Microchannel Geometry Research Articles

The influence of microchannel structure on double emulsion droplets formation

Microfluidics for the preparation of double emulsion droplets for a wide range of applications in medicine, agronomy, and materials science. The requirements for droplet preparation vary in different fields, and these needs can be met by altering fluid parameters, microchannel geometries, and other means. Therefore, this paper explores the influence of microfluidic device geometry on double-emulsion droplet molding using a VOF approach. The influence of variations in the length between the inject tube of the inner phase and inlet of the collection tube, the outlet width, the angle between phases, and the outlet angle on the double-emulsion droplet formation distance as well as on the area and other parameters were emphasized. The results show that increasing the length between the inner phase injection tube and inlet of the collection tube helps to improve the stability of droplet formation; the change of the outlet width has less effect on droplet formation; and the change of the angle between the phases tends to lead to the shift of the droplet formation mode, which has a greater effect on droplet formation. This study can provide a reference for optimizing the microchannel structure.

Flexible droplet microfluidic devices for tuneable droplet generation

Droplet microfluidics is a promising technology for applications that require precise handling of minute fluid volumes. This technology has broad applications in the pharmaceutical industry, food and beverage, and material synthesis. Droplet size and frequency are the two key parameters in these applications. Current droplet microfluidic devices are rigid platforms where the geometry and dimensions of microchannels are fixed after the device fabrication. This poses a significant limitation when adjusting the droplet generation characteristics. Repetitive design, fabrication, and testing are often needed to achieve the optimal microchannel dimensions. To overcome this bottleneck, we develop a proof-of-concept stretchable microfluidic device that can control droplet size and generation frequency by precisely controlling the microchannel dimensions in real-time. Theoretical analysis, numerical modelling, and experimental characterisation were conducted to study the influence of device lateral stretching on these characteristics. We found that the lateral stretching of the device increased the droplet diameter and spacing but reduced the droplet generation frequency. Droplet diameter and spacing increased by ∼20 %, and droplet frequency decreased by ∼45 % when the device was strained up to 25 %. We believe this innovative, flexible droplet microfluidic platform will provide an alternative way to precisely control the droplet formation by modifying the channel dimensions on-site and in real-time.

High-precision ultrasonic atomization using oscillating microchannels: Interplay of three-dimensional vibrational modes and droplet ejection mechanisms

Ultrasonic atomization is a critical process for producing micrometer-diameter droplets, widely utilized in aerosol drug delivery, spectrometry, and printing. The geometry of the vessel containing the fluid being atomized and the oscillations of its sidewalls play a crucial role in controlling the wave patterns and hence the droplet ejection process, especially at actuation frequencies exceeding 1 MHz. However, the mechanisms behind droplet ejection under high-frequency ultrasonic actuation remain poorly understood. We employ oscillating high-aspect-ratio Silicon microchannels to create ideal conditions where capillary forces, microchannel geometry, and oscillatory motion work together to precisely confine a liquid film and generate droplets with controlled diameters. We show that the three-dimensional vibrations of the microchannels, particularly the interplay between actuation frequency, amplitude, and channel geometry, can be used to effectively tune the ligament development and droplet breakup. This understanding allows us to establish conditions to reduce actuation power and hence minimize heating, control shear stresses and tune the droplet size on-demand without compromising uniformity and throughput.

Droplet Microfluidics for High-Throughput Screening and Directed Evolution of Biomolecules.

Directed evolution is a powerful technique for creating biomolecules such as proteins and nucleic acids with tailor-made properties for therapeutic and industrial applications by mimicking the natural evolution processes in the laboratory. Droplet microfluidics improved classical directed evolution by enabling time-consuming and laborious steps in this iterative process to be performed within monodispersed droplets in a highly controlled and automated manner. Droplet microfluidic chips can generate, manipulate, and sort individual droplets at kilohertz rates in a user-defined microchannel geometry, allowing new strategies for high-throughput screening and evolution of biomolecules. In this review, we discuss directed evolution studies in which droplet-based microfluidic systems were used to screen and improve the functional properties of biomolecules. We provide a systematic overview of basic on-chip fluidic operations, including reagent mixing by merging continuous fluid streams and droplet pairs, reagent addition by picoinjection, droplet generation, droplet incubation in delay lines, chambers and hydrodynamic traps, and droplet sorting techniques. Various microfluidic strategies for directed evolution using single and multiple emulsions and biomimetic materials (giant lipid vesicles, microgels, and microcapsules) are highlighted. Completely cell-free microfluidic-assisted in vitro compartmentalization methods that eliminate the need to clone DNA into cells after each round of mutagenesis are also presented.

Using Microfluidics to Align Matrix Architecture and Generate Chemokine Gradients Promotes Directional Branching in a Model of Epithelial Morphogenesis.

The mechanical cue of fiber alignment plays a key role in the development of various tissues in the body. The ability to study the effect of these stimuli in vitro has been limited previously. Here, we present a microfluidic device capable of intrinsically generating aligned fibers using the microchannel geometry. The device also features tunable interstitial fluid flow and the ability to form a morphogen gradient. These aspects allow for the modeling of complex tissues and to differentiate cell response to different stimuli. To demonstrate the abilities of our device, we incorporated luminal epithelial cysts into our device and induced growth factor stimulation. We found the mechanical cue of fiber alignment to play a dominant role in cell elongation and the ability to form protrusions was dependent on cadherin-3. Together, this work serves as a springboard for future potential with these devices to answer questions in developmental biology and complex diseases such as cancers.

Geometry effect on the mass transfer of slug flow in the microchannels with periodic expansion units

Although it was experimentally verified the microchannels with periodic expansion structures can effectively intensify the mass transfer of the slug flow, little research has focused on their geometric design. Hereon, a numerical simulation is conducted to evaluate the impact of microchannel geometry on the mass transfer of slug flow. The square expansion unit performs better than the triangular and circular ones. With the increase of the length, width, and number of the expansion units, and the width of the constriction between the adjacent expansions, the mass transfer performance shows a non-monotonic variation, that is an initial increase followed by a decrease. The underlying reasons are analyzed by evaluating the mixing inside and outside the droplet and the effective mass transfer area. At the preliminarily optimized geometry, a novel mass transfer path, in which not only the droplet caps but also the droplet body becomes the effective mass transfer area, is observed.

3-D numerical simulation of forced convection heat transfer in microscale rectangular channels for low GWP refrigerants

This study presents a numerical evaluation of the performance of low Global Warning Potential (GWP) refrigerants R600a, R290, R1270, and R134a in heat dissipation by forced convection in a microchannel heat sink. The heat sink comprises 50 parallel microchannels with a cross-sectional area equal to 123 × 494 [µm2]. The differential equations describing the physical behavior interaction between the refrigerant and the copper matrix are solved using the finite volume method and the SIMPLEC coupling algorithm. The numerical model is used to predict fluid mechanics and heat transfer for single-phase laminar conditions including the friction factor, the Nusselt number and the average wall temperatures, as well as the whole performance of the microchannel heat sink. The accuracy of the numerical model is validated by comparing the performance of R600a with experimental data, resulting in deviations within the range of the uncertainty of each parameter. The evaluation of the performance of different refrigerants reveals that the friction factor and Nusselt number are significantly influenced by the microchannel geometry and thermal conditions, exhibiting unique yet similar trends for different refrigerants. Furthermore, the analysis of the average wall temperature in the microchannels demonstrates that the hydrocarbon-based refrigerants effectively reduce the temperature of the copper matrix compared to R134a, suggesting a superior performance in the dissipation of heat flux under single-phase laminar conditions.

Simultaneous influence of contact angle, capillary number, and contraction ratio on droplet dynamics in hydrophobic microchannel

In droplet-based microfluidic systems, droplet motion and behavior largely depend on fluid properties, flow conditions, and microchannel geometry. This study presents a model to predict droplet dynamics within a contraction microchannel, considering various factors, such as contact angle (θ), capillary number (Ca), and contraction ratio (C), within defined ranges, through the employment of a three-dimensional numerical simulation method. Droplets may undergo trap, squeeze, or breakup as they traverse the contraction microchannel, contingent upon the aforementioned factors. The transition from trap to squeeze is determined by the critical value of capillary number ( CaI ). Within the squeezed regime, the contact angle influences droplet deformation and velocity within the contraction microchannel. However, when capillary number ( CaII ) in contraction microchannel increases to a specific threshold corresponding to the contraction ratio, the droplet no longer in contact with the wall and becomes independent of the contact angle, a phenomenon referred to as detachment. Moreover, the contact angle’s impact on the transition of droplets from squeeze to breakup is contingent upon the contraction ratio. This study provides a comprehensive overview of droplet behavior within microfluidic systems, offering valuable insights for the optimization and design of microsystems.

Numerical study of the optimization of the thermal and flow characteristics of a collector design of microchannels with two inlets and outlets, containing a nanofluid and a water-based fluid

The effect of aluminum microchannel geometry on the flow properties of water and Al2O3–H2O nanofluid was studied numerically using CFD. For this purpose, we studied three cases of three different shapes of microchannels, the first case is rectangular micro channels, and in the second case, we added obstacles to the bottom and top walls of the microchannels. In the last case, we gradually decreased the hydraulic diameter of the channels at a distance of 2L/3 from the outlet of the microchannel. The channels have two inlets and one outlet. There is also a continuous flow heat source at the bottom of the channels. In this study, the Reynolds number varied from 300 to 600 for the two inlets. We also used water and a nanofluid with a low volume concentration of nanoparticles (2%). We considered that the fluid temperature at the inlet of the microchannels changes from 293 K to 300 K. We relied on the finite volume method with the SIMPLE algorithm in the calculation method. According to the results obtained from this study, the effect of changing the value of the Rayleigh number at the two entrances of the channel was observed on the temperature and velocity of the fluid and on the heat exchange inside the channels in the three cases. We also found that difference of the average of the Nusselt number between the nanofluid and the basic one in the three cases at Re =600 is 1.15, 1.74, and 1.57, respectively. In addition, we found that the shape of the studied micro channels has an effect on the friction factor and thermal resistance.

Impact of surfactant polydispersity on the phase and flow behavior in water: the case of Sodium Lauryl Ether Sulfate

This study delves into the impact of molecular polydispersity on the phase behavior of Sodium Lauryl Ether Sulfate (SLES) surfactant, aiming to deepen understanding of its implications for fundamental science and industrial applications. SLE3S is utilized as a model compound: a comprehensive characterization of molecular polydispersity is conducted using Gas Chromatography–Mass Spectrometry and Nuclear Magnetic Resonance spectroscopy, juxtaposing the findings with those for SLE1S.Our comprehensive investigative approach entails: (i) employing Time-Lapse dissolution experiments in microchannel geometries to observe the dissolution and phase transitions; (ii) utilizing polarized light microscopy, confocal microscopy, and Small Angle X-ray Scattering for microstructure identification assessments; (iii) conducting rheological evaluations at various concentrations and temperatures to determine their effects on the surfactant properties.The findings reveal that SLE3S, being more polydisperse, demonstrates complex phase behavior not observed in the less polydisperse SLE1S. Notably, SLE3S exhibits a unique concentration domain, corresponding to a concentration of about 60 %wt, where hexagonal (H), cubic, and lamellar (Lα) phases coexist, resulting in highly viscoelastic heterogeneous mixtures. This behavior is attributed to the local segregation of surfactant components with varying polarity, underscoring the crucial role of molecular polydispersity in the phase behavior of SLES surfactants.

Experimental investigation of microchannel heat sink performance under non-uniform heat load conditions with different flow configurations

Cooling methods for multiple hotspots with high heat flux pose a reliability threat to electronic devices. This study investigates the microchannel-based heat sink performance under various non-uniform heat load conditions for different geometry with three different flow configurations (I, U and Z). An in-house designed Heater Array Unit (HAU) facilitates the generation of both uniform and non-uniform heat loads using a heater power supply. Two different microchannel geometries were employed, namely, microchannel-1 (MC-1) with channel and fin widths of 0.6 mm and 0.33 mm, respectively, and MC-2 with dimensions of 0.64 mm and 0.572 mm. Each microchannel incorporates three manifold configurations (I, U, and Z). Each flow configuration is regulated by flow control valves. Various non-uniform heat load patterns were considered, including streamline, non-streamline, and across-streamline conditions. To assess the thermal performance of the heat sinks the parameters used are thermal resistance (Rth), Nusselt number (Nu), and temperature non-uniformity (Ѱ). Experimental findings indicate that the MC-2 design with an I flow configuration is more suitable for uniform heat load conditions. On the contrary, for some non-uniform heat load cases MC-1 also showed up as a suitable design over MC-2.

Geometric influence of width ratio and contraction ratio on droplet dynamics in microchannel using a 3D numerical simulation

AbstractMicrochannel geometry is an important factor in determining droplet dynamics in droplet‐based microfluidic systems, much like fluid properties and flow conditions. In this context, two important geometric parameters—the contraction ratio () and the width ratio ()—that are limited to particular value ranges are taken into consideration for evaluation. These parameters interact with the capillary number () and viscosity ratio () to affect different aspects of droplet migration and manipulation, such as trap and squeeze regimes. A theoretical model is proposed, and a three‐dimensional numerical simulation method is used in this work. This model predicts the change from trap to squeeze, which is caused by the interaction of the previously mentioned variables. Interestingly, an inverse correlation exists between the width ratio and the critical capillary number for this transition, which is determined as . Furthermore, the investigation explores the droplet elongation and velocity ratio during their passage through the microchannel. By matching input parameters with microchannel geometry, this information may be useful for the design of microfluidic systems, which would facilitate the careful control and manipulation of droplets.

High-Density Microporous Drainage-Integrating Sheath Flow Generator for Streamlining Microfluidic Cell Sorting Systems.

Tremendous efforts have been made to develop practical and efficient microfluidic cell and particle sorting systems; however, there are technological limitations in terms of system complexity and low operability. Here, we propose a sheath flow generator that can dramatically simplify operational procedures and enhance the usability of microfluidic cell sorters. The device utilizes an embedded polydimethylsiloxane (PDMS) sponge with interconnected micropores, which is in direct contact with microchannels and seamlessly integrated into the microfluidic platform. The high-density micropores on the sponge surface facilitated fluid drainage, and the drained fluid was used as the sheath flow for downstream cell sorting processes. To fabricate the integrated device, a new process for sponge-embedded substrates was developed through the accumulation, incorporation, and dissolution of PMMA microparticles as sacrificial porogens. The effects of the microchannel geometry and flow velocity on the sheath flow generation were investigated. Furthermore, an asymmetric lattice-shaped microchannel network for cell/particle sorting was connected to the sheath flow generator in series, and the sorting performances of model particles, blood cells, and spiked tumor cells were investigated. The sheath flow generation technique developed in this study is expected to streamline conventional microfluidic cell-sorting systems as it dramatically improves versatility and operability.

Nanostructured Substrate-Mediated Bubble Degassing in Microfluidic Systems.

Microfluidic platforms have been widely used in a variety of fields owing to their numerous advantages. The prevention and prompt removal of air bubbles from microchannels are important to ensuring the optimal functioning of microfluidic devices. The entrapment of bubbles in the microchannels can result in flow instability and device performance disruption. Active and passive methods are the primary categories of degassing technologies. Active methods rely on external equipment, and passive methods operate autonomously without any external sources. This study proposed a passive degassing method that employs a nanoscale surface morphology integrated into the substrate of a microfluidic device. Nanostructures exhibit a microchannel geometry and are fabricated based on surface micromachining technology using silver ink and chemical etching. Consequently, the gas permeability is enhanced, resulting in effective degassing through the nanostructure. The performance of this degassing method was characterized under varying substrate permeabilities and input pressure conditions, and it was found that increased permeability facilitates the degassing performance. Furthermore, the applicability of our method was demonstrated by using a serpentine channel design prone to gas entrapment, particularly in the corner regions. The nanostructured substrate exhibited significantly improved degassing performance under the given pressure conditions in comparison to the glass substrate.

Microchannel-based Droplet Generation Using Multiphase Flow: A Review

Microfluidics is a multidisciplinary field that allows for precise control of fluids at a micrometer scale, with the goal of generating encapsulated structures or droplets for specific purposes. However, producing monodispersed droplets remains a challenge, making it necessary for researchers to investigate optimal microchannel geometries and parameters for controlling droplet size. Channel-based geometries, including T-junction, flow-focusing, co-flowing, membrane, and step emulsification, are the most commonly used geometries, each with its own advantages and weaknesses. This literature review aims to highlight assessment methods of microfluidic device performance and physical phenomenon in droplet generation for each channel-based geometry, including recent findings by researchers. Output parameters such as microchannel geometries, flow patterns, and flow regime maps with interpretations can be used to evaluate the optimum input for generating droplets that are suitable for a certain application. With the COVID-19 pandemic affecting the world, there is an opportunity to use microfluidic devices to study SARS-CoV-2 and develop post-pandemic therapeutics. The next challenge in microfluidic device development is producing high-throughput double emulsion droplets with monodispersed size using optimum input parameters to satisfy the drug delivery purpose.

Endothermic and exothermic chemical reactions’ influences on a nanofluid flow across a permeable microchannel with a porous medium

The compact dimensions and large surface area-to-volume ratio characteristic of microchannel geometries can efficiently utilise endothermic and exothermic chemical reactions to enhance heat transfer, control temperature, adjust reactivity, intensify processes and develop innovative thermal management techniques. Becasue of this, the exothermic and endothermic chemical reaction mechanism is explored by considering the titanium oxide (TiO2) nanoparticles with water (H2O) as the base fluid. An incompressible Newtonian fluid flows through the porous microchannel. The pressure gradient factor assumes the flow in the microchannel. Energy and solutal transportation analyses are made with Robin’s boundary conditions and activated energy phenomena. Similar variables are introduced to convert the model into non-dimensional. A numerical scheme, namely the Runge–Kutta-Fehlberg 45th (RKF-45) scheme, is implemented to execute solutions. The results explore that thermal distribution decreases for elevated activation energy values and Biot number values in the exothermic case while enhanced in the endothermic case. The rate of thermal distribution declines with the addition of solid fractions and chemical reaction parameters in the endothermic case; the opposite trend is observed in the exothermic case.

Quantitative study of droplet generation by pressure-driven microfluidic flows in a flow-focusing microdroplet generator

To precisely control the size of droplets is of great importance for the applications of the droplet microfluidics. In a flow-focusing microdroplet generator, the pressure-driven microfluidic device is designed to control the flow rates of the fluids. For a specific geometry of the flow-focusing microchannel, a mathematical model of droplet formation is established, and the nonlinear relation between the droplet length and the driven-pressure ratio can be described by our model. For pressure-driven microfluidic flows, the nonlinear relation between the droplet length and the driving-pressure ratio is measured experimentally in the flow-focusing microchannel. Particularly, by using the closed-loop control method of droplet generation, good agreements are shown between the measured size of droplets and the predicted size of the droplets. As a result, the control precision of the droplet size can be increased drastically by the closed-loop control method of droplet generation. Consequently, monodisperse droplets of extremely small size can be produced in the flow-focusing microdroplet generator.

Mixing performance of T-shaped wavy-walled micromixers with embedded obstacles

This research systematically investigates the impact of microchannel geometry on key parameters governing mixing efficiency and cost. The study focuses on passive T-shaped micromixers with modified sinusoidal wavy walls, analyzing a spectrum of configurations ranging from the raccoon to serpentine by varying the wall phase angles. The traditional T-shaped micromixer serves as a foundational reference, and we systematically vary phase angles, amplitudes, and wavelengths of the wavy walls to comprehensively address all possible configurations. Additionally, different shaped obstacles such as circular, square, diamond, and triangular obstacles are strategically introduced to further enhance mixing performance. The findings reveal intricate relationships and dependencies among geometric factors, shedding light on configurations that significantly enhance mixing efficiencies. Notably, a specific wavy micromixer configuration, characterized by a carefully tuned phase difference, amplitude, and wavelength, exhibits the highest mixing index in the absence of obstacles. The introduction of obstacles, particularly circular ones, further enhances mixing efficiency. As Reynolds (Re) and Schmidt (Sc) numbers increase, the mixing index decreases, and the mixing cost rises. This work adds a quantitative dimension to understanding the interplay between geometric parameters, flow conditions, and mixing performance in passive micromixers with systematic wavy walls and embedded obstacles.

Effect of tool feed rate and tool rotation speed on the microchannel geometry of AA6063-SiC composites machined by micro ED milling

ABSTRACT Miniaturisation of products is an emerging trend in the current scenario. Conventional micromachining of metal matrix composites poses difficulties due to the heterogeneous nature of the material and the presence of hard reinforcement particles. Micro-electric discharge machining has emerged as a proficient method for the precision machining of challenging materials. In this work, micro-ED milling of AA6063–4 wt. % SiC composite was carried out using a graphite electrode. Being the primitive unit of 3D machining, microchannels were machined on the workpiece at different discharge energies, tool feed rates, and tool rotation speeds. The geometrical features of the channel such as channel width, kerf width, taper of the microchannels, and process indicators like tool wear rate and material removal rate were studied. The 6 µm/s tool feed rate, 500 rpm tool rotation speed, and 500 µJ discharge energy provided the desired results for the studied responses.

Improved Thermal Management of Lithium‐Ion Battery Module through Microtexturing of Serpentine Cooling Channels

Elevated battery temperature and its heterogeneity are responsible for many battery‐related problems such as short lifespan and thermal runaway. For longer life and safe functionality of lithium‐ion battery (LIB), both the average battery temperature and temperature heterogeneity must be controlled. The present work analyzes a modified cooling channel design that can control both the average battery temperature and its heterogeneity. The design adopts microtexturing approach for the heat transfer augmentation of the serpentine cooling channels. First, the transient thermal analysis of different geometry of microtextures, viz., microchannels (MCs) and microdimples (MDs), is carried out on an aluminum slab. The rectangular MCs and square MDs are found to be the most efficient among the other considered microtexture geometries. Further, the modified design is employed for a 19‐cell battery pack to analyze the thermal performance, pressure drop, and coolant pump power requirements. A decrease in maximum temperature difference (MTD) by 6.03 % and 3.72% is observed for 380 MCs (rectangular) and 960 MDs (squared), respectively. While MDs improve the temperature homogeneity within the LIBs, the coolant pump power requirement and pressure drop (by ≈15.42%) are higher for it compared to the MC‐based design for a given MTD.