Straight Rectangular Microchannel Research Articles

NUMERICAL STUDY OF PRESSURE DROP AND HEAT TRANSFER IN A STRAIGHT RECTANGULAR AND SEMI CYLINDRICAL PROJECTIONS MICROCHANNEL HEAT SINK

The heat transfer and pressure drop characteristics of a straight rectangular and semi cylindrical projections microchannel heat sink were investigated. The heat sinks, made of copper, were aligned in row as a cluster of 21 microchannels, 231 µm wide and 713 µm deep. Water was used as cooling fluid and was made to flow through the channels. A three dimensional analysis was done for Reynolds number ranging between 200 to 1000 with constant heat flux of 10 6 W/m 2 defined relative to the platform area of the heat sink. The temperature rise and pressure drop of the fluid in straight microchannel heat sink was evaluated using ANSYS-CFX package and was validated using experimental data. Similar analysis was done for semi cylindrical projections microchannel heat sink by solving the conjugate heat transfer problem. A comparison of heat transfer and pressure drop between rectangular and semi cylindrical projections microchannel was done and the results were laid out. Results shows that heat transfer increases with the use of semi cylindrical projections microchannel heat sink.

Shape-based separation of microalga Euglena gracilis using inertial microfluidics

Euglena gracilis (E. gracilis) has been proposed as one of the most attractive microalgae species for biodiesel and biomass production, which exhibits a number of shapes, such as spherical, spindle-shaped, and elongated. Shape is an important biomarker for E. gracilis, serving as an indicator of biological clock status, photosynthetic and respiratory capacity, cell-cycle phase, and environmental condition. The ability to prepare E. gracilis of uniform shape at high purities has significant implications for various applications in biological research and industrial processes. Here, we adopt a label-free, high-throughput, and continuous technique utilizing inertial microfluidics to separate E. gracilis by a key shape parameter-cell aspect ratio (AR). The microfluidic device consists of a straight rectangular microchannel, a gradually expanding region, and five outlets with fluidic resistors, allowing for inertial focusing and ordering, enhancement of the differences in cell lateral positions, and accurate separation, respectively. By making use of the shape-activated differences in lateral inertial focusing dynamic equilibrium positions, E. gracilis with different ARs ranging from 1 to 7 are directed to different outlets.

An experimental and numerical investigation of the use of liquid flow in serpentine microchannels for microelectronics cooling

This paper presents a combined experimental and numerical investigation of single-phase water flow and heat transfer in serpentine rectangular microchannels embedded in a heated copper block. The performance of four different microchannel heat sink (MCHS) configurations are investigated experimentally, the first having an array of straight rectangular microchannels (SRMs), while the other have single (SPSMs), double (DPSMs) and triple path multi-serpentine rectangular microchannels (TPSMs). Three-dimensional conjugate heat transfer models are developed for both laminar and turbulent single-phase water flows in each of these MCHSs and the governing flow and energy equations solved numerically using finite elements. The numerical predictions of pressure drop (ΔP) and average Nusselt number (Nuavg) are in good agreement with experimental data, and indicated that the single path serpentine microchannel (SPSM) leads to a 35% enhancement of the Nuavg at a volumetric flow rate of 0.5l/min and a 19% reduction in total thermal resistance (Rth) compared to the conventional SRM heat sink. However, this enhancement is at the expense of a large (up to ten-fold) increase in ΔP compared to the SRM heat sink, so that a suitable compromise must be struck between heat transfer and pressure drop in practical MCHS designs.

Inertially focused diamagnetic particle separation in ferrofluids

Continuous flow separation of target particles from a mixture is essential to many chemical and biomedical applications. There has recently been an increasing interest in the integration of active and passive particle separation techniques for enhanced sensitivity and flexibility. We demonstrate herein the proof-of-concept of a ferrofluid-based hybrid microfluidic technique that combines passive inertial focusing with active magnetic deflection to separate diamagnetic particles by size. The two operations take place in series in a continuous flow through a straight rectangular microchannel with a nearby permanent magnet. We also develop a three-dimensional numerical model to simulate the transport of diamagnetic particles during their inertial focusing and magnetic separation processes in the entire microchannel. The predicted particle trajectories are found to be approximately consistent with the experimental observations at different ferrofluid flow rates and ferrofluid concentrations.

Viscoelastic Separation of Particles by Size in Straight Rectangular Microchannels: A Parametric Study for a Refined Understanding.

Microfluidic separation of particles has been implemented using a variety of force fields. We demonstrate in this work a continuous sheath-free separation of both a binary and a ternary particle mixture in viscoelastic polymer solutions through straight rectangular microchannels. This label-free separation arises from the flow-induced lift force that directs particles toward size-sensitive focusing positions in a high width/height channel. It is found to be a strong function of multiple experimental parameters, which is systematically investigated in terms of dimensionless numbers. We propose to explain the observed lateral shifting of particle focusing positions as a result of the competing center- (due to fluid elasticity effects) and wall- (due to fluid elasticity and shear thinning effects) directed elastic lift forces. The inertial lift force comes into effect at relatively high flow rates, which appears to reduce the separation efficiency and purity in our experiments.

Continuous sheath-free separation of particles by shape in viscoelastic fluids

Shape is an important indicator of cell type, cycle, and state, etc., and can thus serve as a specific marker for label-free bioparticle separation. We demonstrate in this work a shape-based separation of equal-volumed spherical and peanut particles in viscoelastic fluids through straight rectangular microchannels. This continuous sheath-free separation arises from the shape-dependent equilibrium particle position(s) as a result of the flow-induced elasto-inertial lift and shear thinning effects. A continuous transition from single to dual and to triple equilibrium positions is observed for both types of particles with the increase in flow rate. However, the flow rate at which the transition takes place differs with the particle shape. This phenomenon occurs only in microchannels with a large aspect ratio (width/height) and has not been reported before. It is speculated to correlate with the dissimilar dependences of elastic and inertial lift forces on particle size and flow rate as well as the rotational effects of non-spherical particles.

Effect of junction configurations on microdroplet formation in a T-junction microchannel

This study investigates the dynamic formation process of water microdroplets in a silicon oil flow in a T-junction microchannel. Segmented water microdroplets are formed at the junction when the water flow is perpendicularly injected into the silicon oil flow in a straight rectangular microchannel. This study further presents the effects of the water flow inlet geometry on hydrodynamic characteristics of water microdroplet formation. A numerical multiphase volume of fluid (VOF) scheme is coupled to solve the unsteady three-dimensional laminar Navier-Stokes equations to depict the droplet formation phenomena at the junction. Predicted results on the length and generated frequency of the microdroplets agree well with experimental results in a T-junction microchannel with straight and flat inlets (the base model) for both fluid flows. Empirical correlations are reported between the volumetric flow ratio and the dimensionless microdroplet length or dimensionless frequency of droplet generation at a fixed capillary number of 4.7 · 10−3. The results of this study indicate a reduction in the droplet length of approximately 21% if the straight inlet for the water flow is modified to a downstream sudden contraction inlet for the water flow.

Sheathless Separation of Particles and Cells by Viscoelastic Effects in Straight Rectangular Microchannels

We demonstrate a label-free and sheathless hydrodynamic approach that successfully separate particles and cells by size in straight microchannels. Particles and cells have size-dependent stable equilibrium positions in 0.1wt % polyethylene oxide (PEO, Mw = 4×106 g/mol) solution in a rectangular microchannel: 1μm particles focus at the channel center while 3μm particles focus near the channel side walls, which is completely different from the previously observed focusing at the channel center regardless of particle size. The effect of blockage ratio accounts for the deviation of large particles from the center. Utilizing this size-dependent differential migration, we successfully separate E. coli bacteria and red blood cells (RBCs) with both components tightly focused at different equilibrium positions in a straight microchannel, which has good parallelizability for higher throughput.

Understanding of the liquid overflow behavior inside micro-structured falling film reactors based on a wetting approach

This study investigates the behavior of gravity-driven liquid films inside falling film micro-reactors and focuses on the conditions necessary to obtain and break-up the complete liquid overflow from all of the channels. Micro-structured falling film reactors containing 6–10 straight parallel rectangular micro-channels were designed, constructed and tested. The main objective was to show experimentally the influence of liquid wettability on the hydrodynamic behavior. To achieve this, we explored the fluidic motion as a function of several parameters, including micro-channel size, the structural materials of the reactor (stainless steel, fluorocarbon and silicon oxide), liquid nature (water, water–ethanol mixtures and aqueous solutions of a ionic sodium dodecyl sulfate surfactant) and concentration levels. The major parameters that control the hydrodynamic behavior are the liquid/solid advancing and receding contact angles. The advancing contact angle governs the filling and the overflow of the channels, while the receding contact angle drives the characteristics of the break-up of the total overflow. Through experimentation, we identified a relationship between the contact angles and the resulting liquid flow patterns. We observed that spreading increases with higher ethanol content levels, surfactant concentration or by using a silicon oxide reactor. The flow rate required to achieve or break-up the total overflow decreased as a result. Finally, all of these results can be summarized by two master curves relating the critical flow rates to achieve and break-up the overflow with the advancing and receding contact angles, respectively. The channel dimensions also play a role in the hydrodynamic behavior, since the reduction of the channel width and distance between successive channels leads to a decrease in the critical flow rates.

Exploiting magnetic asymmetry to concentrate diamagnetic particles in ferrofluid microflows

Concentrating particles and cells for measurement or removal is often essential in many chemical and biological applications. Diamagnetic particle concentration has been demonstrated in magnetic fluids using two repulsive or attracting magnets, which in almost all cases are symmetrically positioned on the two sides of the particle-flowing channel. This work studies the effects of magnet asymmetry on the pattern and flow rate of diamagnetic particle concentration in ferrofluid flow through a straight rectangular microchannel. Two attracting permanent magnets with a fixed distance are each embedded on one side of the microchannel with a symmetric or an asymmetric configuration. A pair of symmetric counter-rotating circulations of concentrated particles is formed in the microchannel with a symmetric magnet configuration, which is found to grow in size and progress up the flow. In contrast, the single asymmetric circulation of concentrated particles formed in the microchannel with an asymmetric magnet configuration nearly maintains its size and position. Moreover, the magnet asymmetry is found to increase the ferrofluid flow rate for particle trapping, which is predicted by a three-dimensional theoretical model with a reasonable agreement.

Liquid slip and heat transfer in rotating rectangular microchannels

Although the no-slip boundary condition has been extensively used in conventional fluid flow problems, the recent experimental results show that liquid slip can occur in micro-sized channels with hydrophobic or superhydrophobic surfaces. By exploring the liquid slip phenomena, it is possible to reduce the hydrodynamic resistance inside a microchannel. In this article, we will study the water slip phenomena in rotating microchannels. The rotating microchannel is an essential part of centrifugal microfluidic platform, which has emerged as an advanced technique in biomedical applications and chemical separations. We have numerically investigated the flow and heat transfer inside a straight rectangular microchannel subjected to orthogonal rotation and slip boundary conditions at the channel walls. A pressure based finite volume technique in a staggered grid was applied to solve the steady incompressible Navier–Stokes and energy equations. A grid independence study was performed and the code was validated against benchmark problems. The numerical results showed that, for an orthogonally rotating microchannel, different slip velocities were induced at the left and right walls whereas identical slip velocities were produced in the bottom and top walls. In the fully developed region, the effect of secondary flow increased with the rotation but decreased with the slip length. A reduction of about 28.5–36% in hydrodynamic resistance was observed using a slip length of 10μm for channel aspect ratio α (=width/height)=1.0−20.0. Correlations for friction relation (fRe) as a function of slip length (λ) and rotational Reynolds number (Reω) was proposed. The combined effect of slip length and rotation on convective heat transfer parameters was also studied. It was found that, the liquid slip can increase or decrease the Nusselt number depending on the secondary flow effect and the aspect ratio of the microchannel.

Magnetic concentration of particles and cells in ferrofluid flow through a straight microchannel using attracting magnets

Concentrating particles to a detectable level is often necessary in many applications. Although magnetic force has long been used to enrich magnetic (or magnetically tagged) particles in suspensions, magnetic concentration of diamagnetic particles is relatively new and little reported. We demonstrate in this work a simple magnetic technique to concentrate polystyrene particles and live yeast cells in ferrofluid flow through a straight rectangular microchannel using negative magnetophoresis. The magnetic field gradient is created by two attracting permanent magnets that are placed on the top and bottom of the planar microfluidic device and held in position by their natural attractive force. The magnet–magnet distance is mainly controlled by the thickness of the device substrate and can be made small, allowing for the use of a dilute ferrofluid in the developed magnetic concentration technique. This advantage not only enables a magnetic/fluorescent label-free handling of diamagnetic particles, but also renders such handling biocompatible.

Analysis of Flow and Thermal Performance of a Water-Cooled Transversal Wavy Microchannel Heat Sink for Chip Cooling

With the increasing output power of the integrated circuit chips, the heat flux involved is being accordingly increased. In such situation, the air has almost reached its limit of cooling capacity, and thus the liquid cooling technology incorporating microchannel heat sinks is desired to cool the electronic chips in order to remove more heat loads. However, these microchannel heat sinks are often designed to be straight with rectangular cross section. In this study, on the basis of a straight microchannel having rectangular cross section, a kind of transversal wavy microchannel is designed and then the laminar flow and heat transfer are investigated numerically. It is shown that for removing the identical load, the transversal wavy microchannel has great potential to reduce pressure drop compared to the straight microchannel, especially for higher wave amplitude at the same Reynolds number, indicating the overall thermal performance of the transversal wavy microchannel is superior to the traditional straight rectangular microchannel. It is suggested such wavy microchannel can be used to cool chips effectively with much smaller pressure drop penalty.

Transient conjugate heat transfer in straight microchannels

Many theoretical and experimental studies have been carried out in order to study the flow and heat transfer in microchannels. In the recent years, numerical simulation has been applied to investigate the problem under a variety of conditions. However, much of the focus has been on steady-state problems and time-dependent transport has received relatively minor attention, despite its importance in practical electronic devices and systems. Employing a versatile commercial code, this paper aims to examine heat removal from straight rectangular microchannels affected by a time-dependent heat flux input. Both cosinusoidal variation and step-change application and removal of a uniform heat source are studied to determine the response time of the system. For the fluid phase, the two-dimensional momentum and energy equations are solved, considering temperature dependent properties and viscous dissipation. The effects of the amplitude of the heat flux variation, inlet velocity, and geometry, including the thickness of the heat sink, are investigated. Channels of smaller width are found to be more sensitive to the heat flux source, especially for higher input values. The velocity represents the most important parameter for channels of greatest width considered here, as it directly affects the fluid dynamics and the pressure drop when a time-dependent heat source is applied to the system.

Three-dimensional magnetic focusing of particles and cells in ferrofluid flow through a straight microchannel

Focusing particles and cells into a tight stream is often required in order for continuous flow detection, counting and sorting. So far a variety of particle focusing methods have been developed in microfluidic devices, among which magnetic focusing is still relatively new. We develop in this work an approach to embedding symmetrically two repulsive permanent magnets about a straight rectangular microchannel in a PDMS-based microfluidic device. The closest distance between the magnets is limited only by the sizes of the embedded and holder magnets involved in the fabrication process. The developed device is used to implement and investigate the three-dimensional magnetic focusing of polystyrene particles in ferrofluid microflow with both the top- and side-view visualizations. The effects of flow speed and particle size on the particle focusing effectiveness are studied. The developed device is also applied to magnetically focus yeast cells in ferrofluid, which proves to be biocompatible as verified by a cell viability test. In addition, an analytical model is developed and found to be able to predict the experimentally observed particle and cell focusing behaviors with reasonable agreement.

Deformability-selective particle entrainment and separation in a rectangular microchannel using medium viscoelasticity

We report a new phenomenon that particles are selectively entrained along the corners of a straight rectangular microchannel according to their deformabilities under viscoelastic flows of a polymer solution. The mechanism behind this phenomenon is elucidated by the competition between the wall lift force induced by the particle deformability and the elastic force of the medium at the corners. On the basis of these findings, we devise a novel label-free deformability-selective cell-separation method, which achieves the high-purity separation of rigid particles and rigidified red blood cells (RBCs) from fresh RBCs in a single step, without any active components such as an electric force or sophisticated channel design. Furthermore, our novel method can be used directly to isolate white blood cells (WBCs) from diluted whole blood with a high enrichment ratio (>300) by utilizing difference in cell deformability.

Sheathless elasto-inertial particle focusing and continuous separation in a straight rectangular microchannel

Particle focusing in planar geometries is essentially required in order to develop cost-effective lab-on-a-chips, such as cell counting and point-of-care (POC) devices. In this study, a novel method for sheathless particle focusing, called "Elasto-Inertial Particle Focusing", was demonstrated in a straight microchannel. The particles were notably aligned along the centerline of the straight channel under a pressure-driven flow without any additional external force or apparatus after the addition of an elasticity enhancer: PEO (poly(ethylene oxide)) (∼O(100) ppm). As theoretically predicted (elasticity number: El≈O(100)), multiple equilibrium positions (centerline and corners) were observed for the viscoelastic flow without inertia, whereas three-dimensional particle focusing only occurred when neither the elasticity nor the inertia was negligible. Therefore, the three-dimensional particle focusing mechanism was attributed to the synergetic combination of the elasticity and the inertia (elasticity number: El≈O(1-10)). Furthermore, from the size dependence of the elastic force upon particles, we demonstrated that a mixture of 5.9 and 2.4 µm particles was separated at the exit of the channel in viscoelastic flows. We expect that this method can contribute to develop the miniaturized flow cytometry and microdevices for cell and particle manipulation.

Microfluidic on-chip fluorescence-activated interface control system.

A microfluidic dynamic fluorescence-activated interface control system was developed for lab-on-a-chip applications. The system consists of a straight rectangular microchannel, a fluorescence excitation source, a detection sensor, a signal conversion circuit, and a high-voltage feedback system. Aqueous NaCl as conducting fluid and aqueous glycerol as nonconducting fluid were introduced to flow side by side into the straight rectangular microchannel. Fluorescent dye was added to the aqueous NaCl to work as a signal representing the interface position. Automatic control of the liquid interface was achieved by controlling the electroosmotic effect that exists only in the conducting fluid using a high-voltage feedback system. A LABVIEW program was developed to control the output of high-voltage power supply according the actual interface position, and then the interface position is modified as the output of high-voltage power supply. At last, the interface can be moved to the desired position automatically using this feedback system. The results show that the system presented in this paper can control an arbitrary interface location in real time. The effects of viscosity ratio, flow rates, and polarity of electric field were discussed. This technique can be extended to switch the sample flow and droplets automatically.

On the calibration of astigmatism particle tracking velocimetry for microflows

Astigmatism particle tracking velocimetry (APTV) is a method to determine three components (3C) of the velocity field in a volume (3D) using a single camera. The depth position of the particles is coded by optical distortions caused by a cylindrical lens in the optical setup. This technique is particularly suited for microfluidic applications as measurement errors due to spatial averaging and depth of correlation, typically encountered with μPIV approaches, are eliminated so that the measurement precision is enhanced. Unfortunately, the current state of the technique is limited by the small measurement region achievable with the current calibration procedures as well as by higher order image aberrations (Cierpka et al 2010 Meas. Sci. Technol. 21 045401). In order to extend the size of the measurement volume and to account for all image aberrations, a new intrinsic calibration procedure, based on the imaging function of the particles, is proposed in the paper at hand. It provides an extended measurement depth, taking into account all image aberrations. In this work, the calibration procedure was applied to a μPIV arrangement but could also be implemented on macroscopic experimental setups. The calibration procedure is qualified with synthetic data as well as Poiseuille flow in a straight rectangular micro-channel with a cross-sectional area of 200 × 500 µm2. The three-dimensional velocity distribution of the whole channel was resolved via APTV with uncertainties of 0.9% and 3.7% of the centerline velocity, uc, for the in-plane and out-of-plane components, respectively. Further investigations using different cylindrical-lens focal lengths, magnifications and particle sizes provide information about achievable measurement depths and help to design and adapt the optimal system for the desired experiment.

Microfluidic switch based on combined effect of hydrodynamics and electroosmosis

This paper presents theoretical and experimental investigations on valveless microfluidic switch using the coupled effect of hydrodynamics and electroosmosis. Switching of a non-conducting fluid stream is demonstrated. The first part of the investigation focused on flow switching of a non-conducting fluid, while the second part focused on switching of aqueous liquid droplets in a continuous oil stream. Two sheath streams (aqueous NaCl and glycerol) and a sample stream (silicon oil) are introduced by syringe pumps to flow side by side in a straight rectangular microchannel. External electric fields are applied on the two sheath streams. The switching process using electroosmotic effect for different flow rate and viscosity of sample stream is investigated. The results indicate that the switching response time is affected by the electric fields, flow rate, and viscosity of the sample. At constant inlet volumetric flow rates, the sample streams or droplets can be delivered to the desired outlet ports using applied voltages.