Microchannel Walls Research Articles

Microfluidic preparation of double emulsions using a high aspect ratio double co-flow device

An innovative microfluidic device for one-step production of W/O/W and O/W/O double emulsions is introduced. The proposed geometry inspired by a combination of single and two-step methods consists of two same-level co-flow configurations with a high aspect ratio of height to the channel's width. The designed geometry offers a mechanism to generate double-emulsion drops without requiring regular partial surface modification methods. It offers a simple, robust, and permanent method to fabricate compound droplets in a profoundly controlled manner. Due to the high aspect ratio of the designed geometry, the inner phase jet stream remains enclosed to the middle phase jet, which prevents it from encountering microchannel walls. Hence, the wettability of the channels' surface does not influence the core droplet, implying the double emulsion formation can be achieved without requiring partial wettability treatment. The effects of continuous phase flow rate on the double emulsion size and formation frequency are experimentally investigated. The proposed microfluidic device is able to fabricate relatively high monodispersed and high throughput double emulsions successfully, i.e., the generation frequency can be reached up to 350 Hz while the monodispersity remains below 6%.

A liquid-liquid microreactor for the intensification of hexavalent chromium removal from wastewaters

There is a lively interest to improve hexavalent chromium removal from aqueous solutions since it is highly toxic and can be found in many industrial wastewaters. In this work, a novel slug flow microreactor (SFMR) was used as a new-generation contactor to intensify the Cr (VI) extraction. The applied SFMR was a T-shaped microreactor with a circular microchannel. The microchannel wall wettability profile was altered using a microchannel made of alternate PTFE and stainless steel parts, leading to a significant hydrodynamic change along with slug coalescence and elongation. At low total flow rates, the wall wettability alteration enhanced the Cr (VI) extraction; however, it reduced the extraction at the high total flow rates. The observed results were discussed in details and the liable mechanisms were unveiled. The influence of the microreactor size was also studied, showing the remarkable enhancement of the chromium removal by reducing the microreactor diameter. The time required to reach equilibrium was significantly reduced to even almost 3 s by the reduction of the microreactor size, which was 100–1000-folds shorter than that reported for the conventional contacting systems. The silica nanoparticles were then employed to achieve a higher chromium extraction. Hydrophobic silica nanoparticles led to an increase of the Cr (VI) removal; however, hydrophilic nanoparticles showed a negligible influence. Finally, an empirical correlation was proposed to predict mass transfer.

Improvement of oxide layers formed by plasma electrolytic oxidation on cast Al Si alloy by incorporating TiC nanoparticles

The effect of titanium carbide nanoparticles (TiC NPs) on the structure, chemical and phase composition of the oxide layers obtained by plasma electrolytic oxidation (PEO) on the aluminum-silicon alloy (7.5 wt% Si) was investigated. A significant increase in hardness and effective elastic modulus (~1.4 times), wear resistance (~3 times) and corrosion resistance (~10 times) along with a substantial increase of the layer thickness (about 30%) were found to be connected with chemically inert incorporation of TiC NPs into the oxide layer, while the volume fraction of the incorporated particles in the oxide layer was only about 1%. Improvement of the layer properties was concerned with a decrease in the number of pores and cracks, an increase in the layer crystallinity, and a shift in the phase composition towards more stable phases such as mullite. The assumed mechanism of the TiC NPs effect is a decrease in the breakdown voltage of vapor-gas bubbles (VGB) due to sparking (local breakdowns) from cathodic electrolyte onto the TiC nanoparticles incorporated into the microchannel walls. Sparking to these particles can promote ionization of the gaseous medium, providing earlier microarc discharge through the entire VGB. It results in a substantial increase in the number of microarc discharges, which then initiate larger volumes of melted splash metal and subsequently larger volumes of oxide layer. The higher average temperature results in a decrease in the defectiveness of the macrostructure, the removal of gas from the layer, the growth of crystallites, the removal of residual strains, and a stabilization in the phase composition.

Transient Flow Boiling and Maldistribution Characteristics in Heated Parallel Channels Induced by Flow Regime Oscillations

Flow boiling provides an effective means of heat removal but can suffer from thermal and hydrodynamic transients that compromise heat transfer performance and trigger device failure. In this study, the transient flow boiling characteristics in two thermally isolated, hydrodynamically coupled parallel microchannels are investigated experimentally. High-speed flow visualization is synchronized to high-frequency heat flux, wall temperature, pressure drop, and mass flux measurements to provide time-resolved characterization. Two constant and two transient heating conditions are presented. For a constant heat flux of 63 kW/m² into each channel, boiling occurs continuously in both channels and the parallel channel instability is observed to occur at 15 Hz. Time-periodic oscillations in the pressure drop and average mass flux are observed, but corresponding oscillations in the wall temperatures are virtually nonexistent at this condition. At a slightly lower constant heat flux of 60 kW/m², boiling remains continuous in one of the channels, but the other channel experiences time-periodic flow regime oscillations between single-phase and two-phase flow. At this condition, extreme time-periodic wall temperature oscillations are observed in both channels with a long period (~7 s) due to oscillations in the severity of the flow maldistribution. For the transient heating conditions, square-wave heating profiles oscillating between different heat flux levels are applied to the channels. Because of their relatively high frequency, the heating transients are attenuated by the microchannel walls, resulting in effectively constant heating conditions and flow boiling characteristics like that of the aforementioned constant heating conditions. This study illustrates the susceptibility of parallel two-phase heat sinks to flow maldistribution, particularly when undergoing transient flow regime oscillations.

Toward the Development of Rapid, Specific, and Sensitive Microfluidic Sensors: A Comprehensive Device Blueprint.

Recent advances in nano/microfluidics have led to the miniaturization of surface-based chemical and biochemical sensors, with applications ranging from environmental monitoring to disease diagnostics. These systems rely on the detection of analytes flowing in a liquid sample, by exploiting their innate nature to react with specific receptors immobilized on the microchannel walls. The efficiency of these systems is defined by the cumulative effect of analyte detection speed, sensitivity, and specificity. In this perspective, we provide a fresh outlook on the use of important parameters obtained from well-characterized analytical models, by connecting the mass transport and reaction limits with the experimentally attainable limits of analyte detection efficiency. Specifically, we breakdown when and how the operational (e.g., flow rates, channel geometries, mode of detection, etc.) and molecular (e.g., receptor affinity and functionality) variables can be tailored to enhance the analyte detection time, analytical specificity, and sensitivity of the system (i.e., limit of detection). Finally, we present a simple yet cohesive blueprint for the development of high-efficiency surface-based microfluidic sensors for rapid, sensitive, and specific detection of chemical and biochemical analytes, pertinent to a variety of applications.

Dimensionless analysis on liquid–liquid two-phase flow patterns in a numbered-up microfluidic device

Flow patterns of six liquid–liquid systems in a numbered-up microfluidic set-up were studied in this work. Chloroform-water, butyl acetate-water, ethyl acetate-water, kerosene-water, butanol-water and octanol–water were chosen as test liquids. Three distinct flow regimes were observed in the parallelized microfluidic device: slug flow, droplet flow and parallel flow. A uniform flow distribution was achieved in the scaled-out microfluidic device for all observed flow regimes. Flow non-uniformity was determined based on both the volumetric flow rate and slug size/droplet size/interface position for each microchannel. The two-phase interface's position relative to the microchannel walls was considered a criterion for flow non-uniformity measurement in parallel flow. A comprehensive dimensionless analysis was also performed to determine which dimensionless numbers are appropriate for presenting universal flow maps. Re-0.31We0.41, We0.07Ca0.31, Re0.07Ca0.41, Ca0.5Oh-0.14, We0.25Oh0.31 and Re0.5Oh0.82 were identified as the suitable combinations of dimensionless numbers for proposing generalized flow pattern maps for the numbered-up set-up. For the slug and droplet flow regimes, slug and droplet length and velocity were also investigated.

Hydraulic microactuator with coarse/fine drive-switching mechanism

For precise physical and chemical analyses of cells in microfluidic devices, they should be located near the sensors on a microchannel wall. To transport cells close to the sensors precisely, a hydraulic microactuator with a coarse/fine drive-switching mechanism was developed. The microactuator has a movable thin film with micropillars, and switches drive modes through the contacts of the micropillars into a microchannel wall. A coarse approach and a fine adjustment of the position of cells on the thin film toward the microchannel wall by switching the drive modes was achieved in the microactuator. The displacement of the microactuator was measured at 1 µm resolution using the focus degrees obtained using the two-dimensional fast Fourier transformed images. The displacement ratio of the coarse to fine drive mode achieved using the microactuator was 13.9. Using the microactuator, cyanobacteria were introduced into the device and were transported 53 µm from the bottom to the top of the microchannel in the coarse drive mode, and the position of the cyanobacteria was precisely adjusted near the microchannel wall in the fine drive mode.

Analysis of the forced convection of two-phase Ferro-nanofluid flow in a completely porous microchannel containing rotating cylinders

In the present work, the forced convection of nanofluid flow in a microchannel containing rotating cylinders is investigated in different geometries. The heat flux applied to the microchannel wall is 10,000 W m−2. The effects of Reynolds number, the volume fraction of nanoparticles, and the porosity percentage of the porous medium are investigated on the flow fields, temperature, and heat transfer rate. Reynolds number values vary from Re = 250–1000, non-dimensional rotational velocities 1 and 2, respectively, and volume fraction of nanoparticles 0–2%. The results show that increasing the velocity of rotating cylinders increases the heat transfer; also, increasing the Reynolds number and volume fraction of nanoparticles increases the heat transfer, pressure drop, and Cf,ave. By comparing the porosity percentages with each other, it is concluded that due to the greater contact of the nanofluid with the porous medium and the creation of higher velocity gradients, the porosity percentage is 45% and the values of are 90% higher than the porosity percentage. Comparing porosity percentages with each other, at porosity percentage 90% is greater than at porosity percentage 45%. On the other hand, increasing the Reynolds number reduces the entropy generation due to heat transfer and increases the entropy generation due to friction. Increasing the volume fraction of nanoparticles increases the entropy generations due to heat transfer and friction.

Numerical Analysis of the Heterogeneity Effect on Electroosmotic Micromixers Based on the Standard Deviation of Concentration and Mixing Entropy Index

One approach to achieve a homogeneous mixture in microfluidic systems in the quickest time and shortest possible length is to employ electroosmotic flow characteristics with heterogeneous surface properties. Mixing using electroosmotic flow inside microchannels with homogeneous walls is done primarily under the influence of molecular diffusion, which is not strong enough to mix the fluids thoroughly. However, surface chemistry technology can help create desired patterns on microchannel walls to generate significant rotational currents and improve mixing efficiency remarkably. This study analyzes the function of a heterogeneous zeta-potential patch located on a microchannel wall in creating mixing inside a microchannel affected by electroosmotic flow and determines the optimal length to achieve the desired mixing rate. The approximate Helmholtz–Smoluchowski model is suggested to reduce computational costs and simplify the solving process. The results show that the heterogeneity length and location of the zeta-potential patch affect the final mixing proficiency. It was also observed that the slip coefficient on the wall has a more significant effect than the Reynolds number change on improving the mixing efficiency of electroosmotic micromixers, benefiting the heterogeneous distribution of zeta-potential. In addition, using a channel with a heterogeneous zeta-potential patch covered by a slip surface did not lead to an adequate mixing in low Reynolds numbers. Therefore, a homogeneous channel without any heterogeneity would be a priority in such a range of Reynolds numbers. However, increasing the Reynolds number and the presence of a slip coefficient on the heterogeneous channel wall enhances the mixing efficiency relative to the homogeneous one. It should be noted, though, that increasing the slip coefficient will make the mixing efficiency decrease sharply in any situation, especially in high Reynolds numbers.

Water–copper nanofluid flow in flat and ribbed microchannels: numerical modeling and optimization

PurposeThis paper aims to simulate the nanofluid forced convection in a microchannel. According to the results, at high Reynolds numbers and higher nanofluid volume fractions, an increase in the rib height and slip coefficient further improved the heat transfer rate. The ribs also affect the flow physics depending on the Reynolds number so that the slip velocity decreases with increasing the nanofluid volume fraction and rib height.Design/methodology/approachForced heat transfer of the water–copper nanofluid is numerically studied in a two dimensional microchannel. The effects of the slip coefficient, Reynolds number, nanofluid volume fraction and rib height are investigated on the average Nusselt number, slip velocity on the microchannel wall and the performance evaluation criterion.FindingsIn contrast, the slip velocity increases with increasing the Reynolds number and slip coefficient. Afterwards, a non-parametric function estimation is performed relying on the artificial neural network.Originality/valueFinally, the Genetic Algorithm was used to establish a set of optimal decision parameters for the problem

Enhanced Enzymatic Synthesis of Nicotinamide in Laminar Flow Intensified Microreactors: Models and Simulations

The intracellular level of nicotinamide adenine dinucleotide (NAD+) decreases with age, and this depletion seems to worsen during complex SARS-CoV-2 infections. Increasing the level of NAD+ via nicotinamide and nicotinamide riboside kinase routes may re-establish the antiviral poly-ADP-ribose polymerase functions of infected elderly persons and may potentially restrict SARS-CoV-2 infection by boosting innate immunity. In this work, we simulated nicotinamide synthesis in laminar flow microreactors with nitrile hydratase enzyme uniformly attached on the microchannel wall and/or immobilized on the surface of microparticles dispersed in the liquid phase. Hydrodynamic/diffusional limitations of laminar flow microreactors are greatly alleviated in intensified flow microreactors and microreactors with an enhanced enzymatic process, which greatly improves when nitrile hydratase loading on the surface of microparticles increases (more enzyme works as a free solution-phase enzyme). Enhanced nicotinamide synthesis in intensified flow microreactors generates high conversions even if an enzymatic module (attached enzyme on the channel wall or microparticle surface) does no work at optimal parameters. ©

Influences of microparticle radius and microchannel height on SSAW-based acoustophoretic aggregation

The use of acoustic waves for microfluidic aggregation has become widespread in chemistry, biology and medicine. Although numerous experimental and analytical studies have been undertaken to study the acoustophoretic aggregation mechanisms, few studies have been conducted to optimise the device design. This paper presents a numerical investigation of the acoustophoresis of microparticles suspended in compressible liquid. The wall of the rectangular microchannel is made of Polydimethylsiloxane (PDMS), and Standing Surface Acoustic Waves (SSAW) are introduced into the channel from the bottom wall. First, the relative amplitude of the acoustic radiation force and the viscous drag force is evaluated for particles of different radii ranging from 0.1μm to 15μm. Only when the particle size is larger than a critical value can the particles accumulate at acoustic pressure nodes (PNs). The efficiency of the particle accumulation depends on the microchannel height, so an extensive parametric study is then undertaken to identify the optimum microchannel height. The optimum height, when normalised by the acoustic wavelength, is found to be between 0.57 and 0.82. These findings provide insights into the design of acoustophoretic devices.

Label-Free, Color-Indicating, Polarizer-Free Dye-Doped Liquid Crystal Microfluidic Polydimethylsiloxane Biosensing Chips for Detecting Albumin.

We reveal a novel design for dye-doped liquid crystal (DDLC) microfluidic biosensing chips in the polydimethylsiloxane material. With this design chip, the orientation of DDLCs was affected by the interface between the walls of the channels and DDLCs. When the inside of a channel was coated with an N,N-dimethyl-n-octadecyl-3-aminopropyltrimethoxysilyl chloride (DMOAP) alignment layer, the DDLCs oriented homeotropically in a homeotropic (H) state under cross-polarized microscopy. After immobilization of antigens with antibodies on the alignment layer-coated microchannel walls, the optical intensity of the DDLC change from the dark H state to the bright planar (P) state. Using pressure-driven flow, the binding of antigens/antibodies to the DDLCs could be detected in an experimental sequential order. The microfluidic DDLCs were tested by detecting bovine serum albumin (BSA) and its immune-responses of antigens/antibodies. We proved that this immunoassay chip was able to detect BSA antigens/antibodies pairs with the detection limit about 0.5 µg/mL. The novel DDLC chip was shown to be a simple, multi-detection device, and label-free microfluidic chips are presented.

Selective catalytic hydrogenation of the triple bond in acetylenic alcohols in a microcapillary reactor for fine organic synthesis

The purpose of this work is to present our latest achievements in the design, synthesis and development of a new catalytic microreactor for the processes of selective and continuous synthesis of substances with added value that are used in fine organic synthesis and pharmaceutical industries. In this advanced device, metal nanoparticles are embedded in mesoporous materials, which are deposited in the form of highly active and selective catalytic films on the walls of capillary microchannels with diameters of 220 and 530 μm. In such microreactor systems, there is no subsequent separation of the catalyst, which facilitates continuous operation. Potentially the most stable coatings are crystalline or amorphous titanium dioxide based metal oxides synthesized by the sol-gel method and containing embedded catalytically active polymetallic nanoparticles. The catalytic properties and operational stability of capillary microreactors were investigated at atmospheric pressure and compared with conventional industrial batch reactors. The developed Pd50Zn50 /TiO2 coating showed a higher activity (1.5 gMBE·gPd–1·s–1) and selectivity (96.7 %) in comparison with the coatings described earlier in the literature and with the industrial Lindlar catalyst, and retained high catalytic performance after 88 hours of reaction.

Profiling a soft solid layer to passively control the conduit shape in a compliant microchannel during flow.

The shape of a microchannel during flow through it is instrumental to understanding the physics that govern various phenomena ranging from rheological measurements of fluids to separation of particles and cells. Two commonly used approaches for obtaining a desired channel shape (for a given application) are (i) fabricating the microchannel in the requisite shape and (ii) actuating the microchannel walls during flow to obtain the requisite shape. However, these approaches are not always viable. We propose an alternative, passive approach to a priori tune the elastohydrodynamics in a microsystem toward achieving a predetermined (but not prefabricated) flow geometry when the microchannel is subjected to flow. That is, we use the interaction between a soft solid layer, the viscous flow beneath it, and the shaped rigid wall above it to tune the fluid domain's shape. Specifically, we study a parallel-wall microchannel whose top wall is a slender soft coating of arbitrary thickness attached to a rigid platform. We derive a nonlinear differential equation for the soft coating's fluid-solid interface, which we use to infer how to achieve specific conduit shapes during flow. Using this theory, we demonstrate the tuning of four categories of microchannel geometries, which establishes, via a proof-of-concept, the viability of our modeling framework. We also explore slip length patterning on the rigid bottom wall of the microchannel, a common technique in microfluidics, as an additional "handle" for microchannel shape control. However, we show that this effect is much weaker in practice.

Molecular dynamics simulation of water-based Ferro-nanofluid flow in the microchannel and nanochannel: Effects of number of layers and material of walls

Due to the increasing development of nanotechnology and its wide applications, the flow of a nanofluid in a duct is also optimal geometric construction of ducts in the fabrication and production of various ducts to increase efficiency in nanofluid behavior are essential. In this paper, by using the molecular dynamics (MD) simulation process, the effect of Fe3O4 nanoparticles on the behavior of water-based fluid is investigated. Physical parameters such as total temperature, potential energy, fluid, nanofluid density profiles, fluid velocity, nanofluid profiles, and fluid and nanofluid temperature profiles are reported. Also, the effect of the number of layers and wall material on fluid flow is investigated. Therefore, the channel wall material in the following simulations will be considered as platinum, copper, and iron. Over time, the temperature of atomic structures reaches 300 K, which indicates the temperature stability in the simulated atomic structures. The results show that by increasing the number of wall layers in nanochannels and similar microchannels, interactions between fluid particles and walls increase. As these interactions increase, the accumulation of fluid particles in the vicinity of the channel walls increases, which increases the density of the shelves adjacent to the channel walls. Also, by changing the microchannel material from copper to iron and platinum, the number of interactions between particles in the present structures increases. This increase in the number of interactions between the particles present in the microchannel wall and the base fluid causes the maximum density to be observed in the platinum microchannel.

Two-and-three-dimensional analysis of Joule and viscous heating effects on MHD nanofluid forced convection in microchannels

This paper aims to study viscous heating, Joule heating and their simultaneous impacts on Al2O3-water nanofluid laminar flow and thermal characteristics in the presence of a magnetic field inside microchannels. The flow field and heat transfer are modeled two- and three-dimensionally by employing the finite volume method. To investigate the influence of viscosity variation with temperature on viscous heating, a temperature-dependent viscosity correlation specifically presented for Al2O3-water is employed. Predicted results indicate that viscosity changes due to temperature changes are negligible and although viscous heating can be reasonably ignored in the absence of the magnetic field, it must be taken into account when the magnetic field is applied. At the Hartmann number of 20, near microchannel walls, the velocity gradient rises up to 165% where the viscous dissipation significantly increases the temperature. In addition, when the magnetic field is applied, the 2D model underestimates the temperature compared to the 3D model where the effects of maximum velocity and velocity gradient on the energy equation and heat dissipation exist. The temperature profiles demonstrate that at the Hartmann number of 20, the effects of Joule heating or viscous heating increase the maximum dimensionless temperature by a factor of about 2.5 and by the rise of the Hartmann number, the dimensionless temperature increases and the effects of Joule and viscous heating become more significant. All in all, it is concluded that the heat generated by viscous and Joule heating can remarkably decrease the cooling performance of microchannels in the presence of magnetic fields.

Experimental Study of the Deposition of Magnetic Particles on the Walls of Microchannels.

This study analyzes experimentally the deposition of magnetic beads on the walls of a square microchannel by the action of a nearby cubical magnet. The deposition has been studied for different magnetic bead sizes, flow rates, magnetic conditions and with solutions of magnetic and non-magnetic particles. Images of the time evolution of the deposition under the different conditions have been analyzed to determine the spatial distribution of the accumulation and the growth rate of the depositions. It has been found that the way in which the magnetic beads are deposited on the walls of the microchannel depends strongly on their size and the magnetic configuration. The accumulation of the major part of particles is on the wall closest to the magnet and, depending on the size of the particles, near the magnet leading and trailing edges or near the center of the magnet. The experiments with magnetic and non-magnetic particles revealed the screening effect of the non-magnetic particles on the deposition. In this case, the non-magnetic particles displace the deposition toward the region near the center of the magnet and near the trailing edge.

The thermal –fluid investigation of effects of different wall boundary conditions on platinum catalytic micro-channel combined with a thermoelectric system

In this work, the effects of different regimes of boundary conditions such as convective heat transfer coefficient, insulated wall jointed by slip flow, thermal jump and slip-jump cases are investigated. In all cases of solution, an output voltage/power is achieved from a thermoelectric that is mounted on upper wall of catalytic micro-channel. The solution of this problem is done with coupled fluid (transport species model) and solid (wall of micro-channel and bottom legs of thermoelectric) sections. Furthermore, the electrical values are calculated with an algebraic code prepared into FORTRAN software. The variations of normalized temperature are compared with experimental data that show acceptable agreements with maximum error of 38% for non-slip and 18% for slip boundary conditions. Therefore, the slip wall boundary condition is more precise than non-slip condition. The values of generated power of thermoelectric have the maximum difference of 18% between slip and non-slip condition at inlet velocity of 0.3 m/s. The complete conversion of methane is occurred in insulated wall condition while the lowest value is achieved at thermal-jump condition with 85.46% methane conversion. Increasing inlet velocity the generated electrical power of thermoelectric decreases that is as results of decreasing the average temperature of hot legs of thermoelectric. Furthermore, non-continuity of temperature and continuity of velocity profiles increases output voltage and generated power of thermoelectric.

Interplay of conducting and non-conducting walls on hydromagnetic natural convection flow in a vertical micro-channel with Hall current

Theoretical investigation on the interaction between conducting and non-conducting walls on hydromagnetic natural convection flow of viscous incompressible and electrically conducting fluid through a vertical micro-channel taking into account the effects of induced magnetic field in presence of Hall current is presented. Governing coupled equations responsible for the flow are obtained when either the micro-channel walls are electrically conducting or are electrically non-conducting. Using the method of undetermined coefficients, exact solution are obtained and presented in dimensionless form subject to relevant boundary conditions. Expressions for fluid velocity, induced magnetic field, skin friction, volume flow rate and induced current density in both primary and secondary flow directions are also obtained. Effects of some governing parameters like Hall current parameter, rarefaction parameter and Hartmann number on the different flow situations are given using the aid of line graphs and Tables. The main conclusion of the present analysis is that, in the existence of rarefaction parameter, primary fluid velocity could be enhanced with the increase in Hall parameter when the micro-channel walls are either insulated or when the left micro-channel wall is electrically conducting. Results obtained in this work are relevant in many magnetically controlled devices and could also be used as a benchmark in checking the accuracies of result obtained in some empirical experiments.