Flow In Microchannel Research Articles

Computational study of hydrothermal characteristics in straight microchannels with base triangular and wavy profile designs for electronic cooling

In the current study, computational analysis of the hydrothermal behavior of fluid flow in straight microchannels (MCs) with base triangular and wavy profile designs was conducted and the results were compared with that of straight MCs. The computational study was conducted in the 100–300 Reynolds number (Re) range. According to computational analysis, the thickness of the boundary layer grows as the fluid moves through straight MC, resulting in lower heat dissipation. Continual thinning and thickening of the boundary layer is seen to form in straight MC with triangular base and wavy profile designs. The fluid passing through the MC material can transfer heat more effectively as a result. Additionally, in straight MC with base wavy profiles designs, it was observed that there were more Dean vortices, reduction in stagnation zones, and lowering of channel base temperature. From the current study, it was noticed that straight MC with base wavy profiles designs showed higher heat transfer (0.5% to 21.14%), lower pressure drop (18.65% to 14.25%) compared to straight MC at Re range from 100 to 300, and higher overall performance factor compared to other designs.

Effects of wall conductance on transient MHD free convection flow in a permeable microchannel with induced magnetic field

Effects of wall conductance on transient MHD free convection flow in a permeable microchannel with induced magnetic field

An Eight-Step Continuous-Flow Total Synthesis of Vitamin B1

Vitamin B1 is widely applied in the healthcare and food industry as an antineuritic and antioxidant to maintain the normal functioning of nerve conduction, the heart, and the gastrointestinal tract. This study reports on an integrated eight-step continuous-flow synthesis of vitamin B1 from commercially available 2-cyanoacetamide. The proposed continuous-flow process is based on advances in chemistry, engineering, and equipment design, and affords improved performance and safety compared with batch-mode manufacturing. Several challenges were precisely investigated and controlled, including mixing, unexpected clogging, solvent switches, an exothermic reaction, and the prevention of side reactions, using various micro-channel flow reactors, mixers, separators, and continuous filters. Vitamin B1 was produced with a separated yield of 47.7% and high purity, with a total residence time of about 3.5 h. This eight-step continuous-flow protocol enables technology involving up to six of the key principles of green chemistry. Hence, the application of flow technology is of paramount importance for improving security, reducing waste, and, in particular, improving the efficiency of batch operations that require several days for manufacturing.

Evolution of focused streams for viscoelastic flow in spiral microchannels

Particle migration dynamics in viscoelastic fluids in spiral channels have attracted interest in recent years due to potential applications in the 3D focusing and label-free sorting of particles and cells. Despite a number of recent studies, the underlying mechanism of Dean-coupled elasto-inertial migration in spiral microchannels is not fully understood. In this work, for the first time, we experimentally demonstrate the evolution of particle focusing behavior along a channel downstream length at a high blockage ratio. We found that flow rate, device curvature, and medium viscosity play important roles in particle lateral migration. Our results illustrate the full focusing pattern along the downstream channel length, with side-view imaging yielding observations on the vertical migration of focused streams. Ultimately, we anticipate that these results will offer a useful guide for elasto-inertial microfluidics device design to improve the efficiency of 3D focusing in cell sorting and cytometry applications.

Achieving ultra-uniform temperature distribution with a heterogeneous wettability surface for two-phase flow in microchannels

Microchannel-based two-phase schemes have been proposed as an efficient solution for thermal management. However, the non-uniform temperature distribution in flow boiling limits phase-change performance and impedes commercial applications. In this study, we introduce a novel surface featuring heterogeneous wettability, comprising upstream superhydrophobic, middle plain, and downstream superhydrophilic regions. Hierarchical nanowires are employed to achieve the desired wettability, which can promote bubble departure and enhance capillary pumping. This wettability distribution facilitates upstream bubble nucleation and downstream liquid supply, significantly improving temperature uniformity. Moreover, surface contact angle variations effectively contribute to flow behavior improvement by restricting vapor reversal and fostering churn flow. Our results demonstrate that heat aggregation in the middle region and temperature restriction in the downstream region cause the high-temperature region to migrate upstream. This temperature redistribution stimulates axial heat transfer, substantially suppressing inlet subcooling and restricting temperature gradients. Compared to the plain surface, the heterogeneous wettability surface afforded an 87.8% reduction in temperature difference between Tw1 and Tw5 at approximately 120 °C, suggesting an ultra-uniform temperature distribution. Additionally, the average heat transfer coefficient showed a 35.1% enhancement rate at a wall heat flux of 100 W/cm2, and the performance evaluation criterion showed a 44.2% enhancement rate at an effective heat flux of 190 W/cm2. These results indicate that the proposed surface can efficiently regulate temperature distribution, maintain performance stability, and enhance heat transfer. The heterogeneous wettability surface has considerable potential for practical applications in high-power component heat dissipation.

An open microfluidic design for contact angle measurement

Spontaneous capillary flow in open microchannels is a phenomenon driven by surface energies. The contact angle that the liquid forms with the channel's substrate material and the cross-section of the microchannel decide whether liquid from a connected reservoir will automatically fill the channel or not. In this work we show how this behavior can be used to design a passive contact angle measurement device (CAMD) based on parabolic open microgrooves. To that end, we present a theory of open capillary flow in such microgrooves and compare the results to minimal energy surface simulations. Additionally, we discuss that the condition for capillary flow of curved microchannels is essentially equal to the condition for their straight counterparts having the same cross-section. Lastly, we present two demonstrators of our CAMD made out of micromilled poly(methyl methacrylate). The devices consist of five open microchannels with different cross-sections which are connected to a common liquid reservoir. We show how the behavior of a liquid placed into that reservoir can be used to evaluate the contact angle between the liquid and the substrate material. A comparison to conventional contact angle goniometry shows that our approach is able to successfully estimate contact angles with an accuracy of 10° by design which can be improved by employing a greater number of microchannels. Since our devices were automatically designed and can be tuned to specific applications, this provides an easy approach to include contact angle measurement into existing lab-on-a-chip devices.

Three-Dimensional Simulation of Singleand Multi-Phase Flows in Roughness Microchannels

Three-Dimensional Simulation of Singleand Multi-Phase Flows in Roughness Microchannels

Analysis of the slip flow in the hydrodynamic entrance region of a 2D microchannel

Two-dimensional developing flow in the entrance of a microchannel has been studied numer-ically. Due to its nature, a microchannel can be used in tight space applications and the length of channel can get very small values. Furthermore, if the hydrodynamic development length of flow in microchannel has comparably the same value with the channel length, the channel entrance parameters play critical role on the flow performance of a microscale channel. Lattice Boltzmann Method (LBM) was considered for studying and simulating the developing slip flows through a rectangular microchannel. A unique computational code for this study was developed by using LBM. The slip velocity boundary condition along with Knudsen number values in the slip flow regime was used for this model. The bounce-back boundary condition was considered at the top and bottom walls of the microchannel. The effects of the Reyn-olds numbers (1-100) and Knudsen numbers (0.001, 0.01, 0.1) on the hydrodynamic entrance length has been examined. The numerical results have been compared with the previous stud-ies in the literature and the similarities have been found satisfactory. The entrance length is found to be increasing with the increase of Reynolds and Knudsen numbers. A correlation for hydrodynamic development length as a function of Knudsen and Reynolds numbers was obtained by using the data extracted from LBM simulations performed in this study.

Fabrication of monodisperse droplets and microcapsules using microfluidic chips: a review of methodologies and applications

Abstract Microfluidics has been applied in the preparation of monodisperse droplets and microcapsules due to its high encapsulation efficiency, its ability to create uniform particle sizes, and its capacity to control core–shell ratio and structure. To bring to the fore methodologies for the fabrication and application of monodisperse microcapsules using microfluidics, we present a review of the design, structure, materials, and surface modification techniques of various microfluidic chips. The review also covers fabrication methods, operating parameters and regulation methods of single and multiple monodisperse emulsion droplets fabricated from various microfluidic devices. Our findings show that particle size of monodisperse droplets depend mainly on microchannel characteristic size and flow rate, with particle size increasing with larger microchannel but decreasing with higher continuous phase flow rate. We additionally reviewed and compared various fabrication methods for monodisperse microcapsules, such as interfacial polymerization, free-radical polymerization, ionic cross-linking, and solvent evaporation. We further reviewed and examined the application of monodisperse microcapsules in biology applications, food engineering, composite materials development, and pharmaceutical industry. We found that high-throughput microfluidics for scale-up monodisperse microcapsule preparation towards uniform degradation and targeted release properties of monodisperse microcapsules would be key innovative direction for future applications.

Exact solution of the Graetz–Brinkman problem extended to non-Newtonian nanofluids flow in elliptical microchannels

Exact solution of the Graetz–Brinkman problem extended to non-Newtonian nanofluids flow in elliptical microchannels

Experimental Study of Mass Transfer in a Plug Regime of Immiscible Liquid–Liquid Flow in a T-Shaped Microchannel

In the presented work, the influence of parameters such as the total flow rate of phases, the ratio of flow rates, and residence time on mass transfer during the two-phase flow of immiscible liquids in a T-shaped microchannel was investigated using the micro-LIF technique. The study focused on the plug flow regime, where a 70% water–glycerol solution was used as the dispersed phase, and tri-n-butyl phosphate (TBP) was used as the carrier phase. We determined the transition boundary between the dispersed and parallel flow patterns and calculated the plug length and velocities to develop a mass transfer model. Furthermore, we measured the partition coefficient for the set of liquids used in the experiments and analyzed the concentration fields inside the slugs of the continuous phase at various distances downstream of the T-junction. Using the obtained data, we determined the extraction efficiency and overall volumetric mass transfer coefficient and established dependencies demonstrating the effect of the flow-rate ratio, total flow rate, and the residence time on mass transfer rate and extraction efficiency. Finally, we developed a model for the overall volumetric mass transfer coefficient corresponding to the set of liquids used with an R-squared value of 0.966.

Thermal and hydrodynamic characteristics of single-phase flow in manifold microchannels with countercurrent regions

Manifold microchannel heat sink is promising to dissipate high heat fluxes for electronic devices due to its high surface to volume ratio. Traditional Z-type manifold encounters fluid flow maldistribution and a high temperature difference. In this work, two inlets and countercurrent flows are introduced into the manifold arrangement to alleviate these problems in the Z-type manifold microchannels. Six cases with different manifold arrangements are numerically investigated using single-phase HFE-7100 at mass flow rates ranging from 56.8 to 117.78 mg/s. The results show that the maximum temperature, average temperature, and temperature difference of the revised arrangements are all lower than these of the traditional Z-type manifold arrangement. The minimum temperature difference for an ideal multi inlet manifold arrangement can be less than 0.2 K, while the value is about 5.3 K for the traditional Z-type manifold arrangement. The revised arrangement can also improve the uniformity of mass flux distributions. The ratio of the maximum to the minimum mass flux within microchannels is 12.83 for the traditional Z-type manifold, while this value is about 5.82 for cases with two inlets and countercurrent zones. A longer flow path and a higher local pressure drop in the revised arrangements results in higher pressure drops than the traditional Z-type manifold microchannels. Compared with the traditional Z-type arrangement, heat transfer coefficients and performance evaluation criterion are higher for the improved cases. The presence of a countercurrent zone can improve the comprehensive performance of the manifold microchannel. New criterion based on the thermal resistance calculated at the maximum temperature is proposed to evaluate the cooling performance of electronics.

Quantification of interfacial mass transfer during droplet formation in a microfluidic T-junction using VOF

Taylor flow in microchannels is characterized by interfacial mass transfer rates which are two to three orders of magnitude larger than in conventional reactors. During the droplet formation stage vortices are observed which further contribute to interfacial mass transfer. In this work, we quantify the contribution of the droplet formation stage to the overall mass transfer by integrating a compressive continuum species transfer model into a volume-of-fluid (VOF) solver. It is observed that the interfacial mass transfer depends solely on the vortex intensity. Moreover, altering the flow rate ratio and viscosity significantly affects the interfacial mass transfer performance. Overall, the interfacial mass transfer during the droplet formation stage contributes up to ∼70% of the overall mass transfer at large relative velocities between the dispersed and continuous phases. The present findings will guide applications involving interfacial mass transfer in microchannels.

Pickering emulsion enhanced interfacial catalysis under Taylor flow in a microchannel reactor

There are still technical challenges associated with surface fouling and clogging in microchannels when conducting multiphase catalytic reactions involving solid particles. In this study, a Pickering emulsion was employed to enhance the oxidation of benzyl alcohol (BNOH) catalyzed by Pd/SiO2 nanoparticles under Taylor flow in a microchannel reactor. The reaction characteristics of benzyl alcohol/water Pickering emulsion in a microchannel reactor were studied experimentally. The characteristic results showed that the O/W Picking emulsion system was superior to the W/O Picking emulsion system for the catalytic oxidation reaction. The influences of a wide range of operational variables were characterized on the conversion of reactants and the selectivity of the desired product. The optimized reaction conditions included: reaction temperature 403 K, CB 5 wt%, O2/BNOH (mol/mol) 0.76 and tube length 8 m. Under the optimal operating conditions, the conversion of BNOH and the selectivity of benzaldehyde were found to be 86.53% and 99.79%, respectively. In addition, the interfacial transport-reaction mechanism in Pickering emulsions under Taylor flow in microchannels was proposed from the perspective of multiphase mass transfer. This development provided significant insights and guiding for effectively performing multiphase catalytic reactions involving solid catalyst particles in microchannel reactors.

Bacteria separation and enrichment using viscoelastic flows in a straight microchannel

Bacteria occupy a large domain of prokaryotic microorganisms on earth and play a significant role in public health. In this study, we achieve continuous size-based bacteria separation using viscoelastic flows in a microchannel. Three types of bacteria that differ in size, including Lactobacillus rhamnosus (L. rhamnosus), Staphylococcus epidermidis (S. epidermidis) and Escherichia coli (E. coli) are employed. First, the separation of L. rhamnosus and S. epidermidis is investigated. Under the combined effects of inertial lift and elastic forces, the larger L. rhamnosus cells are driven toward the channel centre while the smaller S. epidermidis cells flow in narrow streams along the channel sidewalls. The results show that the separation performance of the viscoelastic microfluidic device is affected by four main factors: channel length, total flow rate (TFR), sheath-to-sample flow rate ratio (FRR) and polyethylene oxide (PEO) concentration. Furthermore, the separation of L. rhamnosus and E. coli is demonstrated. This work overcomes the issues like shape irregularity and size variation in bacterial samples and achieves the separation of two types of bacteria in a size-dependent manner. We expect that this work will facilitate the future studies of bacterial interspecies communication and interactions as well as the development of biology, biomedicine and biotechnology.

A review of frictional pressure drop characteristics of single phase microchannels having different shapes of cross sections

Abstract This paper theoretically studied pressure drop variation in microchannels having different cross sections (circular, rectangular, square, trapezoidal, triangular, elliptical, parallel plate, co-centric circles, hexagonal, wavy, smoothed or rounded corners cross sections, and rhombus) for single phase Newtonian fluid (gas and liquid) flow. Based on 41 years (approximately) prior literature (1981–till now), 249 articles were studied and number of correlations of pressure drop calculation in microchannels with or without friction factor equation for four cross sections i.e., rectangular, square, circular, trapezoidal, wavy and triangular is collected and also mentioned their limitations at one place. Other than these four cross sections, there is very few experimental/numerical works was present in the literature. A comparable study was performed for laminar as well as turbulent friction factor to calculate the pressure drop with the help of classical theory for gas and liquid flow in microchannels with circular and rectangular cross sections. Results show wonderful outcomes i.e., correlations of laminar pressure drop study can be extendable for transition and turbulent regime in both types (circular and rectangular) of cross sections of microchannels. In different types of flow regime, it is suggested that for each type of cross section (circular and rectangular) we can go for single correlation for gas/liquid system. It is also investigated that the macro channels pressure drop equations can be used for microchannels up to the certain values of Reynolds number. Basically, this paper provides all possible equations of friction factor related to the microchannels that helps to calculate the pressure drop, is collected at one platform also compared their deviation with conventional channels.

Cell spheroids culture array with modifiable chemical gradients.

Cancer cell spheroids have been shown to mimic in vivo tumour microenvironment and are therefore suitable for in vitro drug screening. Microfluidic technology can provide conveniences for spheroid assays such as high-throughput, simplifying manual operation and saving reagent. Here, we propose a concentration gradient generator based on microfluidic technology for cell spheroid culture and assay. The chip consists of upper microchannels and lower microwells. After partitioning HepG2 suspension into the microwells with concave and non-adhesive bottoms, spheroids can spontaneously form. By controlling the fluid replacement and flow in microchannels, the doxorubicin solution is diluted automatically into a series of concentration gradients, which spanning more than one order of magnitude. And then the effect of doxorubicin on spheroids is measured in situ by fluorescent staining. This chip provides a very promising approach to achieve the high-throughput and standardized anti-cancer drug screening in future.

Optimised Electro-Osmotic Flow in Rectangular Microchannels with Smoothed Corners

Electro-osmotic flows are a means of circulating polar fluids through microchannels without resorting to mechanical pumping. The lack of moving parts, of noise and the ease of integration in silicon chips make them an interesting option for microchip cooling and miniaturized total analysis systems. This paper describes an optimization in terms of first- and second-law analysis of the cross-section of a microchannel subject to electro-osmotic flow. Starting from rectangular cross-sections of different aspect ratios, its corners are progressively smoothed and the resulting Poiseuille and Nusselt numbers computed. Performance evaluation criteria are then used to assess the change in, among others, heat transfer rate, temperature difference between wall and bulk fluid, and equivalent pumping power. The entropy number is also computed and the results commented. It is found that the latter criterion highlights a configuration of minimum entropy generation, whereas the trends of the heat transfer and temperature difference are opposite to that of the equivalent pumping power.

A novel microparticle size sorting technology based on sheath flow stable expansion regimes

Previously we have originally reported and experimentally explored the stable expansion phenomenon of sheath flow in straight microchannels. The stable expansion phenomenon differs from the focused stable thread and diffusion, which are expected to appear based on current understanding. It happens within a specific range of Reynolds numbers in microchannels with specific aspect ratios. However, the mechanism insight and potential applications remained poorly understood. In this study, a comprehensive numerical model, involving laminar flow, diluted species transportation, and particle tracing, was established to provide a global transparent insight. Based on the simulation results, the root cause and mechanism of the stable expansion phenomenon were revealed as a dumbbell-shape deformation of the buffer–sample–buffer interface. In addition, a microparticle size sorting application based on these findings was proposed and conducted in both simulation and experiment. The results showed that 1 and 15 μm microparticles can be totally separated with nearly 100% purity. According to the advantages and potentials of this technique, it can be promising to approach a label-free microparticle sorting with a high throughput manner.

Effects of channel wall wettability on gas–liquid dynamics mass transfer under Taylor flow in a serpentine microchannel

The wall wettability of microchannels plays an important role in the gas–liquid mass transfer dynamics under Taylor flow. In this study, we regulated the contact angle of the wall surface through surface chemical grafting polymerization under controlled experimental conditions. The dynamic changes of CO2 bubbles flowing along the microchannel were captured by a high-speed video camera mounted on a stereo microscope, whilst a unit cell model was employed to theoretically investigate the gas–liquid mass transfer dynamics. We quantitatively characterized the effects of wall wettability, specifically the contact angle, on the formation mechanism of gas bubbles and mass transfer process experimentally. The results revealed that the gas bubble velocity, the overall volumetric liquid phase mass transfer coefficients (kLa), and the specific interfacial area (a) all increased with the increase of the contact angle. Conversely, gas bubble length and leakage flow decreased. Furthermore, we proposed a new modified model to predict the gas–liquid two-phase mass transfer performance, based on van Baten's and Yao's models. Our proposed model was observed to agree reasonably well with experimental observations.