Optimization of Newtonian fluid pressure in microcantilever integrated flexible microfluidic channel for healthcare application

The demand for microfluidic pressure sensors is ever-increasing in various industries due to their crucial role in controlling fluid pressure within microchannels. While syringe pump setups have been traditionally used to regulate fluid pressure in microfluidic devices, they often result in larger setups that increase the cost of the device. To address this challenge and miniaturize the syringe pump setup, the researcher introduced integrated T-microcantilever-based microfluidic devices. In these devices, microcantilevers are incorporated, and their deflections correlate with the microchannel's pressure. When the relative pressure of fluid (plasma) changes, the T-microcantilever deflects, and the extent of this deflection provides information on fluid pressure within the microchannel. In this work, finite element method (FEM) based simulation was carried out to investigate the role of material, and geometric parameters of the cantilever, and the fluid viscosity on the pressure sensing capability of the T-microcantilever integrated microfluidic channel. The T-microcantilever achieves a maximum deflection of 127μm at a 5000μm/s velocity for Young's modulus(E) of 360 kPa of PDMS by employing a hinged structure. On the other hand, a minimum deflection of 4.05 × 10-5μm was attained at 5000μm/s for Young's modulus of 1 TPa for silicon. The maximum deflected angle of the T-cantilever is 20.46° for a 360 kPa Young's modulus while the minimum deflection angle of the T-cantilever is measured at 13.77° for 900 KPa at a fluid velocity of 5000μm s-1. The T-cantilever functions as a built-in microchannel that gauges the fluid pressure within the microchannel. The peak pressure, set at 8.86 Pa on the surface of the cantilever leads to a maximum deflection of 0.096μm (approximately 1μm) in the T-cantilever at a 1:1 velocity ratio. An optimized microfluidic device embedded with microchannels can optimize fluid pressure in a microchannel support cell separation.

An embedded obstacle type micromixer-concentration gradient generator based on capillary driven

An embedded obstacle-type micromixer-concentration gradient generator based on capillary self-driven is proposed and studied. Herringbone structure (HS) for mixing and palisade-shape small channels at the outlet are designed in the device (named HS). Simulation and experimentation are done to study the liquid mixing efficiency in the small channels and concentration gradient at the outlet, and the experimental results agree with the simulation results. For three cases of liquid dripping (sequential, reverse, and delayed drippings), mixing analysis shows that the mixing efficiency increases along both mixing channel and palisade length, and is high in the middle small channel of the palisade-shape area and low on both sides. An obvious concentration gradient at the outlet can form compared with the device without the palisade-shape area. Finally, water pH value detection is done as one of the applications of HS. This study can provide guidance for the application of HS in biochemical detection, cell research, drug screening, etc. based on the capillary-driven effect.

Experimental and Theoretical Investigation on Heat Transfer Enhancement in Micro Scale Using Helical Connectors

Microscale electronics have become increasingly more powerful, requiring more efficient cooling systems to manage the higher thermal loads. To meet this need, current research has been focused on overcoming the inefficiencies present in typical thermal management systems due to low Reynolds numbers within microchannels and poor physical properties of the working fluids. For the first time, this research investigated the effects of a connector with helical geometry on the heat transfer coefficient at low Reynolds numbers. The introduction of a helical connector at the inlet of a microchannel has been experimentally tested and results have shown that this approach to flow augmentation has a great potential to increase the heat transfer capabilities of the working fluid, even at low Reynolds numbers. In general, a helical connector can act as a stabilizer or a mixer, based on the characteristics of the connector for the given conditions. When the helical connector acts as a mixer, secondary flows develop that increase the random motion of molecules and possible nanoparticles, leading to an enhancement in the heat transfer coefficient in the microchannel. Otherwise, the heat transfer coefficient decreases. It is widely known that introducing nanoparticles into the working fluids has the potential to increase the thermal conductivity of the base fluid, positively impacting the heat transfer coefficient; however, viscosity also tends to increase, reducing the random motion of molecules and ultimately reducing the heat transfer capabilities of the working fluid. Therefore, optimizing the effects of nanoparticles characteristics while reducing viscous effects is essential. In this study, deionized water and deionized water-diamond nanofluid at 0.1 wt% were tested in a two-microchannel system fitted with a helical connector in between. It was found that the helical connector can make a great heat transfer coefficient enhancement in low Reynolds numbers when characteristics of geometry are optimized for given conditions.

Portable Sensor for Aflatoxin B1 Based on the Regulation of Resistance of a Microchannel Using a Multimeter as Readout

Changes in the charge density on the inner surface of the microchannel can modulate the ion concentration at the tip, thus causing changes in the resistance of the system. In this study, this property is adopted to construct a portable sensor using a multimeter and aflatoxin B1 (AFB1) is used as the model target. Initially, the cDNA/aptamer complex is modified in the microchannel. The inner microchannel surface's charge density is then altered by the recognition of the target, leading to a change in the system's resistance, which can be conveniently monitored using a multimeter. Critical parameters influencing the performance of the system are optimized. Under optimum conditions, the resistance is linearly related to the logarithm of AFB1 concentration in the range of 100 fM-10 nM and the detection limit is 46 fM (S/N = 3). The resistive measurement is separated from the recognition reaction of the target, reducing the matrix interference during the detection process. This sensor boasts high sensitivity and specificity coupled with commendable reproducibility and stability. It is applied to assay the AFB1 content successfully in an actual sample of corn. Moreover, this approach is cost-effective, user-friendly, and highly accurate.

Percutaneous microchannel unilateral approach bilateral micro decompression for adjacent segmental degeneration after lumbar fusion at 10 years: a case report and review of literature

Background

Adjacent segmental degeneration after lumbar fusion is one of the common long-term complications after lumbar fusion. With the continuous development of adjacent segmental degeneration, patients who fail conservative treatment often need reoperation to relieve symptoms. In recent years, the technique of bilateral microdecompression through unilateral approach under microchannel has been widely used in the treatment of lumbar degenerative diseases. However, the efficacy of this procedure for adjacent-segment degeneration after lumbar fusion has not been established. Here, we report a case of bilateral microscopic decompression via a unilateral approach through a microchannel in a patient with adjacent segmental degeneration after lumbar fusion.

Case report

A 70-year-old male patient was admitted to hospital because of lumbago accompanied by left lower extremity pain, numbness and weakness for 2 years, which aggravated for 2 months. Ten years ago, he underwent PLIF for lumbar spinal stenosis, and recovered well after the operation. According to imaging data and physical examination, the diagnosis was adjacent segmental degeneration after lumbar fusion. Bilateral microdecompression was performed through a unilateral approach under a microchannel. Good clinical outcomes was observed through 1-year postoperative follow-up.

Conclusions

This report reports the successful treatment of a patient with ASD 10 years after lumbar fusion. Bilateral microdecompression via a unilateral approach under a microchannel is a safe and effective method for the treatment of ASD after lumbar fusion with good surgical outcomes.

Impact of skin hydration on patterns of microthermal injury produced by fractional CO2 laser

OBJECTIVES

The impact of skin hydration on patterns of thermal injury produced by ablative fractional lasers (AFLs) is insufficiently examined under standardized conditions. Using skin with three different hydration levels, this study assessed the effect of hydration status on microchannel dimensions generated by a fractional CO2 laser.

METHODS

A hydration model (hyperhydrated-, dehydrated- and control) was established in ex vivo porcine skin, validated by changes in surface conductance and sample mass. After, samples underwent AFL exposure using a CO2 laser (10,600 nm) at two examined pulse energies (10 and 30 mJ/mb, fixed 10% density, six repetitions per group). Histological assessment of distinct microchannels (n = 60) determined three standardized endpoints in H&E sections: (1) depth of microthermal treatment zones (MTZs), (2) depth of microscopic ablation zones (MAZs), and (3) coagulation zone (CZ) thickness. As a supplemental in vivo assessment, the same laser settings were applied to hyperhydrated- (7-h occlusion) and normohydrated forearm skin (no pretreatment) of a human volunteer. Blinded measurement of MAZ depth (n = 30) was performed using noninvasive optical coherence tomography (OCT).

RESULTS

Modest differences in microchannel dimensions were shown between hyperhydrated, dehydrated and control skin at both high and low pulse energy. Compared to controls, hyperhydration led to median reductions in MTZ and MAZ depth ranging from 5% to 8% (control vs. hyperhydrated at 30 mJ/mb; 848 vs. 797 µm (p < 0.003) (MAZ); 928 vs. 856 µm (p < 0.003) (MTZ)), while 14%-16% reductions were shown in dehydrated skin (control vs. dehydrated at 30 mJ/mb; MAZ: 848 vs. 727 µm (p < 0.003); MTZ: 928 vs. 782 µm (p < 0.003)). The impact of skin hydration on CZ thickness was in contrast limited. Corresponding with ex vivo findings, hyperhydration was similarly associated with lower ablative depth in vivo skin. Thus, median MAZ depth in hydrated skin was 10% and 14% lower than in control areas at 10 and 30 mJ/mb pulse energy, respectively (10 mJ: 210 vs. 180 µm (p < 0.001); 30 mJ: 335 vs. 300 µm (p < 0.001)).

CONCLUSION

Skin hydration status can exert a minimal impact on patterns of microthermal injury produced by fractional CO2 lasers, although the clinical implication in the context of laser therapy requires further study.

Alternating Current Electroosmotic Flow of Maxwell Fluid in a Parallel Plate Microchannel with Sinusoidal Roughness

The EOF of a viscoelastic Maxwell fluid driven by an alternating pressure gradient and electric field in a parallel plate microchannel with sinusoidal roughness has been investigated within the Debye-Hückel approximation based on boundary perturbation expansion and separation of variables. Perturbation solutions were obtained for the potential distribution, the velocity and the mean velocity, and the relation between the mean velocity and the roughness. There are significant differences in the velocity amplitudes of the Newtonian and Maxwell fluids. It is shown here that the velocity distribution of the viscoelastic fluid is significantly affected by the roughness of the walls, which leads to the appearance of fluctuations in the fluid. Also, the velocity is strongly dependent on the phase difference θ of the roughness of the upper and lower plates. As the oscillation Reynolds number ReΩ increases, the velocity profile and the average velocity um(t) of AC EOF oscillate rapidly but the velocity amplitude decreases. The Deborah number De plays a similar role to ReΩ, which makes the AC EOF velocity profile more likely to oscillate. Meanwhile, phase lag χ (representing the phase difference between the electric field and the mean velocity) decreases when G and θ are increased. However, for larger λ (e.g., λ > 3), it almost has no phase lag χ.

Experimental Study of Flow Boiling Regimes Occurring in a Microfluidic T-Junction

Microchannel flow boiling is an efficient cooling method for high-heat-flux electronic devices. To understand the flow boiling regime in a T-shaped microchannel, this paper prepared T-shaped microchannels of different sizes and designed an experimental platform for the visualization of flow boiling in microchannels, and aimed to study the evolution characteristics of two-phase flow patterns in T-shaped microchannels. The influences of the flow rate and channel size on the boiling flow pattern inside a T-shaped microchannel were experimentally observed and quantitatively described. The results indicate that the occurrence position of the vaporization core gradually migrates from branch channel to main channel as the wall temperature increases. The flow boiling at the bifurcation of the T-shaped microchannel mainly includes the extrusion fracture flow, bubble flow, plug-annular alternating flow and annular flow, in which the annular flow can be further divided into the intermittent annular flow and the stable annular flow. In addition, a high flow rate and small channel size can lead to the disappearance of the bubble flow, and the presence of the bubble flow delays the appearance of the annular flow.

Dislodgement of Hydrogen Bubbles in Microchannels with Embedded Pillars: An Analytical, Experimental, and Numerical Study

Microchannels with integrated pillars have enhanced the production capabilities and performance of various applications due to their high surface-to-volume ratio. However, emerging gas bubbles can become trapped, potentially limiting the functionality or efficiency of the device when scaled down to the low-micrometer scale. Understanding the conditions required to dislodge these bubbles is thus critical for optimizing microfluidic devices with complex physical behaviors. Here an analytical model is presented that outlines the dislodgment conditions and driving forces for such gas-liquid flows. These terms are derived from the gas-liquid interface properties, geometry, and processing parameters. As the density of the pillar arrangement is scaled down, the resistance to bubble dislodgment typically increases. Nevertheless, the bubble is compelled to dislodge at lower pressure loads when critical volumes are reached. This newly discovered effect is particularly noticeable in densely packed arrays and can be explained by the interplay of increased surface tension, geometrical restrictions, and volume-preserving forces. The analytical terms and effects are validated through novel experimental and numerical methods tailored for microchannels in the low-micrometer scale, showing strong agreement.

A Novel Swept-Back Fishnet-Embedded Microchannel Topology

High in reliability, multi in function, and strong in tracking and detecting, active phased array antennas have been widely applied in radar systems. Heat dissipation is a major technological barrier preventing the realization of next-generation high-performance phased array antennas. As a result of the advancement of miniaturization and the integration of microelectronics technology, the study and development of embedded direct cooling or heat dissipation has significantly enhanced the heat dissipation effect. In this paper, a novel swept-back fishnet-embedded microchannel topology (SBFEMCT) is designed, and various microchannel models with different fishnet runner mesh density ratios and different fishnet runner layers are established to characterize the chip Tmax, runner Pmax, and Vmax and analyze the thermal effect of SBFEMCT under these two operating conditions. The Pmax is reduced to 72.37% and 57.12% of the original at mesh density ratios of 0.5, 0.25, and 0.125, respectively. The maximum temperature reduction figures are average with little change in maximum velocity and a small increase in maximum pressure drop across the number of fishnet runner layers from 0 to 4. This paper provides a study of the latest embedded thermal dissipation from the dimension of a single chip to provide a certain degree of new ideas and references for solving the thermal technology bottleneck of next-generation high-performance phased array antennas.

Electromagnetohydrodynamic (EMHD) Flow in a Microchannel with Random Surface Roughness

This study investigates the effect of small random transverse wall roughness on electromagnetohydrodynamic (EMHD) flow is in a microchannel, employing the perturbation method based upon stationary random function theory. An exact solution of a random corrugation function ξ, which is a measure of the flow rate deviated from the case without the roughness of two plates, is obtained by integrating the spectral density. After the sinusoidal, triangular, rectangular, and sawtooth functions that satisfy the Dirichlet condition are expanded into the Fourier sine series, the spectral density of the sine function is used to represent the corrugation function. Interestingly, for sinusoidal roughness, the final expression of the corrugation function is in good agreement with our previous work. Results show that no matter the shape of the wall roughness, the flow rate always decreases due to the existence of wall corrugation. Variations of the corrugation function and the flow rate strongly depend on fluid wavenumber λ and Hartmann number Ha. Finally, the flow resistance is found to become small, and the flow rate increases with roughness that is in phase (θ = 0) compared with the one that is out of phase (θ = π).

Thermal Performance Optimization of Integrated Microchannel Cooling Plate for IGBT Power Module

In high-integration electronic components, the insulated-gate bipolar transistor (IGBT) power module has a high working temperature, which requires reasonable thermal analysis and a cooling process to improve the reliability of the IGBT module. This paper presents an investigation into the heat dissipation of the integrated microchannel cooling plate in the silicon carbide IGBT power module and reports the impact of the BL series micropump on the efficiency of the cooling plate. The IGBT power module was first simplified as an equivalent-mass block with a mass of 62.64 g, a volume of 15.27 cm3, a density of 4.10 g/cm3, and a specific heat capacity of 512.53 J/(kg·K), through an equivalent method. Then, the thermal performance of the microchannel cooling plate with a main channel and a secondary channel was analyzed and the design of experiment (DOE) method was used to provide three factors and three levels of orthogonal simulation experiments. The three factors included microchannel width, number of secondary inlets, and inlet diameter. The results show that the microchannel cooling plate significantly reduces the temperature of IGBT chips and, as the microchannel width, number of secondary inlets, and inlet diameter increase, the junction temperature of chips gradually decreases. The optimal structure of the cooling plate is a microchannel width of 0.58 mm, 13 secondary inlets, and an inlet diameter of 3.8 mm, and the chip-junction temperature of this structure is decreased from 677 °C to 77.7 °C. In addition, the BL series micropump was connected to the inlet of the cooling plate and the thermal performance of the microchannel cooling plate with a micropump was analyzed. The micropump increases the frictional resistance of fluid flow, resulting in an increase in chip-junction temperature to 110 °C. This work demonstrates the impact of micropumps on the heat dissipation of cooling plates and provides a foundation for the design of cooling plates for IGBT power modules.

Simulations of Flows via CFD in Microchannels for Characterizing Entrance Region and Developing New Correlations for Hydrodynamic Entrance Length

Devices with microchannels are relatively new, and many correlations are not yet developed to design them efficiently. In microchannels, the flow regime is primarily laminar, where entrance length may occupy a significant section of the flow channel. Therefore, several computational fluid dynamic simulations were performed in this research to characterize the developing flow regime. The new correlations of entrance length were developed from a vast number of numerical results obtained from these simulations. A three-dimensional laminar flow for 37 Reynolds numbers (0.1, 0.2, …, 1, 2, …, 10, 20, …, 100, 200, …, 1000), primarily in low regime with water flow through six rectangular microchannels (aspect ratio: 1, 0.75, 0.5, 0.25, 0.2, 0.125), has been modeled, conducting 222 simulations to characterize flow developments and ascertain progressive velocity profile shapes. Examination of the fully developed flow condition was considered using traditional criteria such as velocity and incremental pressure drop number. Additionally, a new criterion was presented based on fRe. Numerical results from the present simulations were validated by comparing the fully developed velocity profile, friction factor, and hydrodynamic entrance length for Re > 100 in rectangular channels, for which accurate data are available in the literature. There is a need for hydrodynamic entrance length correlations in a low Reynolds number regime (Re < 100). So, the model was run numerous times to generate a vast amount of numerical data that yielded two new correlations based on the velocity and fRe criteria.

Numerical Investigation of Fluid Flow and Heat Transfer in High-Temperature Wavy Microchannels with Different Shaped Fins Cooled by Liquid Metal

The microchannel heat sink has been recognized as an excellent solution in high-density heat flux devices for its high efficiency in heat removal with limited spaces; however, the most effective structure of microchannels for heat dissipation is still unknown. In this study, the fluid flow and heat transfer in high-temperature wavy microchannels with various shaped fins, including the bare wavy channel, and the wavy channel with circular, square, and diamond-shaped fins, are numerically investigated. The liquid metal-cooled characteristics of the proposed microchannels are compared with that of the smooth straight channel, with respect to the pressure drop, average Nusselt number, and overall performance factor. The results indicate that the wavy structure and fin shape have a significant effect on the heat sink performance. Heat transfer augmentation is observed in the wavy channels, especially coupled with different shaped fins; however, a large penalty of pressure drops is also found in these channels. The diamond-shaped fins yield the best heat transfer augmentation but the worst pumping performance, followed by the square-, and circular-shaped fins. When the Re number increases from 117 to 410, the Nu number increases by 61.7% for the diamond fins, while the ∆p increases as much as 7.5 times.

Investigation of the effects of hydrophobic surfaces on thermohydraulic characteristics and entropy generation of hybrid nanofluids with magnetic properties in a micro-heat sink under a magnetic field

The cooling process of the devices is a big challenge in the electronic industry, and most of the process units (graphical are central) experience defects under harsh temperature conditions, so dissipating generated heat under various working conditions should seriously be studied. This study investigates the magnetohydrodynamics of hybrid-ferro nanofluids in the presence of hydrophobic surfaces in a micro-heat sink. To scrutinize this study, a Finite Volume Method (FVM is applied. The ferro nanofluid includes water as base fluid and Multiwall Carbon Nanotubes (MWCNTs) and Fe3O4 as nano-additives, which are used in three concentrations (0, 1 and 3%). The other parameters such as Reynold number (5-120), Hartmann number (magnitude of the magnetic field from 0 to 6) and hydrophobicity of surfaces are considered to be scrutinized for their impacts on heat transfer and hydraulic variables as well as entropy generation ones. The outcomes indicate that increasing the level of hydrophobicity in surfaces leads to improve heat exchange and reduces the pressure drop simultaneously. Likewise, it decreases the frictional and thermal types of entropy generations. Intensifying the magnitude of the magnetic field enhances the heat exchange as much as the pressure drop. In the same result, it can decrease the thermal term in entropy generation equations for the fluid, but it increases the frictional one and adds a new term named magnetic entropy generation. Incrementing Reynolds number improves the convection heat transfer parameters although it intensifies the pressure drop in the length of the channel. Also, the thermal and frictional kinds of entropy generation decrease and increase with increasing the flow rate (Reynold number).

Thermal Analysis of a Reactive Variable Viscosity TiO2-PAO Nanolubricant in a Microchannel Poiseuille Flow

This paper examines the flow structure and heat transfer characteristics of a reactive variable viscosity polyalphaolefin (PAO)-based nanolubricant containing titanium dioxide (TiO₂) nanoparticles in a microchannel. The nonlinear model equations are obtained and numerically solved via the shooting method with Runge-Kutta-Fehlberg integration scheme. Pertinent results depicting the effects of emerging thermophysical parameters on the reactive lubricant velocity, temperature, skin friction, Nusselt number and thermal stability criteria are presented graphically and discussed. It is found that the Nusselt number and thermal stability of the flow process improve with exothermic chemical kinetics, Biot number, and nanoparticles volume fraction but lessen with a rise in viscous dissipation and activation energy.

Acoustic Streaming Efficiency in a Microfluidic Biosensor with an Integrated CMUT

The effect of microchannel height on acoustic streaming velocity and capacitive micromachined ultrasound transducer (CMUT) cell damping was investigated. Microchannels with heights ranging from 0.15 to 1.75 mm were used in experiments, and computational microchannel models with heights varying from 10 to 1800 micrometers were simulated. Both simulated and measured data show local minima and maxima of acoustic streaming efficiency associated with the wavelength of the `bulk acoustic wave excited at 5 MHz frequency. Local minima occur at microchannel heights that are multiples of half the wavelength (150 μm), which are caused by destructive interference between excited and reflected acoustic waves. Therefore, microchannel heights that are not multiples of 150 μm are more favorable for higher acoustic streaming effectiveness since destructive interference decreases the acoustic streaming effectiveness by more than 4 times. On average, the experimental data show slightly higher velocities for smaller microchannels than the simulated data, but the overall observation of higher streaming velocities in larger microchannels is not altered. In additional simulation, at small microchannel heights (10-350 μm), local minima at microchannel heights that are multiples of 150 μm were observed, indicating the interference between excited and reflected waves and causing acoustic damping of comparatively compliant CMUT membranes. Increasing the microchannel height to over 100 μm tends to eliminate the acoustic damping effect as the local minima of the CMUT membrane swing amplitude approach the maximum value of 42 nm, which is the calculated amplitude of the freely swinging membrane under the described conditions. At optimum conditions, an acoustic streaming velocity of over 2 mm/s in a 1.8 mm-high microchannel was achieved.

Micro Electro-Osmotic Thrusters of Power-Law Fluids for Space Propulsion

In this article, electro-osmotic thrusters (EOTs), which are full of non-Newtonian power-law fluids with a flow behavior index n of the effective viscosity, are theoretically investigated in a microchannel. Different values of the flow behavior index represent two kinds of non-Newtonian power-law fluids, pseudoplastic fluids (n < 1) and dilatant fluids (n > 1), which have not yet been considered to be used as propellants in micro-thrusters. Analytical solutions of the electric potential and flow velocity are obtained using the Debye-Hückel linearization assumption and the approximate scheme of hyperbolic sine function. Then, thruster performances of power-law fluids, including specific impulse, thrust, thruster efficiency, and thrust-to-power ratio, are explored in detail. Results show that these performance curves strongly depend on the flow behavior index and electrokinetic width. It is noted that the non-Newtonian pseudoplastic fluid is most suitable as a propeller solvent in micro electro-osmotic thrusters owing to its improving or optimizing deficiencies in the performances of the existing Newtonian fluid thrusters.

A Qualitative Experimental Proof of Principle of Self-Assembly in 3D Printed Microchannels towards Embedded Wiring in Biofuel Cells

A range of biotech applications, e.g., microfluidic benthic biofuel cells, require devices with the simultaneous capabilities of embedded electrical wiring, aqueous fluidic access, 3D arrays, biocompatibility, and affordable upscalability. These are very challenging to satisfy simultaneously. As a potential solution, herein we present a qualitative experimental proof of principle of a novel self-assembly technique in 3D printed microfluidics towards embedded wiring combined with fluidic access. Our technique uses surface tension, viscous flow, microchannel geometries, and hydrophobic/hydrophilic interactions to produce self-assembly of two immiscible fluids along the length of the same 3D printed microfluidic channel. The technique demonstrates a major step towards the affordable upscaling of microfluidic biofuel cells through 3D printing. The technique would be of high utility to any application that simultaneously requires distributed wiring and fluidic access inside 3D printed devices.

Influence of Stress Jump Condition at the Interface Region of a Two-Layer Nanofluid Flow in a Microchannel with EDL Effects

The influence of stress jump conditions on a steady, fully developed two-layer magnetohydrodynamic electro-osmotic nanofluid in the microchannel, is investigated numerically. A nanofluid is partially filled into the microchannel, while a porous medium, saturated with nanofluid, is immersed into the other half of the microchannel. The Brinkmann-extended Darcy equation is used to effectively explain the nanofluid flow in the porous region. In both regions, electric double layers are examined, whereas at the interface, Ochoa-Tapia and Whitaker's stress jump condition is considered. The non-dimensional velocity, temperature, and volume fraction of the nanoparticle profiles are examined, by varying physical parameters. Additionally, the Darcy number, as well as the coefficient in the stress jump condition, are investigated for their profound effect on skin friction and Nusselt number. It is concluded that, taking into account the change in shear stress at the interface has a significant impact on fluid flow problems.

Influence factors of channel geometry for separation of circulating tumor cells by four-ring inertial focusing microchannel

Inertial microfluidics is a high-throughput and high-efficiency cell separation approach to which attention has been progressively paid in recent years. However, research on the influencing factors that compromise the efficiency of cell separation is still lacking. Therefore, the aim of this study was to evaluate the cell separation efficiency by changing the influencing factors. A four-ring inertial focusing spiral microchannel was designed to separate two kinds of circulating tumor cells (CTCs) from blood. Human breast cancer (MCF-7) cells and human epithelial cervical cancer (HeLa) cells enter the four-ring inertial focusing spiral microchannel together with blood cells, and cancer cells and blood cells were separated from each other at the outlet of the channel by inertial force. The cell separation efficiency at the inlet flow rate in the Reynolds number range of 40-52 was studied by changing the influencing factors such as the cross-sectional shape of the microchannel, the average thickness of the cross-section, and the trapezoidal inclination angle. The results showed that the reduction of the channel thickness and the increase of a certain trapezoidal inclination enhanced the cell separation efficiency to a certain extent, the study showed that when the channel inclination was 6 ° and the average channel thickness was 160 μm. The two kinds of CTC cells could be completely separated from the blood and the efficiency could reached 100%.

Efficient Focusing of Aerosol Particles in the Microchannel under Reverse External Force: A Numerical Simulation Study

Focusing aerosol particles efficiently is of great significance for high-precision aerosol jet printing and detection of the airborne target. A new method was proposed herein to achieve the efficient focusing of aerosol particles in the microchannel by using a reverse external force. Considering the slip at the interface between the gas and the aerosol particle, a numerical model of the particle movement in the microchannel was established and simulations were conducted on the gas-particle two-phase flow in the microchannel under the effect of the reverse external force. The results showed that a suitable reverse external force in a similar order of magnitude to the Stokes force can dramatically increase the velocity difference between the particle and the gas, which significantly enhances the Saffman lift force exerted on the aerosol particle. Eventually, the aerosol particle can be efficiently focused at the center of the microchannel in a short channel length. In addition, the influence of the channel geometry, the magnitude, and the direction of the external force on the particle focusing was also studied. This work is of great significance for the precise detection of aerosol particles and the design of nozzles for aerosol jet printing.

Effect of Nanosecond Pulsed Currents on Directions of Cell Elongation and Migration through Time-Lapse Analysis

It is generally known that cells elongate perpendicularly to an electric field and move in the direction of the field when an electric field is applied. We have shown that irradiation of plasma-simulated nanosecond pulsed currents elongates cells, but the direction of cell elongation and migration has not been elucidated. In this study, a new time-lapse observation device that can apply nanosecond pulsed currents to cells was constructed, and software to analyze cell migration was created to develop a device that can sequentially observe cell behavior. The results showed nanosecond pulsed currents elongate cells but do not affect the direction of elongation and migration. It was also found the behavior of cells changes depending on the conditions of the current application.

Strain Sensor-Inserted Microchannel for Gas Viscosity Measurement

Quantifying the viscosity of a gas is of great importance in determining its properties and can even be used to identify what the gas is. While many techniques exist for measuring the viscosities of gases, it is still challenging to probe gases with a simple, robust setup that will be useful for practical applications. We introduce a facile approach to estimating gas viscosity using a strain gauge inserted in a straight microchannel with a height smaller than that of the gauge. Using a constrained geometry for the strain gauge, in which part of the gauge deforms the channel to generate initial gauge strain that can be transduced into pressure, the pressure change induced via fluid flow was measured. The change was found to linearly correlate with fluid viscosity, allowing estimation of the viscosities of gases with a simple device.

A Solution to the Clearance Problem of Sacrificial Material in 3D Printing of Microfluidic Devices

3D-printing is poised to enable remarkable advances in a variety of fields, such as artificial muscles, prosthetics, biomedical diagnostics, biofuel cells, flexible electronics, and military logistics. The advantages of automated monolithic fabrication are particularly attractive for complex embedded microfluidics in a wide range of applications. However, before this promise can be fulfilled, the basic problem of removal of sacrificial material from embedded microchannels must be solved. The presented work is an experimental proof of principle of a novel technique for clearance of sacrificial material from embedded microchannels in 3D-printed microfluidics. The technique demonstrates consistent performance (~40-75% clearance) in microchannels with printed width of ~200 µm and above. The presented technique is thus an important enabling tool in achieving the promise of 3D printing in microfluidics and its wide range of applications.

Engineering Human Mesenchymal Bodies in a Novel 3D-Printed Microchannel Bioreactor for Extracellular Vesicle Biogenesis

Human Mesenchymal Stem Cells (hMSCs) and their derived products hold potential in tissue engineering and as therapeutics in a wide range of diseases. hMSCs possess the ability to aggregate into "spheroids", which has been used as a preconditioning technique to enhance their therapeutic potential by upregulating stemness, immunomodulatory capacity, and anti-inflammatory and pro-angiogenic secretome. Few studies have investigated the impact on hMSC aggregate properties stemming from dynamic and static aggregation techniques. hMSCs' main mechanistic mode of action occur through their secretome, including extracellular vesicles (EVs)/exosomes, which contain therapeutically relevant proteins and nucleic acids. In this study, a 3D printed microchannel bioreactor was developed to dynamically form hMSC spheroids and promote hMSC condensation. In particular, the manner in which dynamic microenvironment conditions alter hMSC properties and EV biogenesis in relation to static cultures was assessed. Dynamic aggregation was found to promote autophagy activity, alter metabolism toward glycolysis, and promote exosome/EV production. This study advances our knowledge on a commonly used preconditioning technique that could be beneficial in wound healing, tissue regeneration, and autoimmune disorders.

Thermal-Hydrodynamic Behavior and Design of a Microchannel Pin-Fin Hybrid Heat Sink

A three-dimensional convective heat transfer model of a microchannel pin-fin hybrid heat sink was established. Considering the non-uniform heat generation of 3D stacked chips, the splitting distance of pin-fins was optimized by minimizing the maximum heat sink temperature under different heat fluxes in the hotspot, the Reynolds numbers at the entrance of the microchannel, and the proportions of the pin-fin volume. The average pressure drop and the performance evaluation criteria were considered to be the performance indexes to analyze the influence of each parameter on the flow performance and comprehensive performance, respectively. The results showed that the maximum temperature of the hybrid heat sink attained a minimum value with an increase in the splitting distance. The average pressure drop in the center passage of the microchannel first increased and then decreased. Furthermore, the optimal value could not be simultaneously obtained with the maximum temperature. Therefore, it should be comprehensively considered in the optimization design. The heat flux in the hotspot was positively correlated with the maximum heat sink temperature. However, it had no effect on the flow pressure drop. When the Reynolds number and the pin-fin diameter increased, the maximum heat sink temperature decreased and the average pressure drop of the microchannel increased. The comprehensive performance of the hybrid heat sink was not good at small Reynolds numbers, but it significantly improved as the Reynolds number gradually increased. Choosing a bigger pin-fin diameter and the corresponding optimal value of the splitting distance in a given Reynolds number would further improve the comprehensive performance of a hybrid heat sink.

Review on Coupled Thermo-Hydraulic Performance of Nanofluids and Microchannels

As electronic components continue to be miniaturized, the heat flux density continues to increase. Scholars have proposed the use of microchannel heat sinks (MCHS) to dissipate heat from devices with high heat flux density, and have pointed out that the heat dissipation capability of MCHS can be improved in two ways: using nanofluids with high thermal conductivity and optimizing the structure of MCHS. In this paper, the thermophysical parameters and thermo-hydraulic performance of nanofluids in microchannels are reviewed. Improving the heat dissipation of MCHS is analyzed and discussed in terms of both thermal properties and flow properties, respectively.

Interface Dynamics and the Influence of Gravity on Droplet Generation in a Y-microchannel

The present experimental investigation is focused on the influence of gravity upon water-droplet formation in a Y-microchannel filled with oil. The flows are in the Stokes regime, with very small capillary numbers and Ohnesorge numbers less than one. The study was performed in a square-cross-section channel, with a = 1.0 mm as the characteristic dimension and a flow rate ratio κ in a range between 0.55 and 1.8. The interface dynamics in the vicinity of breakup and the transitory plug flow regime after the detachment of the droplet were analysed. The dependence of droplet length L was correlated with the channel position against the gravity and κ parameters. The results of the work prove that, for κ=1, the droplet length L is independent of channel orientation.

Real-Time Tunable Optofluidic Splitter via Two Laminar Flow Streams in a Microchannel

This paper reports a tunable optofluidic splitter in which the incident light is split via refraction and reflection at the interface between two laminar flows in a microchannel but with different refractive indices. A Y-junction microchannel is used to demonstrate the continuous tuning of the splitting ratio of optical power by smooth adjustment of the ratio of two flow rates. In addition, it has achieved the tuning of split angle from 5° to 19° by the control of the refractive index contrast. The dynamic response gives a fastest switching frequency of 1.67 Hz between the "wave-guiding" and "splitting" status.

Modeling and Mathematical Investigation of Blood-Based Flow of Compressible Rate Type Fluid with Compressibility Effects in a Microchannel

In this investigation, the compressibility effects are visualized on the flow of non-Newtonian fluid, which obeys the stress-strain relationship of an upper convected Maxwell model in a microchannel. The fundamental laws of momentum and mass conservation are used to formulate the problem. The governing nonlinear partial differential equations are reduced to a set of ordinary differential equations and solved with the help of the regular perturbation method assuming the amplitude ratio (wave amplitude/half width of channel) as a flow parameter. The axial component of velocity and flow rate is computed through numerical integration. Graphical results for the mean velocity perturbation function, net flow and axial velocity have been presented and discussed. It is concluded that the net flow rate and Dwall increase in case of the linear Maxwell model, while they decrease in case of the convected Maxwell model. The compressibility parameter shows the opposite results for linear and upper convected Maxwell fluid.

Analysis of Microchannel Heat Sink of Silicon Material with Right Triangular Groove on Sidewall of Passage

Microchannel heat sink (MCHS) is a promising solution for removing the excess heat from an electronic component such as a microprocessor, electronic chip, etc. In order to increase the heat removal rate, the design of MCHS plays a vital role, and can avoid damaging heat-sensitive components. Therefore, the passage of the MCHS has been designed with a periodic right triangular groove in the flow passage. The motivation for this form of groove shape is taken from heat transfer enhancement techniques used in solar air heaters. In this paper, a numerical study of this new design of microchannel passage is presented. The microchannel design has five variable groove angles, ranging from 15° to 75°. Computational fluid dynamics (CFD) is used to simulate this unique microchannel. Based on the Navier-Stokes and energy equations, a 3D model of the microchannel heat sink was built, discretized, and laminar numerical solutions for heat transfer, pressure drop, and thermohydraulic performance were derived. It was found that Nusselt number and thermo-hydraulic performance are superior in the microchannel with a 15° groove angle. In addition, thermohydraulic performance parameters (THPP) were evaluated and discussed. THPP values were found to be more than unity for a designed microchannel that had all angles except 75°, which confirm that the proposed design of the microchannel is a viable solution for thermal management.

Comparative Study of the Thermal and Hydraulic Performance of Supercritical CO2 and Water in Microchannels Based on Entropy Generation

The excellent thermophysical properties of supercritical CO₂ (sCO₂) close to the pseudocritical point make it possible to replace water as the coolant of microchannels in application of a high heat flux radiator. The computational fluid dynamics (CFD) method verified by experimental data is used to make a comparison of the thermal hydraulic behavior in CO₂-cooled and of water-cooled microchannels. The operation conditions of the CO₂-based cooling cases cover the pseudocritical point (with the inlet temperature range of 306~320 K and the working pressure of 8 MPa), and the water-based cooling case has an inlet temperature of 308 K at the working pressure of 0.1 MPa. The channel types include the straight and zigzag microchannels with 90°, 120°, and 150° bending angles, respectively. The analysis result shows that, only when the state of CO₂ is close to the pseudocritical point, the sCO₂-cooled microchannel is of a higher average heat convection coefficient and a lower average temperature of the heated surface compared to the water-cooled microchannel. The entropy generation rate of the sCO₂-cooled microchannel can reach 0.58~0.69 times that of the entropy generation rate for the water-cooled microchannel. Adopting the zigzag structure can enhance the heat transfer, but it does not improve the comprehensive performance represented by the entropy generation rate in the sCO₂-cooled microchannel.

Gas Selectivity Enhancement Using Serpentine Microchannel Shaped with Optimum Dimensions in Microfluidic-Based Gas Sensor

A microfluidic-based gas sensor was chosen as an alternative method to gas chromatography and mass spectroscopy systems because of its small size, high accuracy, low cost, etc. Generally, there are some parameters, such as microchannel geometry, that affect the gas response and selectivity of the microfluidic-based gas sensors. In this study, we simulated and compared 3D numerical models in both simple and serpentine forms using COMSOL Multiphysics 5.6 to investigate the effects of microchannel geometry on the performance of microfluidic-based gas sensors using multiphysics modeling of diffusion, surface adsorption/desorption and surface reactions. These investigations showed the simple channel has about 50% more response but less selectivity than the serpentine channel. In addition, we showed that increasing the length of the channel and decreasing its height improves the selectivity of the microfluidic-based gas sensor. According to the simulated models, a serpentine microchannel with the dimensions W = 3 mm, H = 80 µm and L = 22.5 mm is the optimal geometry with high selectivity and gas response. Further, for fabrication feasibility, a polydimethylsiloxane serpentine microfluidic channel was fabricated by a 3D printing mold and tested according to the simulation results.

CO2-Laser-Micromachined, Polymer Microchannels with a Degassed PDMS slab for the Automatic Production of Monodispersed Water-in-Oil Droplets

In our recent study, we fabricated a pump/tube-connection-free microchip comprising top and bottom polydimethylsiloxane (PDMS) slabs to produce monodispersed water-in-oil droplets in a fully automated, fluid-manipulation fashion. All microstructures required for droplet production were directly patterned on the surfaces of the two PDMS slabs through CO2-laser micromachining, facilitating the fast fabrication of the droplet-production microchips. In the current extension study, we replaced the bottom PDMS slab, which served as a microfluidic layer in the microchip, with a poly(methyl methacrylate) (PMMA) slab. This modification was based on our idea that the bottom PDMS slab does not contribute to the automatic fluid manipulation and that replacing the bottom PDMS slab with a more affordable and accessible, ready-to-use polymer slab, such as a PMMA, would further facilitate the rapid and low-cost fabrication of the connection-free microchips. Using a new PMMA/PDMS microchip, we produced water-in-oil droplets with high degree of size-uniformity (a coefficient of variation for droplet diameters of <5%) without a decrease in the droplet production rate (~270 droplets/s) as compared with that achieved via the previous PDMS/PDMS microchip (~220 droplets/s).

Enhanced Sensing Performance of a Microchannel-Based Electrochemiluminescence Biosensor for Adenosine Triphosphate via a dsDNA Superstructure Amplification Strategy

The sensing performance of a microchannel-based electrochemiluminescence (ECL) biosensor is related to the change ratio of charge density on the surface of microchannels caused by a target recognition reaction. In this study, adenosine triphosphate (ATP) served as a model target. The dsDNA superstructures containing a capture probe (CP, containing an ATP aptamer sequence) and alternating units of ssDNA probes of P1 and P2, CP/(P1/P2)_n, were grafted onto the inner wall of microchannels first. The CP in dsDNA superstructures captured ATP molecules, causing the release of dsDNA fragments containing alternating units of P1 and P2, (P1/P2)_n. The target recognition reaction significantly changed the charge density of microchannels, which altered the ECL intensity of the (1,10-phenanthroline)ruthenium(II)/tripropylamine system in the reporting interface. The ECL intensity of the constructed system had a linear relationship with the logarithm of ATP concentration ranging from 1 fM to 100 pM with a detection limit of 0.32 fM (S/N = 3). The biosensor was successfully applied to detect ATP in rat brains.

Enhancing the Formation and Stability of Oil-In-Water Emulsions Prepared by Microchannels Using Mixed Protein Emulsifiers

Although natural emulsifiers often have many drawbacks when used alone, their emulsifying ability and stability can usually be improved unexpectedly when used in combination. In this study, monodisperse emulsions stabilized by combining two natural protein emulsifiers, i.e., whey protein isolate (WPI) and sodium caseinate (SC), in different proportions were prepared using microchannel (MC) emulsification. The influences of temperature, pH, ionic strength, and storage time on the microstructure and stability of the emulsions were examined. Analysis of the microstructure and droplet size distribution revealed that the WPI-, SC-, and mixed protein-stabilized emulsions exhibited uniform droplet distribution. The droplet size and ξ-potential of the MC emulsions stabilized by mixed protein emulsifiers were higher than those of the emulsions stabilized by WPI or SC separately. The emulsions stabilized by the two types of proteins and mixed emulsifiers had better stability under high salt concentrations than the synthetic emulsifier Tween 20. WPI-SC-stabilized emulsions were more resistant to high temperatures (70–90°C) and exhibited excellent stabilization than those stabilized by WPI and SC, which was attributed to the more sufficient coverage provided by the two types of protein emulsifier layers and better protein adsorption at the oil-water interface. These results indicate that WPI-SC is a potential stabilizer for MC emulsion requirements. This study provides a basis for the formulation of monodisperse and stable natural emulsion systems.

Numerical Investigation of Special Heat Transfer Phenomenon in Wire-Wrapped Fuel Rod of SFR

Sodium-cooled reactors (SFR) have always been recognized as one of the most promising candidates for the fourth-generation nuclear systems as announced by the Generation-IV International Forum. In the design of SFR, helical wire-wrapped rod is applied to stabilize the structure of the rod bundle and enhance coolant mixing. Although there has been considerable research on SFR in computational fluid dynamics (CFD), the phenomenon of heat transfer has rarely been paid attention to. This article discovered that there exists reversed heat flux from coolant to wrapped wire, which is contrary to our usual understanding. This phenomenon has not been reported in previous CFD calculations. Hence, a solid heat conduction model is proposed to prove this phenomenon and analyze the heat transfer process. The simulation results show that the wrapping wire embedding depth, the shape of the calculation domain and the physical properties of all components have great influence on the magnitude of the reversed heat flux. The present findings will have strong influence on the temperature field and maximum value of the fuel rod as well as profound reference value for future flow calculation, especially in grid generation and treatment of the junction between the winding wire and fuel rod.

Ultrafast DNA Amplification Using Microchannel Flow-Through PCR Device

Polymerase chain reaction (PCR) is limited by the long reaction time for point-of-care. Currently, commercial benchtop rapid PCR requires 30−40 min, and this time is limited by the absence of rapid and stable heating and cooling platforms rather than the biochemical reaction kinetics. This study develops an ultrafast PCR (<3 min) platform using flow-through microchannel chips. An actin gene amplicon with a length of 151 base-pairs in the whole genome was used to verify the ultrafast PCR microfluidic chip. The results demonstrated that the channel of 56 μm height can provide fast heat conduction and the channel length should not be short. Under certain denaturation and annealing/extension times, a short channel design will cause the sample to drive slowly in the microchannel with insufficient pressure in the channel, causing the fluid to generate bubbles in the high-temperature zone and subsequently destabilizing the flow. The chips used in the experiment can complete 40 thermal cycles within 160 s through a design with the 56 µm channel height and with each thermal circle measuring 4 cm long. The calculation shows that the DNA extension speed is ~60 base-pairs/s, which is consistent with the theoretical speed of the Klen Taq extension used, and the detection limit can reach 67 copies. The heat transfer time of the reagent on this platform is very short. The simple chip design and fabrication are suitable for the development of commercial ultrafast PCR chips.

Combined electromechanically driven pulsating flow of nonlinear viscoelastic fluids in narrow confinements

Controlled microscale transport is at the core of many scientific and technological advancements, including medical diagnostics, separation of biomolecules, etc., and often involves complex fluids. One of the challenges in this regard is to actuate flows at small scales in an energy efficient manner, given the strong viscous forces opposing fluid motion. We try to address this issue here by probing a combined time-periodic pressure and electrokinetically driven flow of a viscoelastic fluid obeying the simplified linear Phan-Thien-Tanner model, using numerical as well as asymptotic tools, in view of the fact that oscillatory fields are less energy intensive. We establish that the interplay between oscillatory electrical and mechanical forces can lead to complex temporal mass flow rate variations with short-term bursts and peaks in the flow rate. We further demonstrate that an oscillatory pressure gradient or an electric field, in tandem with another steady actuating force can indeed change the net throughput significantly-a paradigm that is not realized in Newtonian or other simpler polymeric liquids. Our results reveal that the extent of augmentation in the flow rate strongly depends on the frequency of the imposed actuating forces along with their waveforms. We also evaluate the streaming potential resulting from an oscillatory pressure-driven flow and illustrate that akin to the volume throughput, the streaming potential also shows complex temporal variations, while its time average gets augmented in the presence of a time-periodic pressure gradient in a nonlinear viscoelastic medium.

Microchannel-embedded implantable device with fibrosis suppression for prolonged controlled drug delivery

For the prolonged, controlled delivery of systemic drugs, we propose an implantable drug-delivery chip (DDC) embedded with pairs of a microchannel and drug-reservoir serving as a drug diffusion barrier and depot, respectively. We pursued a DDC for dual drugs: a main-purpose drug, diclofenac (DF), for systemic exposure, and an antifibrotic drug, tranilast (TR), for local delivery. Thus, the problematic fibrotic tissue formation around the implanted device could be diminished, thereby less hindrance in systemic exposure of DF released from the DDC. First, we separately prepared DDCs for DF or TR delivery, and sought to find a proper microchannel length for a rapid onset and sustained pattern of drug release, as well as the required drug dose. Then, two distinct DDCs for DF and TR delivery, respectively, were assembled to produce a Dual_DDC for the concurrent delivery of DF and TR. When the Dual_DDC was implanted in living rats, the DF concentration in blood plasma did not drop significantly in the later periods after implantation relative to that in the early periods before fibrotic tissue formation. When the Dual_DDC was implanted without TR, there was a significant decrease in the blood plasma DF concentration as the time elapsed after implantation. Biopsied tissues around the Dual_DDC exhibited a significant decrease in the fibrotic capsule thickness and collagen density relative to the Dual_DDC without TR, owing to the effect of the local, sustained release of the TR.

Automatic microchannel detection using deep learning in intravascular optical coherence tomography images

Microchannel formation is known to be a significant marker of plaque vulnerability, plaque rupture, and intraplaque hemorrhage, which are responsible for plaque progression. We developed a fully-automated method for detecting microchannels in intravascular optical coherence tomography (IVOCT) images using deep learning. A total of 3,075 IVOCT image frames across 41 patients having 62 microchannel segments were analyzed. Microchannel was manually annotated by expert cardiologists, according to previously established criteria. In order to improve segmentation performance, pre-processing including guidewire detection/removal, lumen segmentation, pixel-shifting, and noise filtering was applied to the raw (r,θ) IVOCT image. We used the DeepLab-v3 plus deep learning model with the Xception backbone network for identifying microchannel candidates. After microchannel candidate detection, each candidate was classified as either microchannel or no-microchannel using a convolutional neural network (CNN) classification model. Our method provided excellent segmentation of microchannel with a Dice coefficient of 0.811, sensitivity of 92.4%, and specificity of 99.9%. We found that pre-processing and data augmentation were very important to improve results. In addition, a CNN classification step was also helpful to rule out false positives. Furthermore, automated analysis missed only 3% of frames having microchannels and showed no false positives. Our method has great potential to enable highly automated, objective, repeatable, and comprehensive evaluations of vulnerable plaques and treatments. We believe that this method is promising for both research and clinical applications.

The Streaming Potential of Fluid through a Microchannel with Modulated Charged Surfaces

In this paper, the effects of asymmetrically modulated charged surfaces on streaming potential, velocity field and flow rate are investigated under the axial pressure gradient and vertical magnetic field. In a parallel-plate microchannel, modulated charged potentials on the walls are depicted by the cosine function. The flow of incompressible Newtonian fluid is two-dimensional due to the modulated charged surfaces. Considering the Debye-Hückel approximation, the Poisson-Boltzmann (PB) equation and the modified Navier-Stokes (N-S) equation are established. The analytical solutions of the potential and velocities (u and v) are obtained by means of the superposition principle and stream function. The unknown streaming potential is determined by the condition that the net ionic current is zero. Finally, the influences of pertinent dimensionless parameters (modulated potential parameters, Hartmann number and slip length) on the flow field, streaming potential, velocity field and flow rate are discussed graphically. During the flow process and under the impact of the charge-modulated potentials, the velocity profiles present an oscillating characteristic, and vortexes are generated. The results show that the charge-modulated potentials are beneficial for the enhancement of the streaming potential, velocity and flow rate, which also facilitate the mixing of fluids. Meanwhile, the flow rate can be controlled through the use of a low-amplitude magnetic field.

Design and Fabrication of Double-Layer Crossed Si Microchannel Structure

A four-step etching method is used to prepare the double-layer cross Si microchannel structure. In the first etching step, a <100> V-groove structure is etched on (100) silicon, and the top channel is formed after thermal oxidation with the depth of the channel and the slope of its sidewall being modulated by the etching time. The second etching step is to form a sinking substrate, and then the third step is to etch the bottom channel at 90° (<100> direction) and 45° (<110> direction) with the top channel, respectively. Hence, the bottom channel on the sink substrate is half-buried into the top channel. Undercut characteristic of 25% TMAH is used to perform the fourth step, etching through the overlapping part of the two layers of channels to form a double-layer microchannel structure. Different from the traditional single-layer microchannels, the double-layer crossed microchannels are prepared by the four-step etching method intersect in space but are not connected, which has structural advantages. Finally, when the angle between the top and bottom is 90°, the root cutting time at the intersection is up to 6 h, making the width of the bottom channel 4–5 times that of the top channel. When the angle between the top and bottom is 45°, the root cutting time at the intersection is only 4 h, and due to the corrosion along (111), the corrosion speed of the sidewall is very slow and the consistency of the width of the upper and lower channels is better than 90° after the end. Compared with the same-plane cross channel structure, the semiburied microchannel structure avoids the V-shaped path at the intersection, and the fluid can pass through the bottom channel in a straight line and cross with the top channel without overlapping, which has a structural advantage. If applied to microfluidic technology, high-efficiency delivery of two substances can be carried out independently in the same area; if applied to microchannel heat dissipation technology, the heat conduction area of the fluid can be doubled under the same heat dissipation area, thereby increasing the heat dissipation efficiency.

Electrostatic Potential Analysis in Polyelectrolyte Brush-Grafted Microchannels Filled with Polyelectrolyte Dispersion

In this study, the model framework that includes almost all relevant parameters of interest has been developed to quantify the electrostatic potential and charge density occurring in microchannels grafted with polyelectrolyte brushes and simultaneously filled with polyelectrolyte dispersion. The brush layer is described by the Alexander-de Gennes model incorporated with the monomer distribution function accompanying the quadratic decay. Each ion concentration due to mobile charges in the bulk and fixed charges in the brush layer can be determined by multi-species ion balance. We solved 2-dimensional Poisson–Nernst–Planck equations adopted for simulating electric field with ion transport in the soft channel, by considering anionic polyelectrolyte of polyacrylic acid (PAA). Remarkable results were obtained regarding the brush height, ionization, electrostatic potential, and charge density profiles with conditions of brush, dispersion, and solution pH. The Donnan potential in the brush channel shows several times higher than the surface potential in the bare channel, whereas it becomes lower with increasing PAA concentration. Our framework is fruitful to provide comparative information regarding electrostatic interaction properties, serving as an important bridge between modeling and experiments, and is possible to couple with governing equations for flow field.

Heat Transfer and Fluid Flow Characteristics of Microchannel with Oval-Shaped Micro Pin Fins

A novel microchannel heat sink with oval-shaped micro pin fins (MOPF) is proposed and the characteristics of fluid flow and heat transfer are studied numerically for Reynolds number (Re) ranging from 157 to 668. In order to study the influence of geometry on flow and heat transfer characteristics, three non-dimensional variables are defined, such as the fin axial length ratio (α), width ratio (β), and height ratio (γ). The thermal enhancement factor (η) is adopted as an evaluation criterion to evaluate the best comprehensive thermal-hydraulic performance of MOPF. Results indicate that the oval-shaped pin fins in the microchannel can effectively prevent the rise of heat surface temperature along the flow direction, which improves the temperature distribution uniformity. In addition, results show that for the studied Reynolds number range and microchannel geometries in this paper, the thermal enhancement factor η increases firstly and then decreases with the increase of α and β. In addition, except for Re = 157, η decreases first and then increases with the increase of the fin height ratio γ. The thermal enhancement factor for MOPF with α = 4, β = 0.3, and γ = 0.5 achieves 1.56 at Re = 668. The results can provide a theoretical basis for the design of a microchannel heat exchanger.

Enhanced Fog Harvesting through Capillary-Assisted Rapid Transport of Droplet Confined in the Given Microchannel

A novel integrated bioinspired surface is fabricated by using an innovative capillarity-induced selective oxidation method, to achieve the combination of the fog-collecting characteristics of a variety of creatures, i.e., the micronanostructures of spider silk, the wettable patterns of desert beetle, the conical structure of cactus spine, and the hierarchical microchannel of Sarracenia trichome. The fog is captured effectively via multistructures on the cone tips, and captured droplet is collected and confined in the microchannel to realize rapid transport via the formation of wettable pattern on the surface and the introduction of wettable gradient in the microchannel. Consequently, the fog harvest efficiency reaches 2.48 g/h, increasing to nearly 320% compared to the normal surface. More interestingly, similar to Sarracenia trichome, the surface also presents two transport modes, namely, Mode I (water transport along dry microchannel) and Mode II (succeeding water slippage on the water film). In Mode II, the velocity of 34.10 mm/s is about three times faster than that on the Sarracenia trichome. Such a design of integrated bioinspired surface may present potential applications in high-efficiency water collection systems, microfluidic devices, and others.

Single-step Trypsin Inhibitor Assay on a Microchannel Array Device Immobilizing Enzymes and Fluorescent Substrates by Inkjet Printing

In this paper, we report a single-step trypsin inhibitor assay on a microchannel array device immobilizing enzymes and substrates by inkjet printing. The microdevice is composed of a poly(dimethylsiloxane) (PDMS) microchannel array that immobilizes trypsin and fluorescent substrates as reactive reagents at the two bottom corners of a microchannel. Inkjet printers allow simple, accurate, and position-selective immobilization of reagents as nanoliter spots. Therefore, plural reactive reagents, such as enzymes and substrates, can be separately immobilized at different positions in the same microchannel without mixing, and thus allowing for single-step operation by simply introducing a sample solution through capillary action. Furthermore, reproducible fabrication and mass production of the device could be expected. In this study, the efficiency of an acidic solution as a spotting agent for protease immobilization to prevent decrease in the fluorescence intensity was confirmed. Additionally, single-step trypsin inhibitor screening was performed using three inhibitors. Finally, we investigated the storage stability of the device and confirmed that it remained stable for at least 10 days.

Numerical Analysis of Viscous Dissipation in Microchannel Sensor Based on Phononic Crystal

Phononic crystals with phononic band gaps varying in different parameters represent a promising structure for sensing. Equipping microchannel sensors with phononic crystals has also become a great area of interest in research. For building a microchannels system compatible with conventional micro-electro-mechanical system (MEMS) technology, SU-8 is an optimal choice, because it has been used in both fields for a long time. However, its mechanical properties are greatly affected by temperature, as this affects the phononic bands of the phononic crystal. With this in mind, the viscous dissipation in microchannels of flowing liquid is required for application. To solve the problem of viscous dissipation, this article proposes a simulation model that considers the heat transfer between fluid and microchannel and analyzes the frequency domain properties of phononic crystals. The results show that when the channel length reaches 1 mm, the frequency shift caused by viscous dissipation will significantly affect detecting accuracy. Furthermore, the temperature gradient also introduces some weak passbands into the band gap. This article proves that viscous dissipation does influence the band gap of phononic crystal chemical sensors and highlights the necessity of temperature compensation in calibration. This work may promote the application of microchannel chemical sensors in the future.

Visualization of Local Concentration and Viscosity Distribution during Glycerol-Water Mixing in a Y-Shape Minichannel: A Proof-of-Concept-Study

The work presents an efficient and non-invasive method to visualize the local concentration and viscosity distribution of two miscible and non-reacting substances with a significant viscosity difference in a microchannel with a Y-shape cell. The proof-of-concept setup consists of a near-infrared (NIR) camera and cost-effective dome lighting with NIR light-emitting diodes (LED) covering the wavelength range of 1050 to 1650 nm. Absorption differences of glycerol and water and their mixtures with a mass fraction of glycerol from 0 to 0.95 gGlycgtotal-1 were analyzed in the NIR spectral area. The resulting measurement images were converted in a concentration profile by using absorbance calculated with Lambert-Beer law. A linear behavior between the concentration and the absorption coefficient is demonstrated. The result of local concentration in mass fraction was used to determine the local viscosity and illustrated as distribution images. By variating the fluid parameters, the influences of the highly different original viscosities in the mixing procedure were investigated and visualized.