Straight Rectangular Microchannel Research Articles

Nonlinear Electrophoresis of Microparticles in Shear Thinning Fluids.

The nonlinear electric field dependence of particle electrophoresis has been demonstrated to occur in Newtonian fluids for highly charged particles under large electric fields. It has also been predicted to arise from the rheological effects of non-Newtonian fluids even at small electric fields. We present in this work an experimental verification of nonlinear electrophoresis in shear thinning xanthan gum solutions through a straight rectangular microchannel. The addition of polymer into a Newtonian buffer solution is found to change the electric field dependence from linear to superlinear for electroosmotic, electrokinetic, and electrophoretic velocities. The nonlinear index of each of these electrokinetic phenomena increases with the increasing polymer or buffer concentration, among which electrophoresis exhibits the strongest nonlinearity. Both these observed trends are captured by a dimensionless electrokinetic shear thinning number that depends on the power-law index of fluid viscosity and the Debye length.

Characteristics analysis and structure optimization of a hybrid micro-jet impingement/micro-channel heat sink

To solve the heat dissipation problem of high-power electronics, a novel hybrid micro-jet impingement/micro-channel heat sink is proposed in this work. Different from the continuous and straight rectangular micro-channel, the inclined and interrupted channel walls (fins) are adopted to enhance the fluid disturbance inside the heat sink. To obtain the thermal–hydraulic performance of the hybrid heat sink, an overall 3D modeling and numerical simulations are conducted instead of local cell numerical simulation. In addition, to further improve the heat transfer and flow resistance performance, the effects of variation of structural parameters on heat transfer and flow resistance properties are investigated, including the fin inclination angle, fin length and fin height. The results of structural parametric analysis reveal that there are interactions between different structural parameters, but with different levels of significance for different evaluation indicators. Then, to obtain the optimal combinations of structural parameters for different applications, the multi-objective genetic algorithm coupled with artificial neural network (ANN) is utilized to search for the optimal value of each structural parameter. The heat transfer thermal resistance, pressure drop and mean absolute temperature deviation of the heated surface are utilized as the optimization objectives to evaluate the cooling and resistance characteristics. The optimization results obtained by three-objective optimization and two-objective optimization are analyzed and compared. Finally, seven optimal compromise solutions are selected from the Pareto front by using TOPSIS algorithm with different weight factor matrices. For the optimal compromise solutions, the maximum relative errors between the predicted values of the ANN and numerical simulation values are only 3%, which further demonstrates the accuracy of the artificial neural network model and the effectiveness of using multi-objective optimization to guide heat sink design.

Numerical analysis on thermal-hydraulic performance of optimized microchannel heat sink with slant ribs and quatrefoil rib-elliptical groove complex structures

A novel microchannel heat sink (MCHS-SREGQR) arranged with the spoiler element structure of the slant rib-elliptical groove-quatrefoil rib is proposed to meet the heat dissipation demands of electronic devices with high heat flux. The strategies for heat transfer enhancement are as follows: Firstly, the elliptical grooves are periodically arranged in the sidewalls of the microchannel and the quatrefoil ribs are set one-to-one at the centre of the grooves. The two structures complement each other's strengths and weaknesses. Secondly, the slant ribs are symmetrically arranged along the centreline of the channel. The slant ribs tend to expand along the flow direction, and produce diversion, guiding and gathering effects on the fluid, thus enhancing flow efficiency of the fluid domain in all aspects. Lastly, a novel quatrefoil rib is put forward, with a special structural design that both effectively inhibits the flow dead regions around the corners of the rib and significantly increases fluid turbulence. Three-dimensional numerical simulations are used to evaluate the effects of slant rib relative position, slant rib relative transverse spacing, and overall rib relative height on pressure drop, heat transfer capacity, and temperature field uniformity of the microchannel. Furthermore, the hydrothermal performance of MCHS-SREGQR is analyzed in comparison with a traditional straight rectangular microchannel and six other microchannels arranged with similar spoiler elements. The results indicate that when the Reynolds number is 696, the temperature at the heat source surface of MCHS-SREGQR decreases by 52.3% and the thermal resistance decreases by 66.6%, while the Nusselt number increases by 131% and the temperature uniformity increases by 76.8%. In addition, the field coordination number Fc increases by 131% and the performance evaluation criterion PEC can be achieved as 1.179.

Optimal Design of Microchannel Heat Sink with Staggered Fan-Shaped Cavities and Ribs

In the present study, a three-dimensional numerical simulation is conducted to study the characteristics of fluid flow and heat transfer in a newly designed microchannel heat sink with fan-shaped cavities and ribs (MC-FCFR). The Reynolds number ranges from 147 to 736. A straight rectangular microchannel (MC-RC) is used as the baseline. The geometric parameters include the relative spacing θ, relative width α, and relative height β of the fan-shaped cavities and ribs. The overall performance index (OPI) of the MC-FCFR is evaluated in terms of the friction factor and Nusselt number. The OPI for the MC-FCFR with θ=3.56, α=1.63, and β=0.5 reaches 1.70.

Flow thermohydraulic characterization of open diverging microchannel heat sink for high heat flux dissipation

The future fusion reactor puts forward higher demand for the heat dissipation capability of the divertor, which requires to withstand ultra-high heat flux of up to 20 MW/m2. Our previous study demonstrated that microchannel cooling technology is a promising strategy to remove high heat loads on the target plate of the divertor. However, the high pressure drop of the straight rectangular microchannel limits its practical implementation. In this study, we propose an open diverging microchannel heat sink (ODMHS) to reduce the pressure drop under high heat flux and high flow rate conditions. The effects of the width and height of expanding channel on the heat transfer performance and pressure drop were first studied numerically. The results indicated that expanding channel width resulted in a reduction of heat transfer coefficient, whereas the expansion of the channel height led to the enhancement of heat transfer performance. Regardless of the divergence of channel width or height, the enlargement of the channel dimension can effectively reduce the pumping power. Subsequently, experimental tests were carried out in ODMHS with a width ratio and a height ratio of 1.33 and 1.30, respectively. The results demonstrated that the ODMHS can withstand heat flux of up to 15.6 MW/m2 at 3 L/min. This study provides important insights for high heat flux dissipation on the target plate of the divertor by rational design of microchannel structures.

Hydrothermal performance analysis of microchannel heat sink with embedded module with ribs and pin-fins

A new microchannel heat sink with embedded modules with ribs and pin-fins is proposed as an effective solution to realize microchannel heat dissipation within low-temperature co-fired ceramic substrates. Enhance heat dissipation with the following strategies: 1. microchannel cooling within the low-temperature co-fired ceramic substrate to shorten the heat transfer path; 2. embedding embedded modules in the substrate to reduce the overall thermal resistance; 3. creating ribs and pin-fins on the embedded modules to enhance convective heat transfer. The impacts of relative rib height, relative pin-fin height, and relative number of auxiliary channels on the heat transfer performance, temperature uniformity, and pressure drop were investigated. The performance of the proposed design has been compared to the performance of similar designs like straight rectangular microchannels, microchannels with pin-fins embedded modules, and microchannels with ribs embedded modules. Under the condition of Reynolds number 458, the heat transfer performance is improved by 63.91%, the temperature uniformity is improved by 67.65% and the pressure drop is increased by only 10.24%. The heat transfer performance of the microchannel heat sink with embedded modules with ribs and pin-fins can be effectively improved by adjusting the height of the pin-fins and the ribs height on the embedded module.

Comparative study on the thermal performance of microencapsulated phase change material slurry in tortuous geometry microchannel heat sink

The microencapsulated phase change material slurry (MPCMS) exhibits improved performance in microchannels as a result of its increased latent heat and thermal conductivity. This study investigates the thermo-hydraulic performance of paraffin encapsulated with titanium and a silica shell in straight, wavy, and zigzag rectangular microchannel configurations. The length of the heat sink is 30 mm and its hydraulic diameter is 0.4 mm. A uniform heat flux is supplied at various mass flow rates of 200–400 ml/min with a Reynolds number corresponding to 400–800 at 10, 20, and 30 W/cm2 respectively. Using prepared MPCMS, the performance of selected microchannels is evaluated. The experimental result demonstrates that the MPCMS improves the heat transfer rate of all configurations relative to deionized water (DI). At conditions of medium heat flux (i.e., 20 W/cm2) and high flow rate (i.e., Re = 780), the low titanium (Ti) encapsulated slurry-based straight microchannel has a higher Nusselt number (i.e., 6.5%) than the DI. This enhancement is further improved by 21.9% and 27% in wavy and zigzag microchannels, respectively. Under the same conditions, the zigzag microchannel with low Ti concentration MPCM (i.e., 1%) demonstrates 77% and 234.7% higher performance than the wavy and straight channels, respectively.

Nonlinear electrophoresis of dielectric particles in Newtonian fluids.

In classical electrokinetics, the electrophoretic velocity of a dielectric particle is a linear function of the applied electric field. Theoretical studies have predicted the onset of nonlinear electrophoresis at high electric fields because of the nonuniform surface conduction over the curved particle. However, experimental studies have been left behind and are insufficient for a fundamental understanding of the parametric effects on nonlinear electrophoresis. We present in this work a systematic experimental study of the effects of buffer concentration, particle size, and particle zeta potential on the electrophoretic velocity of polystyrene particles in a straight rectangular microchannel for electric fields of up to 3kV/cm. The measured nonlinear electrophoretic particle velocity is found to exhibit a 2(±0.5)-order dependence on the applied electric field, which appears to be within the theoretically predicted 3- and 3/2-order dependences for low and high electric fields, respectively. Moreover, the obtained nonlinear electrophoretic particle mobility increases with decreasing buffer concentration (for the same particle) and particle size (for particles with similar zeta potentials) or increasing particle zeta potential (for particles with similar sizes). These observations are all consistent with the theoretical predictions for high electric fields.

Computational study of inertial migration of prolate particles in a straight rectangular channel

Inertial migration of spherical particles has been investigated extensively using experiments, theory, and computational modeling. Yet, a systematic investigation of the effect of particle shape on inertial migration is still lacking. Herein, we numerically mapped the migration dynamics of a prolate particle in a straight rectangular microchannel using smoothed particle hydrodynamics at moderate Reynolds number flows. After validation, we applied our model to 2:1 and 3:1 shape aspect ratio particles at multiple confinement ratios. Their effects on the final focusing position, rotational behavior, and transitional dynamics were studied. In addition to the commonly reported tumbling motion, for the first time, we identified a new logrolling behavior of a prolate ellipsoidal particle in the confined channel. This new behavior occurs when the confinement ratio is above an approximate threshold value of K = 0.72. Our microfluidic experiments using cell aggregates with similar shape aspect ratio and confinement ratio confirmed this new predicted logrolling motion. We also found that the same particle can undergo different rotational modes, including kayaking behavior, depending on its initial cross-sectional position and orientation. Furthermore, we examined the migration speed, angular velocity, and rotation period as well as their dependence on both particle shape aspect ratio and confinement ratio. Our findings are especially relevant to the applications where particle shape and alignment are used for sorting and analysis, such as the use of barcoded particles for biochemical assays through optical reading, or the shape-based enrichment of microalgae, bacteria, and chromosomes.

Length-based separation of Bacillus subtilis bacterial populations by viscoelastic microfluidics

In this study, we demonstrated the label-free continuous separation and enrichment of Bacillus subtilis populations based on length using viscoelastic microfluidics. B. subtilis, a gram-positive, rod-shaped bacterium, has been widely used as a model organism and an industrial workhorse. B. subtilis can be arranged in different morphological forms, such as single rods, chains, and clumps, which reflect differences in cell types, phases of growth, genetic variation, and changing environmental factors. The ability to prepare B. subtilis populations with a uniform length is important for basic biological studies and efficient industrial applications. Here, we systematically investigated how flow rate ratio, poly(ethylene oxide) (PEO) concentration, and channel length affected the length-based separation of B. subtilis cells. The lateral positions of B. subtilis cells with varying morphologies in a straight rectangular microchannel were found to be dependent on cell length under the co-flow of viscoelastic and Newtonian fluids. Finally, we evaluated the ability of the viscoelastic microfluidic device to separate the two groups of B. subtilis cells by length (i.e., 1–5 μm and >5 μm) in terms of extraction purity (EP), extraction yield (EY), and enrichment factor (EF) and confirmed that the device could separate heterogeneous populations of bacteria using elasto-inertial effects.

Separation and Enrichment of Yeast Saccharomyces cerevisiae by Shape Using Viscoelastic Microfluidics.

Yeast Saccharomyces cerevisiae (S. Cerevisiae) is one of the most attractive microbial species used for industrial production of value-added products and is an important model organism to understand the biology of the eukaryotic cells and humans. S. Cerevisiae has different shapes, such as spherical singlets, budded doublets, and clusters, corresponding to phases of the cell cycle, genetic, and environmental factors. The ability to obtain high-purity populations of uniform-shaped S. Cerevisiae cells is of significant importance for a wide range of applications in basic biological research and industrial processes. In this work, we demonstrate shape-based separation and enrichment of S. Cerevisiae using a coflow of viscoelastic and Newtonian fluids in a straight rectangular microchannel. Due to the combined effects of lift inertial and elastic forces, this label-free and continuous separation arises from shape-dependent migration of cells from the Newtonian to the non-Newtonian viscoelastic fluid. The lateral position of S. Cerevisiae cells with varying morphologies is found to be dependent on cell major axis. We also investigate the effects of sheath and sample flow rate, poly(ethylene oxide) (PEO) concentration and channel length on the performance of the viscoelastic microfluidic device for S. Cerevisiae enrichment and separation by shape. Moreover, the separation efficiency, cell extraction yield, and cell viability after sorting operations are studied.

Particle separation in xanthan gum solutions

Label-free separation of particles by an intrinsic property can be implemented in microfluidic devices through either an externally imposed field or an inherent flow-induced force. Among the latter type of passive techniques, elastic or elasto-inertial lift-based particle separation in non-Newtonian fluids has received a rapidly growing interest in the past decade. However, current demonstrations of particle separation in non-Newtonian fluids have all taken place in viscoelastic polymer or biological solutions. We demonstrate for the first time a continuous sheath-free separation of polystyrene particles in the flow of weakly elastic xanthan gum (XG) solution through a simple straight rectangular microchannel. This separation is fundamentally different from that in the flow of viscoelastic solutions. We explain the observed particle migrations in XG solutions using the competition of a strong wall-directed (because of the strong shear thinning effect) and a small center-directed (because of the weak elasticity effect) lateral force induced by normal stresses in a Poiseuille flow.

The motion of rigid particles in the Poiseuille flow of pseudoplastic fluids through straight rectangular microchannels

There has been in the past decade a significantly growing interest in the use of flow-induced lift forces for a passive control of particle motion in microchannels. This nonlinear microfluidic technique can be implemented in both Newtonian and non-Newtonian fluids. The motions of rigid particles in confined flows of viscoelastic fluids with and without shear-thinning effects have each been well studied in the literature. However, a comprehensive understanding of particle motion in inelastic shear-thinning fluids is still lacking. We present herein a systematic experimental study of the motion of rigid particles in the Poiseuille flow of pseudoplastic xanthan gum (XG) solutions through straight rectangular microchannels. We find that the number and location of particle equilibrium positions are both a strong function of channel dimension, particle size and XG concentration. We attempt to explain the experimental observations using the competition of inertial and elastic lift forces acting on particles. Our experimental results imply a potentially high throughput separation of rigid particles by size in XG solutions.

Experimental study of particle electrophoresis in shear-thinning fluids

Electric field is the method of choice in microfluidic devices for precise transport and placement of particles via fluid electroosmosis and particle electrophoresis. However, current studies on particle electrophoresis in microchannels have been focused mainly upon Newtonian fluids though many of the chemical and biological fluids possess non-Newtonian characteristics. Especially lacking is the experimental study of particle electrophoresis in either type of fluids. We report in this work an unprecedented experimental observation that polystyrene particles migrate towards the walls of straight rectangular microchannels in the electroosmotic flow of shear-thinning xanthan gum (XG) solutions. This phenomenon is opposite to the particle focusing along the channel center in our control experiment with the Newtonian base fluid of the XG solutions. It is attributed to a fluid shear thinning-induced lift that overcomes the wall-induced repulsive electrical lift. The parametric effects of the fluid-particle-channel-(electric) field system are systematically investigated.

Revisit of wall-induced lateral migration in particle electrophoresis through a straight rectangular microchannel: Effects of particle zeta potential.

Previous studies have reported a lateral migration in particle electrophoresis through a straight rectangular microchannel. This phenomenon arises from the inherent wall-induced electrical lift that can be exploited to focus and separate particles for microfluidic applications. Such a dielectrophoretic-like force has been recently found to vary with the buffer concentration. We demonstrate in this work that the particle zeta potential also has a significant effect on the wall-induced electrical lift. We perform an experimental study of the lateral migration of equal-sized polystyrene particles with varying surface charges under identical electrokinetic flow conditions. Surprisingly, an enhanced focusing is observed for particles with a faster electrokinetic motion, which indicates a substantially larger electrical lift for particles with a smaller zeta potential. We speculate this phenomenon may be correlated with the particle surface conduction that is a strong function of particle and fluid properties.

On nonequilibrium shrinkage of supercritical CO2 droplets in a water-carrier microflow

We report an experimental study on the hydrodynamic shrinkage of supercritical carbon dioxide (scCO2) microdroplets during a nonequilibrium process. After scCO2 microdroplets are generated by water shearing upon a scCO2 flow in a micro T-junction, they are further visualized and characterized at the midpoint and the ending point of a straight rectangular microchannel (width × depth × length: 150 μm × 100 μm × 1.5 mm). The measured decreases in droplet size by 8%–36% indicate and simply quantify the droplet shrinkage which results from the interphase mass transfer between the droplet and the neighboring water. Using a mathematical model, the shrinkage of scCO2 droplets is characterized by solvent-side mass transfer coefficients (ks: 1.5 × 10−4–7.5 × 10−4 m/s) and the Sherwood number (Sh: 7–37). In general, ks here is two orders of magnitude larger than that of hydrostatic liquid CO2 droplets in water. The magnitude of Sh numbers highlights the stronger effect of local convections than that of diffusion in the interphase mass transfer. Our results, as reported here, have essential implications for scCO2-based chemical extractions and carbon storage in deep geoformations.

Tunable, Sheathless Focusing of Diamagnetic Particles in Ferrofluid Microflows with a Single Set of Overhead Permanent Magnets.

There has been increasing interest in the use of magnetic fluids to manipulate diamagnetic particles in microfluidic devices. Current methods for diamagnetic-particle focusing in magnetic fluids require either a pair of repulsive magnets or a diamagnetic sheath flow. We demonstrate herein a tunable, sheathless focusing of diamagnetic particles in a microchannel ferrofluid flow with a single set of overhead permanent magnets. Particles are focused into a single stream near the bottom wall of a straight rectangular microchannel, where a magnetic-field minimum is formed as a result of the magnetization of the ferrofluid. This focusing can be readily switched off and on by removing and replacing the permanent magnets. More importantly, the particle-focusing position can be tuned by shifting the magnets with respect to the microchannel. We perform a systematic experimental study of the parametric effects of the fluid-particle-channel system on diamagnetic-particle focusing in terms of a defined particle-focusing effectiveness.

Fluid rheological effects on particle migration in a straight rectangular microchannel

There has recently been a significantly increasing interest in the passive manipulation of particles in the flow of non-Newtonian fluids through microchannels. However, an accurate and comprehensive understanding of the various fluid rheological effects on particle migration is still largely missing. We present in this work a systematic experimental study of both the individual and the combined effects of fluid inertia, elasticity, and shear thinning on the motion of rigid spherical particles in a straight rectangular microchannel. We first study the sole effect of each of these rheological properties in a Newtonian fluid, purely elastic (i.e., Boger) fluid, and purely shear-thinning (i.e., pseudoplastic) fluid, respectively. We then study the combined effects of two or all of these rheological properties in a pseudoplastic fluid and two types of elastic shear-thinning fluids, respectively. We find that the fluid elasticity effect directs particles toward the centerline of the channel while the fluid shear-thinning effect causes particle migration toward both the centerline and corners. These two effects are combined with the fluid inertial effect to understand the particle migration in inertial pseudoplastic and viscoelastic fluid flows.

Surface-conduction enhanced dielectrophoretic-like particle migration in electric-field driven fluid flow through a straight rectangular microchannel

An electric field has been extensively used to manipulate fluids and particles via electrokinetic flow in microchannels and nanochannels for various lab-on-a-chip applications. Recent studies have demonstrated the action of a dielectrophoretic-like lift force on near-wall particles in an electrokinetic flow due to the particles’ modifications of the field-line structure adjacent to a planar wall. This work presents a fundamental investigation of the lateral migration of dielectric particles in the electrokinetic flow of buffer solutions of varying molar concentrations through a straight rectangular microchannel. We find that the particle migration-induced electrokinetic centerline focusing is significantly enhanced with the decrease of the buffer concentration. This observed phenomenon may be attributed to the increased surface conduction effect in a lower-concentration buffer that yields a larger Dukhin number, Du. It seems qualitatively consistent with a recent theoretical study that predicts a greater wall-induced electrical lift with the increasing value of the Dukhin number for Du ≥ 1.

Numerical study of enhanced mixing in pressure-driven flows in microchannels using a spatially periodic electric field.

We propose an innovative mechanism for enhancing mixing in steady pressure driven flow of an electrolytic solution in a straight rectangular microchannel. A transverse electric field is used to generate an electroosmotic flow across the cross-section. The resulting flow field consists of a pair of helical vortices that transport fluid elements along the channel. We show, through numerical simulations, that chaotic advection may be induced by periodically varying the direction of the applied electric field along the channel length. This periodic electric field generates a longitudinally varying, three-dimensional steady flow, such that the streamlines in the first half of the repeating unit cell intersect those in the second half, when projected onto the cross-section. Mixing is qualitatively characterized by tracking passive particles and obtaining Poincaré maps. For quantification of the extent of mixing, Shannon entropy is calculated using particle advection of a binary mixture. The convection diffusion equation is also used to track the evolution of a scalar species and quantify the mixing efficiency as a function of the Péclet number.