Straight Microchannel Research Articles

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.

Numerical investigation into the thermo-fluid performance of wavy microchannels with superhydrophobic walls

As an efficient cooling method, wavy microchannel heat sink (W-MCHS) with superhydrophobic walls is suggested as an alternative to a straight microchannel heat sink (S-MCHS) with conventional walls. Navier-Stokes and Energy equations with slip boundary conditions (velocity slip and temperature jump) are solved to study the hydraulic and thermal performances of the microchannels. It is observed that based on the “goodness factor” criterion, the overall performance of the new heat sink design is improved by 47.3%. In fact, the pressure drop reduction accompanying the use of superhydrophobic walls outweighs the thermal performance degradation due to the presence of trapped layers of air. It is also observed that with an increase in the waviness of the channel (either by an increase in the wave amplitude or a decrease in the wavelength), the pressure drop reduction by the use of superhydrophobic walls intensifies as the maximum velocity shifts from the centre of the channel towards the crest regions. This, in turn, increases the velocity gradient at the wall which excites the velocity slip and results in a better hydraulic performance. On the other hand, the thermal performance is deteriorated to a greater extent (compared to the hydraulic improvement) when the channel waviness increases, due to the elongated flow path and the related increase in the trapped layers of air. As a result, as long as the cooling performance of the heat sink with superhydrophobic walls is concerned, microchannels with lower waviness are desirable.

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.

Separation of Motile Euglena Using Microchannel

Euglena is a unique microorganism that combines characteristics of animals and plants and expected as new biofuel and microbial actuator. When the euglena is used for these purposes, high motility cells must be important. Conventionally, when taking out target microorganism , micropipette was used but it was time-consuming and there was a risk of damaging microorganism. In this study, we propose using microchannel to separate motile euglena efficiently and to avoid damaging. In order to achieve this purpose, first, we investigated the behavior of Euglena in flow. We quantitatively evaluated Euglena’s swimming mode and diffusion coefficient in a straight microchannel（width1000µm，hight100µm） using image analysis. We found that as the flow velocity rises，fraction of Euglena which swims parallel in flow increases. In addition, we found that Euglena has rheotaxis. Rheotaxis fraction increases rapidly around flow rate of 80µm/s and decreases over flow rate of 120µm/s. We analyzed Euglena’s diffusion coefficient with Particle Tracking Velocimetry (PTV) using ImageJ. Euglena’s diffusion coefficient rises as the flow velocity increases, but it decreases at flow rate of 60µm/s. This phenomenon seems to be due to Euglena’s rheotaxis. As a summary so far, we found that Euglena was strongly influenced by flow velocity, and the motion depends on the flow rate. Based on these experimental results, we made separating microchannel by soft lithography that using 3D printed mold. As a result, we succeeded in separating motile Euglena which is 12% higher on average swimming speed.

Propagation and extinction behavior of methane/air premixed flames through straight and converging-diverging microchannels

Propagation and extinction behavior of a CH4/air premixed flame passing through straight and converging-diverging (C-D) microchannels (diameters ranging from 1 to 10 mm) were investigated both experimentally and numerically. The dynamic behavior of flame propagation inside the channels was experimentally studied by using CH∗ chemiluminescence and direct imaging. Three patterns of flame propagation were observed, i.e., the flame can survive, partially extinguish and then re-ignite downstream, or completely extinguish. Regime diagrams showing these different patterns as functions of channel geometry and equivalence ratio were generated for both the straight and C-D channels. Numerical simulations were carried out to explain the experimentally observed flame dynamics inside the channels using detailed CH4/air chemistry. In general, flames were easier to extinguish in C-D channels than in straight channels for a fixed channel diameter and equivalence ratio. Additionally, flames were harder to survive in C-D channels with larger exit-to-throat area ratios. Both heat loss and flame stretch were key factors that can generally cause flame extinction in narrow C-D channels. Heat loss was found to be the primary reason for flame extinction inside the microchannel in comparison with the stretch effect.

Pulsatile flow micromixing coupled with ICEO for non-Newtonian fluids

The most biofluids/polymers, which are extensively used in lab-on-a-Chip devices exhibit non-Newtonian characteristics, whose mixing behaviors become crucial for the most engineering applications. For this purpose, the present numerical study investigates the performance of pulsatile flow micromixer coupled with induced-charge electro-osmosis (ICEO) for non-Newtonian fluids, such as power-law and Carreau. The mixing characteristics of Newtonian fluids are also presented and compared to non-Newtonian cases. The straight microchannel geometry is modified using rectangular obstruction with various heights to increase stirring performance for non-Newtonian fluids. The magnitude of the external electric field, the frequency of pulsatile flow and the width of rectangular obstruction are systematically varied for a comprehensive parametric evaluation. Time-dependent patterns of streamline and corresponding concentration maps are illustrated for qualitative presentation. Moreover, mixing index values are plotted versus obstruction width, the frequency of pulsatile flow and imposed electric field strength for quantitative analysis. It is observed that pulsatile flow with ICEO yields reasonable performance for all type of fluids. However, a rectangular obstruction is required to obtain maximum performance of mixing of non-Newtonian fluids for the given time, t = 10 s and length, l = 0.7 mm. It can be reported that rectangular obstruction has a significant effect on mixing performance for current conditions.

Novel thermal model of microchannel cooling system designed for fast simulation of liquid-cooled ICs

The research and design of liquid-cooled integrated circuits (IC) relies heavily on accurate simulation. Ideally, finite-element-method (FEM) based tools should be used for this purpose. However, in most cases a fully coupled thermo-fluidic simulation of complex ICs is very long, mostly due to the fact that the simulation of fluid dynamics is very time consuming. Therefore, in this paper we propose a novel model for thermal simulation of ICs cooled by integrated microchannels which significantly reduces the simulation time. The new approach is based on removing the fluid from the model and treating the solid-liquid boundary as a convective boundary, described by the convection equation. It is shown that the proposed model offers very good accuracy in steady-state, with errors below 3 °C in every chip point. In transient domain the results are equally good, with the maximum error around 3.3 °C (6% relative error). Moreover, the simulation times have been reduced by about two orders of magnitude with respect to fully coupled FEM simulation. However, the proposed model has also important limitations: currently ICs with only one layer of straight microchannels are supported and the model requires recalculation if some chip parameters are changed.

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.

Process intensification of honeycomb fractal micro-reactor for the direct production of lower olefins from syngas

Inspired by the nature fractal structure, a novel honeycomb fractal micro-reactor was designed and applied to intensify both heat and mass transfer in the exothermic conversion of syngas to olefins. The results show that the honeycomb fractal micro-reactor provides more even distribution of temperature profiles, a narrower reactant residence time distribution, and relative smaller pressure drop per unit length, compared with parallel straight microchannel reactor and mini-fixed bed reactor under the same reaction conditions. Owning to the reinforcement of flow separation and convergence in the honeycomb structure, the contact between reactants and catalyst particles were enhanced, leading to a better heat and mass transfer. Under the current range of temperature, pressure and standard GHSV, the honeycomb fractal micro-reactor leads to supreme CO conversion and lower olefins yield.

Experimental investigations on biopolymer in enhancing the liquid flow in microchannel

AbstractDrag reduction has been incorporated in various industrial fields. Most of the works proved that drag reduction is efficient in turbulent flow. It is also observed that polymers can enhance the laminar flow which could be a milestone in medical field. In this work, five straight microchannels with fixed depth of 100, 50, and 60 μm in width and 200, 300, 400, and 500 μm in length were designed and fabricated using direct writing method. Xanthan gum as bio‐based drag reducing additive was chosen and diluted with deionized water to investigate its feasibility in enhancing the laminar flow in the microchannel. Eight different concentrations of xanthan gum ranging from 20 to 500 ppm were used to evaluate the effect of concentration on drag reduction performance using pressure measurement. The maximum flow increment of 34.90% was achieved by utilizing 500 ppm of xanthan gum at an operating pressure of 100 mbar in the microchannel with a width of 500 μm.

Numerical Study on Single Flowing Liquid and Supercritical CO2 Drop in Microchannel: Thin Film, Flow Fields, and Interfacial Profile

Taylor segments, as a common feature in two- or multi-phase microflows, are a strong flow pattern candidate for applications when enhanced heat or mass transfer is particularly considered. A thin film that separates these segments from touching the solid channel and the flow fields near and inside the segment are two key factors that influence (either restricting or improving) the performance of heat and mass transfer. In this numerical study, a computational fluid dynamics (CFD) method and dense carbon dioxide (CO2) and water are applied and used as a fluid pair, respectively. One single flowing liquid or supercritical CO2 drop enclosed by water is traced in fixed frames of a long straight microchannel. The thin film, flow fields near and within single CO2 drop, and interfacial distributions of CO2 subjected to diffusion and local convections are focused on and discussed. The computed thin film is generally characterized by a thickness of 1.3~2.2% of the channel width (150 µm). Flow vortexes are formed within the hydrodynamic capsular drop. The interfacial distribution profile of CO2 drop is controlled by local convections near the interface and the interphase diffusion, the extent of which is subject to the drop size and drop speed as well.

Heat Transfer Characteristics and Flow Visualization during Flow Boiling of Acetone in Semi-Open Multi-Microchannels

ABSTRACTExperimental results of flow boiling characteristics and flow patterns with acetone in two different microchannel heat sinks are presented in this paper. A semi-open microchannel heat sink and a straight microchannel heat sink with 19 parallel microchannels each were designed and tested. The semi-open microchannels have a channel width of 0.8 mm, fin width of 0.4 mm, and pedestal height of 0.2 mm and the straight microchannels have a rectangular cross section of 0.8 mm × 1 mm. The experimental heat fluxes ranged from 0 to 90 kW/m2, vapor quality ranged from 0.05 to 0.5, mass fluxes ranged from 4.34 to 15.62 kg/m2·s and the inlet temperatures were 20 and 30°C, respectively. Compared to those in the straight microchannels, flow boiling heat transfer coefficients can be improved by up to 36.2%. Furthermore, flow patterns were observed with a speed video camera. The flow boiling heat transfer mechanisms are analyzed according to the observed flow patterns.

Heat Transfer Characteristics of CO2 Desorption from N‐Methyldiethanolamine Solution in a Microchannel Reactor

AbstractThe heat transfer performance and energy consumption of CO2 desorption from rich N‐methyldiethanolamine (MDEA) solution were determined experimentally in a straight microchannel reactor. Nucleate boiling was found to be the dominant heat transfer mechanism in this experiment. The heat transfer coefficients were strongly dependent on the heat flux. The solution flow rate was the most influential factor on the heat flux, followed by desorption temperature, MDEA concentration, and CO2 loading. In addition, an empirical correlation was proposed to predict the experimental heat transfer coefficients.

Thermal-hydraulic performance of a tapered microchannel

Tapered microchannel is a channel configuration design (CCD) which has gained attention owing to the superior performance in promoting uniform temperature distribution in a microchannel. In this study, the thermal-hydraulic performance of four different tapered channels is evaluated against a straight microchannel. The average hydraulic diameter, conductive heat transfer in the substrate material and convective heat transfer area are kept constant for the microchannels. The experiment is conducted for steady-state convective heat transfer using distilled water as the working fluid for Reynolds number range of 1300–3400 and a constant heat flux of 53.0 W/cm2. Results show that highly-tapered microchannels promote early turbulence at lower Reynolds numbers with better heat transfer and much greater pressure drop. All the tapered microchannels possess a thermal-hydraulic performance index which is less than 1, implying that a tapered microchannel yields a lower heat transfer capability given the same pumping power as a straight microchannel.

A comparative study of flow boiling HFE-7100 in silicon nanowire and plainwall microchannels

Extensive experimental investigations along with high speed visualizations have been performed to assess the flow boiling characteristics in Silicon Nanowire (SiNW) microchannels. Experiments have been also performed in Plainwall microchannels to compare their performances with SiNW configurations. HFE-7100 has been used as the working fluid and experiments are conducted in a forced convection loop at mass flux range of 400–1600 kg/m2s. Arrays of microchannel consist 5 (five) parallel straight microchannels with Width, Depth and Length dimension of 220 μm, 250 μm and 10 mm respectively. Flow boiling performances including heat transfer coefficient (HTC), pressure drop, two-phase flow instabilities and critical heat flux (CHF) have been studied in both the Silicon plainwall (smooth inner surface) and Silicon Nanowire (silicon nanostructured inner surface) microchannels. High speed flow visualizations have been performed at up to 70,000 frames per s (fps) to understand the difference in boiling mechanisms between Plainwall and SiNW. SiNW performs significantly enhanced HTC (up to 400% improvement), reduces flow boiling instabilities and pressure drop (up to 70% reduction) compared to Plainwall microchannels. However, little/insignificant effect of nanostructured surface has been observed on CHF. In addition, a major difference in two-phase flow regime development has been observed between the SiNW and Plainwall microchannels during flow visualization. Specifically, SiNWs introduce explosive bubble nucleation, reduce intermittent flow regimes (slug/churn), improve rewetting, maintain thin liquid film and thus, improve system performances.

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.

Parametric study on mixing process in an in-plane spiral micromixer utilizing chaotic advection

Recent advances in the field of microfabrication have made the application of high-throughput microfluidics feasible. Mixing which is an essential part of any miniaturized standalone system remains the key challenge. This paper proposes a geometrically simple micromixer for efficient mixing for high-throughput microfluidic devices. The proposed micromixer utilizes a curved microchannel (spiral microchannel) to induce chaotic advection and enhance the mixing process. It is shown that the spiral microchannel is more efficient in comparison to a straight microchannel, mixing wise. The pressure drop in the spiral microchannel is only slightly higher than that in the straight microchannel. It is found that the mixing process in the spiral microchannel enhances with increasing the inlet velocity, unlike what happens in the straight microchannel. It is also realized that the initial radius of the spiral microchannel plays a prominent role in enhancing the mixing process. Studying different cross sections, it is gathered that the square cross section yields a higher mixing quality.

Numerical Study of Heat Transfer Enhancement of Roll-to-Roll Microchannel Heat Exchangers

The heat transfer performance of two roll-to-roll microchannel heat exchangers with square cross section and side length ranging from 0.2 mm to 0.5 mm were investigated via numerical studies. In order to assess the heat transfer enhancement, equivalent straight channel heat exchangers were also researched numerically as comparisons. For the roll-to-roll devices, numerical studies demonstrated that there were two reasons for heat transfer enhancement. First, when the average Dean number of the fluid was greater than approximately 10, Dean vortices started to form within the roll-to-roll microchannels, enhancing the convective heat transfer between channels. Second, the compact roll-to-roll structure of the heat exchangers increased the area of heat transfer compared with straight microchannel equivalents, and thus promoted the conductive heat transfer. Numerical simulations noted both higher Nusselt numbers and higher thermal performance factors (TPF) for roll-to-roll microchannel heat exchangers compared with equivalent straight channels and were employed to optimize both the microchannel cross section dimensions and the wall thickness between channels. In addition, the swirling strength and the heat transfer area were also calculated to characterize the convective and conductive heat transfer, respectively, allowing for a comparison between two roll-to-roll microchannel heat exchanger designs.

Non-lithographic copper-wire based fabrication of micro-fluidic reactors for biphasic flow applications

The search for a cost-effective fabrication method for cross-linked polydimethylsiloxane (PDMS) based microreactors remains an attractive area of research. In this work, a simple and cost-effective method for fabricating microchannel with different geometrical configurations such as straight, helical and straight channel with gradual 90° bend has been offered. In order to construct the microchannels slender copper wire of 0.21 mm diameter has been planted inside a block of PDMS and then it has been easily drawn out by applying a little tensile force. Two different types of inlet sections (L and T types) have been constructed for each type of microchannel. Moreover, detailed flow visualization studies (flow patterns) using a biphasic system, toluene-water-acetic acid have been performed for straight and helical microchannels for mass transfer application. The flow patterns have been observed by varying a wide range of inlet velocities of both the phases from 0.0079 to 0.047 m/s. The effect of surface and viscous forces are dominant over other forces such as inertia and gravity, which is evident from the range of slug flow in both the microchannels. The overall volumetric mass transfer coefficient is found to be in the range of 0.06–0.6 s−1, which is significantly higher than the reported results. This copper wire-based non-lithographic technique is flexible and convenient in comparison with conventional lithographic methods and may enable the production of miniature components for chemical and biochemical systems, involving continuous flow systems usually comprising of two or more phases.

Effects of geometry factors on microvortices evolution in confined square microcavities

Recently, microcavities have become a central feature of diverse microfluidic devices for many biological applications. Thus, the flow and transport phenomena in microcavities characterized by microvortices have received increasing research attention. It is important to understand thoroughly the geometry factors on the flow behaviors in microcavities. In an effort to provide a design guideline for optimizing the microcavity configuration and better utilizing microvortices for different applications, we investigated quantitatively the liquid flow characteristics in different square microcavities located on one side of a main straight microchannel by using both microparticle image velocimetry (micro-PIV) and numerical simulation. The influences of the inlet Reynolds numbers (with relatively wider values Re = 1–400) and the hydraulic diameter of the main microchannel (DH = 100, 133 μm) on the evolution of microvortices in different square microcavities (100, 200, 400 and 800 μm) were studied. The evolution and characteristic of the microvortices were investigated in detail. Moreover, the critical Reynolds numbers for the emergence of microvortices and the transformation of flow patterns in different microcavities were determined. The results will provide a useful guideline for the design of microcavity-featured microfluidic devices and their applications.