thermal performance is achieved at low roughness height.