Microscale Channels Research Articles

Flow boiling heat transfer of R134a and low GWP refrigerants in a horizontal micro-scale channel

The present paper presents an investigation of the effects of the refrigerant type on the heat transfer coefficient during flow boiling inside micro-scale channels. Experimental results for R134a, R1234ze(E), R1234yf and R600a for flow boiling in a circular channel with internal diameter of 1.1mm are presented. The experimental database comprises 3409 data points covering mass velocities ranging from 200 to 800kg/m2s, heat fluxes from 15 to 145kW/m2, saturation temperatures of 31 and 41°C, and vapor qualities from 0.05 to 0.95. The experimental data were parametrically analysed and the effects of the experimental parameters (heat flux, mass velocity, saturation temperature and working fluid) identified. Subsequently, the experimental data were compared against the most quoted predictive methods from literature, including macro and micro-scale methods. Based on the broad database obtained in the present study, an updated version of the predictive method of Kanizawa et al. (2016) was proposed. The updated version provided accurate predictions of the present experimental database, predicting more than 97% and 86% of the results within error bands of ±30 and ±20%, respectively.

Fully developed mixed convection of nanofluids in microtubes at constant wall temperature: Anomalous heat transfer rate and thermal performance

This is a theoretical investigation on fully developed mixed convective flow of nanofluids inside microtubes subjected to a constant wall temperature (CWT). The modified Buongiorno model is used for the nanofluids which fully accounts for the distribution of nanoparticles concentration on thermophysical properties. The effect of nanoparticles migration originating from the nano-scale diffusivities including thermophoretic diffusion (temperature-gradient driven force) and Brownian diffusion (concentration-gradient driven force) on the thermophysical characteristics of nanofluids has been considered. A Navier's slip condition is considered at the wall to model the non-equilibrium region at the fluid-solid interface in micro-scale channels. A scale analysis is performed to estimate the relative significance of the pertaining parameters that should be included in the governing equations. The effects of pertinent parameters including the ratio of Brownian motion to thermophoresis (NBT), slip parameter (λ), mixed convective parameter (Nr), and bulk mean nanoparticle volume fraction (ϕB) on the flow and thermal fields are investigated. The figure of merit (FoM) is used to measure the thermal performance of equipment and finding the optimum thermal condition. It is shown that increasing the buoyancy force would enhance the heat transfer rate, especially for the larger nanoparticles. Also, larger nanoparticles enhance the thermal performance based on a required heat transfer rate with the lowest penalty in the pressure drop.

Apparatus for measuring pressure-driven transport through channels at high Knudsen numbers.

The pressure-driven gas flow through micro- and nano-porous structures is particularly interesting for innovative technologies such as microelectromechanical and nano-mechanical-electrical systems. The classical continuum assumption breaks down for rarefied flow through channels with a characteristic dimension comparable to the mean free path of the gas. Theories based on molecular interactions have been formulated to predict the flow at high Knudsen numbers. Measuring rarefied gas flow experimentally is a challenge since only a few studies have been able to determine flowrates in the molecular flow regime. Here we present the design of an experimental apparatus, which can be used to measure the flow of gases through nano- and microscale channels in the flow regimes where molecular effects are critical. The equations used to design the apparatus are given, focusing on the slip and transition flow regimes (together sometimes called "Intermediate flow regime"). A channel with a diameter of 325 μm ± 5μm and a length of 2 mm was tested experimentally with the apparatus for a wide range of Knudsen numbers (10-2 < Kn < 1 × 105) demonstrating its suitability through the slip and transition regime (2.23 × 10-2 < Kn < 2.26).

Optical feedback interferometry for microscale-flow sensing study: numerical simulation and experimental validation.

Optical feedback interferometry (OFI) performance for microscale-flow sensing is studied theoretically and experimentally. A new numerical modeling approach for OFI flow meter spectrum reproduction is presented in this work to study the optical effect on the signal due to the micro-scale channel geometry. Two well-defined frequency peaks are found in the OFI spectrum, this phenomenon can be attributed to the reflection of the forward scattered light on the channel rear interface. The flow rate measurement shows good accuracy over a range of fluid velocities from 16.8 mm/s to 168 mm/s, thus providing a promising tool to study and to optimize the OFI microfluidic sensor system.

Processing gain and noise in multi-electrode impedance cytometers: Comprehensive electrical design methodology and characterization

We introduce a novel technique for minimizing false positives and improving the fidelity of signals detected with impedance cytometry. We make use of interdigitated multi-finger electrodes for introducing redundancy and robustness in impedance based detection of micro- and nanoparticles. Multi-electrode impedance sensors provide multiple peaks and unique signatures for single cells and particles, and can be used in conjunction with match filters to provide processing gain. This approach is particularly useful in minimizing false positives, which is particularly difficult for signals with low signal-to-noise ratio. The unique signature provided by multi-electrode sensors results in higher signal-power compared to single electrode pair sensors, and can also result in more fidelity in distinguishing particles from noise. This technique holds the potential of using micro-scale channels for detection of nanoscale particles such as viruses and even individual proteins. Previous work has shown extraction of signal from noise using multi-electrode sensors. To the best of our knowledge, this manuscript presents the first comprehensive attempt to characterize processing gain improvement and system noise in multi-electrode sensors by directly comparing simultaneous measurements obtained from sensors fabricated along the same channel. By measuring simultaneously the electrode pairs along a single channel, consistency in flow rate and buffer conditions is ensured, while other experimental assay-to-assay variations are minimized. We achieve high SNR for particles as small as 1um in a 30μm wide by 10 diameter tall pore. SNR and noise measurements for different electrode configurations and excitation frequencies is thoroughly studied. The results of this study demonstrated that while increasing number of electrodes resulted in increased processing gain, noise levels also increased with larger number of electrode fingers, thus total SNR remained relatively unchanged. Our investigations showed that this increase in noise is likely due to electrochemical reactions at the surface of the electrode dominating the total noise.

Turbulence in microfluidics: Cleanroom-free, fast, solventless, and bondless fabrication and application in high throughput liquid-liquid extraction

This paper addresses an important breakthrough in the deployment of ultra-high adhesion strength microfluidic technologies to provide turbulence at harsh flow rate conditions. This paper is only, to our knowledge, the second reporting on the generation of high flow rate-assisted turbulence in microchannels. This flow solves a crucial bottleneck in microfluidics: the generation of high throughput homogeneous mixings. We focused on the fabrication of bulky polydimethylsiloxane (PDMS) microchips (without any interfaces) rather than the laborious surface modifications that were employed in the first reporting about turbulence-assisted microfluidics. The fabrication is cleanroom-free, simple, low-cost, fast, solventless, and bondless requiring only a laboratory oven. More specifically, our method relies on the shaping of a nylon scaffold, cure of PDMS with embedded nylon, and removal of this scaffold. The scaffold was obtained by manually wrapping nylon threads. The withdrawing out of the scaffold was completed in few seconds using only a plier. Such microchannels endured flow rates of up to 60.0 mL min−1 with a strikingly low elastic deformation. The importance in producing turbulence into microscale channels was successfully shown in liquid-liquid extractions. The great energy dissipation rate relative to the turbulence created high throughput and efficient extractions in microfluidics for the first time. The residence time was only 0.01 s at 25.0 mL min−1 (total flow rate of the immiscible phases). In addition, the partition coefficient determined in a single run was similar to that obtained by the conventional batch shake-flask method that was realized in triplicate.

Interferometric measurement and numerical comparisons of supersonic heat transfer flows in microchannel

With the fast development of electronic systems and the ever-increasing demand of thermally “smart” design in space and aeronautic engineering, the heat transfer innovations and high heat flux challenges have become a hot topic for decades. This study is aimed at the effective cooling heat transfer design by super-/sub-sonic air flow in microscale channels for high heat flux devices. The design is based on the low temperature flows with supersonic expansion in microscale, which yields a compact and simple design. By careful microelectromechanical process, microscale straight and bumped channels (with simple arc curve) are fabricated and experimentally tested in this study. The microscale flow field and density distributions under new designs are visualized quantitatively by an advanced phase-shifting interferometer system, which results are then compared carefully with numerical simulations. In this study, large differences between the two designs in density distribution and temperature changes (around 50K) are found. The high heat flux potential for supersonic microchannel flows is realized and discussion into detail. It is confirmed that the bump design contributes significantly to the heat transfer enhancement, which shows potential for future application in novel system designs.

The extended Graetz problem for micro-slit geometries; analytical coupling of rarefaction, axial conduction and viscous dissipation

In order to support the recent MEMS and Lab-on-a-chip technologies, we studied heat transport in micro-scale slit channel gas flows. Since the micro convection transport phenomena diverges from conventional macro-scale transport due to rarefaction, axial conduction and viscous heating, an accurate understanding requires a complete coupling of these effects. For such cases, we studied heat transfer in hydrodynamically developed, thermally developing gas flows in micro-slits at various flow conditions. The analytical solution of the energy equation considered both the heat conduction in the axial direction and heat dissipation of viscous forces. Furthermore, updated boundary conditions of velocity slip and temperature jump were applied based on Knudsen number of flow in order to account for the non-equilibrium gas dynamics. Local Nusselt number (Nu) values were calculated as a function of Peclet (Pe), Knudsen (Kn) and Brinkman (Br) numbers which were selected carefully according to possible micro-flow cases. Strong variation of Nu in thermal development length was found to dominate heat transfer behavior of micro-slits with short heating lengths for early slip flow regime. For this instance, influence of axial conduction and viscous dissipation was equally important. On the other hand, high Kn slip flow suppressed the axial conduction while viscous heating in a small surface-gas temperature difference case mostly determined the fully developed Nu and average heat transfer behavior as a function of Kn value.

Towards Single-Step Biofabrication of Organs on a Chip via 3D Printing

Organ-on-a-chip engineering employs microfabrication of living tissues within microscale fluid channels to create constructs that closely mimic human organs. With the advent of 3D printing, we predict that single-step fabrication of these devices will enable rapid design and cost-effective iterations in the development stage, facilitating rapid innovation in this field.

Modelling of the flow-rate dependent partial thermal resistance of integrated microscale cooling structures

As microscale cooling structures are integral parts of modern chip level cooling concepts, the understanding of the behavior of different assemblies is desirable to create a novel co-design concepts where the thermal design is embedded into the conventional IC design flow. The first step of understanding is to create a general formula which describes the heat transfer mechanism and can be implemented in IC CAD environments. This paper presents an analytical study of this mechanism in microscale channel based cooling structures. A closed analytical formula is also given to calculate the partial thermal resistance. Numerical analyses are also conducted and the results are compared against the analytical ones.

Microfluidic assay for precise measurements of mouse, rat, and human neutrophil chemotaxis in whole-blood droplets.

Animal models of human disease differ in innate immune responses to stress, pathogens, or injury. Precise neutrophil phenotype measurements could facilitate interspecies comparisons. However, such phenotype comparisons could not be performed accurately with the use of current assays, as they require the separation of neutrophils from blood using species-specific protocols, and they introduce distinct artifacts. Here, we report a microfluidic technology that enables robust characterization of neutrophil migratory phenotypes in a manner independent of the donor species and performed directly in a droplet of whole blood. The assay relies on the particular ability of neutrophils to deform actively during chemotaxis through microscale channels that block the advance of other blood cells. Neutrophil migration is measured directly in blood, in the presence of other blood cells and serum factors. Our measurements reveal important differences among migration counts, velocity, and directionality among neutrophils from 2 common mouse strains, rats, and humans.

3D macroporous electrode and high-performance in lithium-ion batteries using SnO2 coated on Cu foam.

A three-dimensional porous architecture makes an attractive electrode structure, as it has an intrinsic structural integrity and an ability to buffer stress in lithium-ion batteries caused by the large volume changes in high-capacity anode materials during cycling. Here we report the first demonstration of a SnO2-coated macroporous Cu foam anode by employing a facile and scalable combination of directional freeze-casting and sol-gel coating processes. The three-dimensional interconnected anode is composed of aligned microscale channels separated by SnO2-coated Cu walls and much finer micrometer pores, adding to surface area and providing space for volume expansion of SnO2 coating layer. With this anode, we achieve a high reversible capacity of 750 mAh g−1 at current rate of 0.5 C after 50 cycles and an excellent rate capability of 590 mAh g−1 at 2 C, which is close to the best performance of Sn-based nanoscale material so far.

Multi-Dimensional Nanostructures for Microfluidic Screening of Biomarkers: From Molecular Separation to Cancer Cell Detection.

Rapid screening of biomarkers, with high specificity and accuracy, is critical for many point-of-care diagnostics. Microfluidics, the use of microscale channels to manipulate small liquid samples and carry reactions in parallel, offers tremendous opportunities to address fundamental questions in biology and provide a fast growing set of clinical tools for medicine. Emerging multi-dimensional nanostructures, when coupled with microfluidics, enable effective and efficient screening with high specificity and sensitivity, both of which are important aspects of biological detection systems. In this review, we provide an overview of current research and technologies that utilize nanostructures to facilitate biological separation in microfluidic channels. Various important physical parameters and theoretical equations that characterize and govern flow in nanostructure-integrated microfluidic channels will be introduced and discussed. The application of multi-dimensional nanostructures, including nanoparticles, nanopillars, and nanoporous layers, integrated with microfluidic channels in molecular and cellular separation will also be reviewed. Finally, we will close with insights on the future of nanostructure-integrated microfluidic platforms and their role in biological and biomedical applications.

Review: General Issues and Correlations for Predicting Flow Boiling Heat Transfer Coefficients in Micro-Scale Channels

A review study was performed for basic heat transfer mechanism and quantitative analysis of correlations for flow boiling heat transfer in micro-scale channels. Several criteria for determining threshold diameter for micro-scale channels were discussed and the concept of confinement number was commented. The distinctive feature of flow boiling in micro-scale channels were considered and it was found out that the effect of the heat flux, latent heat, viscous force, surface tension, and inertial force was more significant. Important dimensionless parameters were summarized and it was pointed out that the boiling number, capillary number, and Weber number could be expected to play important roles at flow boiling in micro-scale channels. 17 correlations for flow boiling in micro-scale channels were reviewed in this study, and they were categorized by three types of correlations such as an equivalent Nusselt number correlation, a correlation with superposition of nucleate and convection boiling mechanism, and a flow pattern-based correlation. The predicted values by the correlations were compared with 536 experimental data from four different literatures and a correlation with smallest prediction errors was found. Some correlations showed distinct trends of convection heat transfer coefficient (h) change with respect to the variation of vapor quality. The trends are categorized by three trends such as noticeable increase of h with the increase of vapor quality and significant continuous decrease after dryout point, minor increase and decrease or decrease and increase of h, and gradual and continuous decrease of h with the increase of vapor quality. For each trend of h change, recommendable correlations and their basic equation forms were proposed to compare the prediction results with experimental data or to develop a new correlation by modifying existing correlations.

Thermal management in System-on-Package structures by applying microscale heat sink. Part I: Consideration of the appropriate channel length of microscale heat sink(s)

Since the first microchannel heat sink was introduced, intense research on microchannel based heat sinks has been conducted to study the heat transfer mechanism and fluid flow characteristics in microscale channel structures. In this paper the determination of proper channel length of the microscale heat sink(s) is presented. The main aim is to introduce a methodology to define the maximum applicable channel length in order to achieve the maximum heat transfer from the substrate to the fluid without realizing unnecessarily long channels. It is important because superfluous parts of the channel(s) from the heat transfer point of view occupy important areas where further channel(s) can be realized. The proposed characteristic channel length depends on the fluid properties, the flow rate and the channel geometries. This approach could be more important in the case of the more-than-Moore 3D integration (e.g.: System-in-Packages, stacked dies structure, etc.) where removing the heat from the inner layers or forming homogenous temperature distribution within a selected die is the most up-to-date question nowadays. The determination of the proper length of the channel(s) gives a basis to design the optimal architecture of microscale heat sink structures. The commonly applied microscale cooling structures with radial arrangement and one inlet usually reach the highest achievable heat transfer at low pressure drop but occupy more surface area than needed at applied flow rates.

A Mechanically Actuating Carbon-Nanotube Fiber in Response to Water and Moisture.

A new family of hierarchically helical carbon-nanotube fibers with many nano- and micro-scale channels has been synthesized. They demonstrate remarkable mechanical actuations in response to water and moisture. The water or moisture is first rapidly transported through the trunk micron-scale channels and then efficiently infiltrates into the interconnected capillary nanoscale channels, similar to the blood flow in our body. Therefore, rapid and large contraction and rotation of the fiber occurs with a high reversibility. These mechanically actuating fibers are promising for various applications, and smart windows and louvers have been investigated as two demonstrations.

Design and Characteristics of Dual-Core Photonic Crystal Fiber Doppler Differential Velocimeter

We present the design and characteristics of laser Doppler differential velocimeter using a dual-core photonic crystal fiber (DC-PCF). Transmission modes and mode interference characteristics of DC-PCF were investigated. Results show that higher order modes and cladding modes are excited, which can be cut off by bending the DC-PCF. The independent fundamental modes of two cores are clearly obtained. The mode interference near the distal end of DC-PCF was observed, and the interference process from independent faculae to the interaction of two core beams is presented. The velocimeter demonstrates a good working distance (∼30 µm) and a small measuring volume (∼273.4 µm3), which is well suited for flow measurement in micro-scale fluidic channels. The velocity measurement is successfully demonstrated by observing the frequency modulation and the spacing of the fringe projected interference. The measurements are verified and in good agreement with the results obtained by the light gate sensor.

Mesosponge Optical Sinks for Multifunctional Mercury Ion Assessment and Recovery from Water Sources.

Using the newly developed organic-inorganic colorant membrane is an attractive approach for the optical detection, selective screening and removal, and waste management recovery of highly toxic elements, such as Hg(II) ions, from water sources. In the systematic mesosponge optical sinks (MOSs), anchoring organic colorants into 3D, well-defined cage cavities and interconnected tubular pores (10 nm) in the long microscale channels of membrane scaffolds enhances the requirements and intrinsic properties of the hierarchal membrane. This scalable design is the first to allow control of the multifunctional processes of a membrane in a one-step screening procedure, such as the detection/recognition, removal, and filtration of ultratrace Hg(II) ions, even from actual water sources (i.e., tap, underground). The selective recovery, detection, and extraction processes of Hg(II) ions in a heterogeneous mixture with inorganic cations and anions as well as organic molecules and surfactants are mainly dependent on the structure of the colorant agent, the pH conditions, competitive ion-system compositions and concentrations, and Hg-to-colorant binding events. Our result shows that the solid MOS membrane arrays can be repeatedly recycled and retain their hierarchal mesosponge sink character, avoiding fouling via the precipitation of metal salts as a result of the reuse cycle. The Hg(II) ion rejection and the permeation of nonselective elements based on the membrane filtration protocol may be key considerations in water purification and separation requirements. The selective recovery process of Hg(II) ions in actual contaminated samples collected from tap and underground water sources in Saudi Arabia indicates the practical feasibility of our designed MOS membrane arrays.

Continuous medium exchange and optically induced electroporation of cells in an integrated microfluidic system

Optically induced electroporation (OIE) is a promising microfluidic-based approach for the electroporation of cell membranes. However, previously proposed microfluidic cell-electroporation devices required tedious sample pre-treatment steps, specifically, periodic media exchange. To enable the use of this OIE process in a practical protocol, we developed a new design for a microfluidic device that can perform continuous OIE; i.e., it is capable of automatically replacing the culture medium with electroporation buffers. Integrating medium exchanges on-chip with OIE minimises critical issues such as cell loss and damage, both of which are common in traditional, centrifuge-based approaches. Most importantly, our new system is suitable for handling small or rare cell populations. Two medium exchange modules, including a micropost array railing structure and a deterministic lateral displacement structure, were first adopted and optimised for medium exchange and then integrated with the OIE module. The efficacy of these integrated microfluidic systems was demonstrated by transfecting an enhanced green fluorescent protein (EGFP) plasmid into human embryonic kidney 293T cells, with an efficiency of 8.3%. This result is the highest efficiency reported for any existing OIE-based microfluidic system. In addition, successful co-transfections of three distinct plasmids (EGFP, DsRed and ECFP) into cells were successfully achieved. Hence, we demonstrated that this system is capable of automatically performing multiple gene transfections into mammalian cells. A microfluidic device offers a more streamlined approach for introducing foreign genes into mammalian cells. So-called ‘lab-on-a-chip’ systems could simplify and improve the efficiency of many biological techniques by employing microscale channels and valves to precisely control the movement of fluids, cells and reagents. Researchers led by Gwo-Bin Lee of National Tsing Hua University, Hsinchu, have devised such a system to manage the targeted genetic modification of cultured cells via a process known as electroporation. Lee and colleagues constructed microfluidic devices that employ polymeric structures to direct a continuously flowing stream of cells out of their culture medium and into a specialized module for light-induced electroporation. Their approach achieves remarkable efficiency in introducing individual genes into human cells, and the researchers were even able to use their device to deliver three different plasmids simultaneously.

Numerical and experimental investigation of heat transfer and fluid flow characteristics in a micro-scale serpentine channel

A combined experimental and numerical investigation is carried out to study the characteristics of laminar flow and forced convection heat transfer in a square cross-section wavy “serpentine” microchannel with the upper wall insulated and other side walls held at constant temperature. Experimental measurements of convective heat transfer and pressure drop are performed for 30/70% and 10/90% by weight mixtures of glycerine/water over a range of Dean number from 0.6 to 80. Complementary three-dimensional computational fluid dynamics numerical simulations are also conducted for the same conditions. The results show that the growth of secondary-flow vortices promotes fluid mixing in the serpentine microchannel and leads to an enhancement of the convective heat transfer. As a consequence the serpentine microchannel is able to enhance the performance of heat transfer relative to a straight microchannel over the entire range of Dean number. Meanwhile, at these values of Prandtl number the relative pressure-drop losses increase with increasing Dean number. These increased pressure-drop losses are rather modest over the whole range of Dean number compared with the significant enhancement in heat transfer.