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This research study provides a detailed analysis of the two-phase flow boiling heat transfer coefficients of R600a and R717. In addition, it proposes novel saturated flow boiling heat transfer correlations for these two refrigerants. In this context, 1306 experimental data points for R600a and 885 data samples for R717 are extracted from the literature works to develop new flow boiling models. Two different correlations are constructed under different forms of baseline frameworks for each refrigerant. The proposed flow boiling model for R600a utilizes the dimensionless numbers and operational parameters of the best performing literature correlations for this refrigerant while the model developed for R717 takes the advantage of piecewise continuous correlation form which successfully trace the tendencies of heat transfer coefficient with varying vapor qualities. It is found that the new model for R600a has a mean absolute error of 12.4% and mean relative error of 0.1, predicting 78.5% of the entire database within +/- 20.0% error band whereas the proposed correlation for R717 has an absolute error value of 17.3% having 65.3% of the data within +/- 20.0% error band, which are much better and accurate estimations compared to those obtained by the existing flow boiling models for R717 refrigerant.

A compressible capacitive mechanical pressure sensor has been developed. Porous polydimethylsiloxane (p-PDMS) has been chosen as dielectric insulator because of its dielectric constant value. Gold nanoparticles have been embedded in p-PDMS to change the dielectric properties and to tune its elasticity. p-PDMS and its nanocomposite have been synthesized using the sugar leaching process. The p-PDMS physical characterization, with and without the gold nanoparticles, has been conducted to investigate its elastic response to compressive stresses as a function of both the polymer preparation thermal treatment and the gold nanoparticle concentration. A sensor operating in a low-pressure range between about 100 Pa and 10 kPa with a strain ranging between about 5% and 95% has been realized. Dielectric constant and electrical resistivity measurements have been performed using samples with a starting volume of the order of 1 cm3. The relationship between the dielectric constant, the electrical resistivity and the compressive stress/strain has been also deduced. The described sensor is flexible, biocompatible, water equivalent and can have applications in biomedicine (orthopedic, dentistry), engineering (stress–strain measurements, robotics), and microelectronics (microbalances, stress test on electronic devices).

The Hypersonic Precooled Combined Cycle Engine (HPCCE), which introduces precooler into traditional hypersonic engine, is regarded as the most promising propulsion system for realizing a single-stage-to-orbit vehicle. The unique demands lead to the application of the compact heat exchangers, which can realize high thrust-to-weight ratio, sufficient specific impulse and high compression ratio. However, it is challenging to accurately manufacture the compact heat exchanger due to its extremely high heat dissipation capacity, remarkable compactness, superior adaptability and harsh operating condition. This review summarizes the precooling schemes of combined cycle propulsions and describes the demands and key issues in the fabrication of a compact heat exchanger for HPCCE. The investigation focuses on the application of various micromanufacturing methods of heat exchangers constructed from tubes of less than 1 mm in diameter and microchannels of less than 200 micrometers. Various micromanufacturing processes, which include microforming, micromachining, stereolithography, chemical etching, 3D printing, joining and other advanced microfabricating processes, were reviewed. In addition, the technologies are compared in terms of dimensional tolerance, material compatibility, and process applicability. Furthermore, the boundaries of the micromanufacturing constraints are specified as references for the design of compact heat exchangers. Ultimately, the technological difficulties and development trends are discussed for the fabrication of compact heat exchangers for HPCCE.

The influence of using nanofluids on heat transfer augmentation and characteristics of fluid flow in rectangular cross sectional microchannel heat sink (MCHS) is numerically investigated for single and double stack microchannels at uniform heat flux, q = 100 W/cm2 and Reynolds number ranging from 200 to 1500. In this study, alumina–water (Al2O3-H2O) nanofluid with different nanoparticle volume concentrations from 1% to 5% was used as a coolant for the MCHS. The three-dimensional steady, laminar flow and heat transfer governing equations are solved by finite volume method using ANSYS-Fluent 18.2. The MCHS performance is evaluated in terms of temperature profile, heat transfer coefficient, pressure drop, pumping power and volume reduction. The results reveal that increasing the nanoparticles concentration improves the cooling process. The maximum heat transfer augmentation in the current study was achieved at the highest concentration of 5%. For example, at this concentration the temperature was reduced by 7.3% using single stack microchannel. However, 33.5% temperature reduction was achieved using double stack microchannels. Also, it results in an increase in the heat transfer coefficient by 13.12% compare to that of the pure water. On the other hand, for a single stack at nanoparticle concentration of 5%, both pressure drop and power increased by 10% and 8%, respectively. Because of the significant influence of the nanofluid on the improvement of the cooling process, an estimation of the volume reduction was made. It was found that 62.6% volume reduction for single stake at concentration 5% can be achieved.

Femtosecond (fs) laser micromachining on polymeric materials is a single-step, and contactless manufacturing technology. Knowing the potential of poly(ionic liquid)s (PILs) and their derived composite materials incorporating ionic liquids (PIL–IL) to design membranes with improved CO2 separation, we here explore for the first time the creation of microchannels on the surface of PIL–IL materials by laser ablation using femtosecond laser radiation. PIL–IL membranes composed of pyrrolidinium-based PILs containing the [NTf2]– and [C(CN)3]– anions and different amounts of their corresponding ILs (40 and 60 wt%) were prepared and micromachined using fs laser pulses varying the pulse repetition rate, scanning speed, and pulse energy. The morphology of the fs laser modified PIL–IL samples was investigated through scanning electron microscopy (SEM), while the influence of the fs laser processing on the membranes structure was analyzed by solid-state nuclear magnetic resonance (ssNMR), Fourier-transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The CO2/N2 and CO2/H2 separation performances of the irradiated membranes were also evaluated and compared to those of the non-irradiated. Depending on the parameters used, fs laser processing was successful in modifying the surface of PIL–IL membranes through the formation of microchannels around 55–60 μm deep. Significant improvements in CO2, N2 and H2 permeabilities were achieved for the irradiated PIL–IL membranes, maintaining their CO2/N2 and CO2/H2 permselectivities.

Thermal management is crucial for the proper functioning of a system whether it is in electronics, process industry, automobile, and renewable devices. Micro-channel heat exchanger has proved to be efficient in heat ejection from renewable systems due to high heat transfer surface to volume ratio. This study focuses on evaluating the cooling performance of straight, wavy, and dual wavy Micro-Channel Heat Exchanger by modelling the heat transfer model in ANSYS Fluent. The incompressible fluid is considered in the laminar regime using alumina-based nanofluids with 1%, 3%, and 6% concentration. The solution is computed by selecting SIMPLE pressure-velocity coupling scheme with second-order momentum and energy discretization. Nusselt number, pressure drop, base temperature, and Thermal Performance Factor (TPF) are used as performance parameters for comparing nanofluids performance at Reynolds number range of 100–900. For straight, wavy, and dual wavy model heat transfer, as well as pressure drop, increased with Reynolds number. It is observed that wavy and dual wavy channels compared to straight channel improved convective heat transfer due to the formation of secondary vortices at the curved section. Dual wavy with wavy base and flat base wall showed highest Nusselt number increase of more than double when compared with straight channel of equal concentration. For 6% nano particles addition in all channels, on average both dual wavy channels showed highest improvement of 8% when compared with 0% concentration channel. Dual wavy channel with a flat base and wavy base reduced the base heater temperature by 10 °C and 9 °C compared to the straight channel. A maximum Thermal Performance Factor of 2.2 is achieved for dual wavy channel with a wavy base configuration with 6% nanoparticles.

Flow boiling heat transfer enhancement using porous coatings in microchannels has been experimentally investigated with HFE-7200. Results of the coated microchannel heat sink were compared to baseline results in a plain, micro-milled copper microchannel heat sink at similar operating conditions, namely inlet pressure of 1 bar, mass flux of 200 – 400 kg/m2 s and inlet subcooling of 10 K at wall heat fluxes between 24.5 kW/m2 to 206.6 kW/m2. Flow visualisation results and SEM surface analysis are presented. The coated surface was densely populated with well-defined cavities between 0.6 µm to 3.3 µm wide. The plain channels had fewer cavities for a given area and these were larger, i.e. up to 6 µm. Bubble generation frequency in the coated channels is significantly higher than in the plain channels due to the presence of more favourable nucleation sites on the coated surface. Flow pattern development occurred similarly in both heat sinks, namely bubbly to slug, churn and annular flow with increasing heat flux, with earlier flow pattern transitions in the coated heat sink. Enhancement in microchannel flow boiling heat transfer was shown to be influenced by mass velocity and may reach up to 44% at low heat fluxes, where the nucleate boiling mechanism is dominant. It diminishes with increasing heat flux, corresponding to nucleate boiling suppression following flow regime transition. Pressure drop increase posed by the coated heat sink is relatively small in terms of overall system power consumption but appears to be influenced by mass flux, pressure oscillations as well as channel rewetting behaviour.

Phase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter Dh whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of Dh = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different Dh, the applied heat and mass flux values varied between 20 to 200 kW/m2 and 150 to 2400 kg/m2s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nuglob was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance R¯l compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the R¯l by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms.

Hydrodynamics plays a major role in transport of heat and mass transfer in microchannels. This includes flow patterns and flow regimes in which the micro-channels are operated. The flow patterns have a major impact the transport properties. Another important aspect is the pressure drop in micro-channels. In the present review, the experimental and Computational Fluid Dynamics (CFD) studies covering all the above aspects have been covered. The effect of geometrical parameters like shape of channel, channel size, material of construction of channels; operating parameters like flow velocity, flow ratio and fluid properties have been presented and analyzed. Experimental and analytical work of different pressure drop models has also been presented. All the literature related to influence of flow patterns on transport properties like volumetric mass transfer coefficients (VMTC) and heat transfer coefficients (HTC) have been presented and analyzed. It is found that most works in Liquid-Liquid Extraction (LLE) systems have been carried out in slug flow and T-junctions. Models for coupled systems of flow and mass transfer have been presented and works carried out for different coupled systems have been listed. CFD simulations match experimental results within 20% deviations in quantitative and qualitative predictions of flow phenomena for most research articles referred in this review. There is a disparity in prediction of a generalized regime map and a generalized regime map for prediction of flow patterns for various systems would need the help of Artificial Intelligence.

This paper investigates the effect of silver nanofluid on a flat-plate pulsating heat pipe (FP-PHP). The simulation was performed using FLUENT 15.0 software, for which a three-dimensional model having a microchannel structure in the condensation section was developed. The developed model adopted the volume of fluid (VOF) method to track the internal vapor-liquid interface of the FP-PHP and to observe the state of the two-phase flow. The result revealed the evident presence of various types of bubble flows, including dispersed bubble flows, slug flows, annular flows, and column flows in the evaporation section. Trends in thermal resistance variation during simulation were studied by changing the volume fraction, liquid filling rate, and heating power. The thermal resistance of the FP-PHP containing silver nanofluid was lower and the FP-PHP exhibited stable state when the heating power reached 120 W. The optimal volume fraction of silver nanofluid was 1%, and the FP-PHP containing sliver nanofluid exhibited increased heat transfer efficiency.

In this study, changes in the droplet formation mechanism and the law of droplet length in a two-phase liquid–liquid system in 400 × 400 μm standard T-junction microchannels were experimentally studied using a high-speed camera. The study investigated the effects of various dispersed phase viscosities, various continuous phase viscosities, and two-phase flow parameters on droplet length. Two basic flow patterns were observed: slug flow dominated by the squeezing mechanism, and droplet flow dominated by the shear mechanism. The dispersed phase viscosity had almost no effect on droplet length. However, the droplet length decreased with increasing continuous phase viscosity, increasing volume flow rate in the continuous phase, and the continuous-phase capillary number Cac. Droplet length also increased with increasing volume flow rate in the dispersed phase and with the volume flow rate ratio. Based on the droplet formation mechanism, a scaling law governing slug and droplet length was proposed and achieved a good fit with experimental data.

Liquid infused surfaces (LIS) achieve non-wetting properties through asperity structures containing pockets of a lubricating liquid rather than air. Although studies have demonstrated several potential applications of LIS, the effect of liquid infused texture on the flow of liquid in microchannels has not been systematically addressed. The present work focuses on the effect of liquid-infused texture geometry and the infused liquid properties on the flow characteristics of a bulk fluid in rectangular and round microchannel geometries. Considering four different textured geometries of transverse ridges, longitudinal ridges, in-lined square posts and staggered squares, a detailed parametric study is presented to assess the resulting drag reduction and friction factor, expressed in terms of the Poiseuille number, on the flow in microchannels. The effect of geometrical parameters is investigated for different asperity solid fraction ( ϕ s ), asperity height ( h s ) and channel thickness ( H ) or tube diameter ( D ), while the effect of the infused fluid is studied in terms of a ratio of its viscosity relative to the viscosity of the flowing bulk fluid. Based on the results of the parametric study an extension of the conventional Moody diagram for non-wetting surfaces is developed. Further, a design of experiments study is conducted to relate the drag reduction and Poiseuille number to the various governing parameters, using which conditions that maximize drag reduction are identified.

This work presents a novel method for the non-invasive, in-line monitoring of mixing processes in microchannels using the Raman photometric technique. The measuring set-up distinguishes itself from other works in this field by utilizing recent state-of-the-art customized photon multiplier (CPM) detectors, bypassing the use of a spectrometer. This addresses the limiting factor of integration times by achieving measuring rates of 10 ms. The method was validated using the ternary system of toluene&ndash, water&ndash, acetone. The optical measuring system consists of two functional units: the coaxial Raman probe optimized for excitation at a laser wavelength of 532 nm and the photometric detector centered around the CPMs. The spot size of the focused laser is a defining factor of the spatial resolution of the set-up. The depth of focus is measured at approx. 85 &micro, m with a spot size of approx. 45 &micro, m, while still maintaining a relatively high numerical aperture of 0.42, the latter of which is also critical for coaxial detection of inelastically scattered photons. The working distance in this set-up is 20 mm. The microchannel is a T-junction mixer with a square cross section of 500 by 500 &micro, m, a hydraulic diameter of 500 &micro, m and 70 mm channel length. The extraction of acetone from toluene into water is tracked at an initial concentration of 25% as a function of flow rate and accordingly residence time. The investigated flow rates ranged from 0.1 mL/min to 0.006 mL/min. The residence times from the T-junction to the measuring point varies from 1.5 to 25 s. At 0.006 mL/min a constant acetone concentration of approx. 12.6% was measured, indicating that the mixing process reached the equilibrium of the system at approx. 12.5%. For prototype benchmarking, comparative measurements were carried out with a commercially available Raman spectrometer (RXN1, Kaiser Optical Systems, Ann Arbor, MI, USA). Count rates of the spectrophotometer surpassed those of the spectrometer by at least one order of magnitude at identical target concentrations and optical power output. The experimental data demonstrate the suitability and potential of the new measuring system to detect locally and time-resolved concentration profiles in moving fluids while avoiding external influence.

In recent years, surface acoustic wave (SAW) devices have demonstrated great potentials and increasing applications in the manipulation of nano- and micro-particles including biological cells with the advantages of label-free, high sensitivity and accuracy. In this letter, we introduce a novel tilted-angle SAW devices to optimise the acoustic pressure inside a microchannel for cancer-cell manipulation. The SAW generation and acoustic radiation force are improved by seamlessly patterning electrodes in the space surrounding the microchannel. Comparisons between this novel SAW device and a conventional device show a 32% enhanced separation efficiency while the input power, manufacturing cost and fabrication effort remain the same. Effective separation of HeLa cancer cells from peripheral blood mononuclear cells is demonstrated. This novel SAW device has the advantages in minimizing device power consumption, lowering component footprint and increasing device density.

The interest in developing efficient nano and pico-satellites has grown in the last 20 years. Secondary propulsion systems capable of serving specific maneuvers are an essential part of these small satellites. In particular, Micro-Electro-Mechanical Systems (MEMS) Vaporizing Liquid Microthrusters (VLM), using water as a propellant, represent today a smart choice in terms of simplicity and cost. In this paper, we first propose a review of the international literature focused on MEMS VLM development, reviewing the different geometries and heating solutions proposed in the literature. Then, we focus on a critical aspect of these micro thrusters: the presence of unstable phenomena. In particular, the boiling instabilities and reverse channel flow substantially impact the MEMS VLMs’ performance and limit their applicability. Finally, we review the research focused on the passive and active control of the boiling instabilities, based on VLM geometry optimization and active heating strategies, respectively. Today, these ones represent the two principal research axes followed by the scientific community to mitigate the drawbacks linked to the use of MEMS VLMs.

In this paper, we present the results of a systematic numerical study of the flow and mixing modes of fluids in micromixers of various configurations, in particular, an analysis of passive micromixers, the most widely used in practice, as well as the main methods to intensify mixing. The advantages of microstructure reactors can significantly reduce reaction times and increase productivity compared to traditional bulk reactors. Four different geometries of micromixers, including the straight T-shaped microchannel, were considered. The effect of the geometrical patterns of micromixers, as well as of the Reynolds number on flow regimes and mixing efficiency were analyzed. The Reynolds number varied from 1 to 300. Unlike other studies, the efficiency of the considered mixers was for the first time compared with the cost of pressure loss during pumping. As a result, the efficiency of the most optimal micromixer in terms of hydraulic mixing and the optimal operation ranges were determined. It was shown that the maximum normalized mixing efficiency in the entire range of Re numbers was noted for mixer, in which a vortex-based intensification of mixing occurs due to the flow swirling in cylindrical chambers. This mixer allows mixing the fluids 600 times more efficiently than a straight T-mixer, while all other conditions being equal.

Thermal performance enhancement in microchannel heat sinks has recently become a challenge due to advancements in modern microelectronics, which demand compatibility with heat sinks able to dissipate ever-increasing amounts of heat. Recent advancements in manufacturing techniques, such as additive manufacturing, have made the modification of the microchannel heat sink geometry possible well beyond the conventional rectangular model to improve the cooling capacity of these devices. One such modification in microchannel geometry includes the introduction of secondary flow channels in the walls between adjacent mainstream microchannels. The present study computationally models secondary flow channels in regular trapezoidal and parallel orientations for fluid circulation through the microchannel walls in a heat sink design. The heat sink is made of silicon wafer, and water is used as the circulating fluid in this study. Continuity, momentum, and energy equations are solved for the fluid flow through the regular trapezoidal secondary flow and parallel secondary flow designs in the heat sink with I-type, C-type, and Z-type inlet–outlet configurations. Plots of velocity contours show that I-type geometry creates optimal flow disruption in the heat sink. Therefore, for this design, the pressure drop and base plate temperatures are plotted for a volumetric flow rate range, and corresponding contour plots are obtained. The results are compared with corresponding trends for the conventional rectangular microchannel design, and associated trends are explained. The study suggests that the flow phenomena such as flow impingement onto the microchannel walls and formation of vortices inside the secondary flow passages coupled with an increase in heat transfer area due to secondary flow passages may significantly improve the heat sink performance.

The effect of surface material on the flow boiling characteristics in a multi-microchannel evaporator is described in this paper. HFE-7100, a dielectric and eco-friendly working fluid, was tested at atmospheric pressure, inlet sub-cooling of 5 K, base heat flux up to 433.5 kW/m2 and mass flux range 50–250 kg/m2s. Two heat sinks made of copper and aluminium were fabricated having the same channel dimensions giving a hydraulic diameter of 0.46 mm with a base area of 500 mm2. The average roughness was measured and the values were 0.286 μm and 0.192 μm for the copper and aluminium, respectively. The effect of surface material was not significant at low heat flux. However, it became significant at moderate and high wall heat fluxes. The flow patterns for the two microchannels were similar and included bubble, slug, churn and annular flow. The experimental results showed that the aluminium surface achieved on the average difference of 12% higher heat transfer coefficients than those found in the copper microchannel. It also gave higher flow boiling pressure drop; the average difference was up-to approximately 30%. The different surface microstructures in the two examined heat sinks could explain the different heat transfer and pressure drop behaviour.

Micromachining has gained considerable interest across a wide range of applications. It ensures the production of microfeatures such as microchannels, micropockets, etc. Typically, the manufacturing of microchannels in bioceramics is a demanding task. The ubiquitous technologies, laser beam machining (LBM) and rotary ultrasonic machining (RUM), have tremendous potential. However, again, these machining methods do have inherent problems. LBM has issues concerning thermal damage, high surface roughness, and vulnerable dimensional accuracy. Likewise, RUM is associated with high machining costs and low material-removal rates. To overcome their limits, a synthesis of LBM and RUM processes known as laser rotary ultrasonic machining (LRUM) has been conceived. The bioceramic known as biolox forte was utilized in this investigation. The approach encompasses the exploratory study of the effects of fundamental input process parameters of LBM and RUM on the surface quality, machining time, and dimensional accuracy of the manufactured microchannels. The performance of LRUM was analyzed and the mechanism of LRUM tool wear was also investigated. The results revealed that the surface roughness, depth error, and width error is decreased by 88%, 70%, and 80% respectively in the LRUM process. Moreover, the machining time of LRUM is reduced by 85%.

Peripheral nerves are basic communication structures guiding motor and sensory information from the central nervous system to receptor units. Severed peripheral nerve injuries represent a large clinical problem with relevant challenges to successful synthetic nerve repair scaffolds as substitutes to autologous nerve grafting. Numerous studies reported the use of hollow tubes made of synthetic polymers sutured between severed nerve stumps to promote nerve regeneration while providing protection for external factors, such as scar tissue formation and inflammation. Few approaches have described the potential use of a lumen structure comprised of microchannels or microfibers to provide axon growth avoiding misdirection and fostering proper healing. Here, we report the use of a 3D porous microchannel-based structure made of a photocurable methacrylated polycaprolactone, whose mechanical properties are comparable to native nerves. The neuro-regenerative properties of the polymer were assessed in vitro, prior to the implantation of the 3D porous structure, in a 6-mm rat sciatic nerve gap injury. The manufactured implants were biocompatible and able to be resorbed by the host&rsquo, s body at a suitable rate, allowing the complete healing of the nerve. The innovative design of the highly porous structure with the axon guiding microchannels, along with the observation of myelinated axons and Schwann cells in the in vivo tests, led to a significant progress towards the standardized use of synthetic 3D multichannel-based structures in peripheral nerve surgery.

International audience; The context of this work is to explore the possibility of making bio-sourced foams at high throughput in microsystems. A fundamental element for the development of new technologies based on biphasic flows in microchannels is a good characterization and knowledge of hydrodynamics. In this contribution, cross slot microsystems are tested as a promoter for the formation of train of bubbles in aqueous solutions of xanthan gum. A flow chart is proposed to present the conditions allowing a dispersed flow regime; i.e. the conditions guaranteeing the complete incorporation of the gas into the liquid phase (no gas pocket) and then the control of the void fraction. The foaming is studied under potential industrial conditions, i.e. operating with liquid flow rates up to 24 L/h. High speed imaging within the channels reveals that the mechanism of formation of bubbles is related to flow focusing. Bubbles are generated alternatively from each of the two opposite gas inlets. Using whey protein isolate to enhance foaming points out that the bubble size may be reduced along a long channel by tip streaming. The effect of the modulation of the viscosity of the liquid base (xanthan gum content) is also discussed.

In this study, Y-bifurcated microchannels fabricated from a {100} silicon in 25 wt% tetramethylammonium hydroxide water solution at the temperature of 80 °C have been presented and analysed. We studied the etching of acute angles with sides along the crystallographic directions in the masking layer where 1 < n < 8. We considered symmetrical acute corners in the masking layer with respect to the crystallographic directions. The angles between the appropriate and crystallographic directions were smaller than 45°. Moreover, we observed asymmetrical acute corners formed by the and crystallographic directions, where m ≠ n. We found that the obtained convex corners were not distorted during etching. Consequently, it is not necessary to apply convex corner compensation. These fabricated undistorted convex corners represent the angles of the bifurcations. The sidewalls of the microchannels are defined by etched planes of the {n11} and {100} families. Analytical relations were derived for the widths of the microchannels. The results enable simple and cost-effective fabrication of various complex silicon microfluidic platforms. This is the peer-reviewed version of the article: Milče M Smiljanić, Žarko Lazić, Milena Rašljić Rafajilović, Katarina Cvetanović Zobenica, Evgenija Milinković and Ana Filipović, J. Micromech. Microeng. 31 017001, DOI: [https://doi.org/10.1088/1361-6439/abcb67] The published version [https://cer.ihtm.bg.ac.rs/handle/123456789/4003]

This study aimed to analyze the two-phase heat transfer in a vertical upward flow configuration under different gravity conditions. The facility is a new version of MicroBo (Microgravity Boiling), the working fluid used is perfluorohexane C6F14 and a 4.0 mm aluminum channel is arranged vertically to perform ebullition process. The platform used to get data in microgravity was the parabolic flight, which represents one of the most widely used platforms despite the period of time available to collect data is very short. The analysis of the results has been carried out following two main approaches: the first, ground-flight data comparison, for the study of variable gravity data collected during the 67th ESA campaign of parabolic flight held in November 2017, while the second, ground data analysis, to analyze the data collected on the ground in the ENEA laboratory with the same facility in December 2018. In particular ground – flight data have been analyzed comparing boiling curves at different flow rates, from a minimum of 5.3 l/h up to 17 l/h, in micro, hyper and normal gravity. The operating inlet pressure during experiments has been fixed at 1.6 bar with a heat flux range of 5.4 - 85.6 kW/m2. For what regard ground data boiling curves, with a flow rate of 10 l/h, are shown with a direct flow pattern visualization. The operating pressure has been fixed at 1.3 bar and 1.8 bar with a heat flux range of 2.5 – 94.6 kW/m2. For the latter analysis, it also has been carried out a qualitative study for the validation of the void fraction model, starting from the visualization of the images of flow patterns recorded during the acquisitions.

In this paper, a new experimental setup was proposed to realize the constant-heat-flux boundary condition based on a counter flow microchannel heat exchanger with the same heat capacity rate of the hot and cold streams. This approach provides a constant fluid temperature gradient along the surfaces. An analytical two-dimensional model was developed to describe the heat transfer processes in the hot stream and the cold stream, respectively. In the experiments, DI-water was employed as the working fluid. Laser induced fluorescence (LIF) method was used to measure the fluid temperature field within the microchannel. Different combinations of flow rates were studied to investigate the heat transfer characteristics in the microchannel. The measured mean temperature distribution matched well with the proposed analytical model. A correlation for Nusselt number (Nu) was proposed based on the experimental Reynolds number (Re

We conducted systematic numerical investigations of the flow characteristics within the entrance region of rectangular microchannels. The effects of the geometrical aspect ratio and roughness on entrance lengths were analyzed. The incompressible laminar Navier&ndash, Stokes equations were solved using finite volume method (FVM). In the simulation, hydraulic diameters (Dh) ranging from 50 to 200 &micro, m were studied, and aspect ratios of 1, 1.25, 1.5, 1.75, and 2 were considered as well. The working fluid was set as water, and the Reynolds number ranged from 0.5 to 100. The results showed a good agreement with the conducted experiment. Correlations are proposed to predict the entrance lengths of microchannels with respect to different aspect ratios. Compared with other correlations, these new correlations are more reliable because a more practical inlet condition was considered in our investigations. Instead of considering the influence of the width and height of the microchannels, in our investigation we proved that the critical role is played by the aspect ratio, representing the combination of the aforementioned parameters. Furthermore, the existence of rough elements obviously shortens the entrance region, and this effect became more pronounced with increasing relative roughness and Reynolds number. A similar effect could be seen by shortening the roughness spacing. An asymmetric distribution of rough elements decreased the entrance length compared with a symmetric distribution, which can be extrapolated to other irregularly distributed forms.

Ultrasound in the presence of gas-filled microbubbles can be used to enhance local uptake of drugs and genes. To study the drug delivery potential and its underlying physical and biological mechanisms, an in vitro vessel model should ideally include 3-D cell culture, perfusion flow, and membrane-free soft boundaries. Here, we propose an organ-on-a-chip microfluidic platform to study ultrasound-mediated drug delivery: the OrganoPlate. The acoustic propagation into the OrganoPlate was determined to assess the feasibility of controlled microbubble actuation, which is required to study the microbubble-cell interaction for drug delivery. The pressure field in the OrganoPlate was characterized non-invasively by studying experimentally the well-known response of microbubbles and by simulating the acoustic wave propagation in the system. Microbubble dynamics in the OrganoPlate were recorded with the Brandaris 128 ultrahigh-speed camera (17 million frames/s) and a control experiment was performed in an OptiCell, an in vitro monolayer cell culture chamber that is conventionally used to study ultrasound-mediated drug delivery. When insonified at frequencies between 1 and 2 MHz, microbubbles in the OrganoPlate experienced larger oscillation amplitudes resulting from higher local pressures. Microbubbles responded similarly in both systems when insonified at frequencies between 2 and 4 MHz. Numerical simulations performed with a 3-D finite-element model of ultrasound propagation into the OrganoPlate and the OptiCell showed the same frequency-dependent behavior. The predictable and homogeneous pressure field in the OrganoPlate demonstrates its potential to develop an in vitro 3-D cell culture model, well suited to study ultrasound-mediated drug delivery.

The ability to enhance heat transfer rates of shear-thinning fluids in microchannel devices is evaluated numerically for steady and time-dependent flows in the range of 400 < Re

This experimental study investigated narrow and high aspect ratio multi-microchannels that were micro-machined in copper with thin separating walls during saturated flow boiling of refrigerants. The hypothesis was that these channels could increase the footprint critical heat flux and support the future development of thermal management systems for power electronics and other electronic packages. The measured footprint critical heat flux was as high as 678.5 W/cm2, which is twice as high as other investigations in the literature concerning saturated flow boiling of refrigerants. Two test samples were fabricated with a footprint area of (10 × 10) mm. The first had 25 rectangular channels (198 μm wide, 1167 μm high). The second had 17 rectangular channels (293 μm wide, 1176 μm high). The experimental investigation covered low-GWP replacement refrigerants (R1234yf, R1234ze(E)) as well as the well-examined R134a serving as a benchmark. A large data bank was obtained with 432 data points covering a wide range of typical inlet subcooling (1.3–14.7) K and mass fluxes (333–1260) kg/m2s as well as two nominal saturation temperatures (30 and 40) °C.The effect of inlet subcooling was found to be consistently significant at the higher mass fluxes. This was contradictory to other investigations that found moderate or insignificant effects. The result is suggested to be attributed to the two-phase stability induced by the upstream throttle valve, inlet orifices, isolated refrigerant in the inlet and outlet plenums as well as the short heated length causing higher importance of orifice to channel pressure drop importance. Finally, a new modified Katto and Ohno correlation including the effect of subcooling was proposed, achieving a 4.0% mean average error and predicting 93.3% of the data points within 10% error.

Blood coagulation is a critical and complex reaction that involves various chemical substances, such as prothrombin, fibrinogen, and fibrin. The process can be divided into three main steps, namely the formation of the prothrombin activator, conversion of prothrombin to thrombin, and conversion of fibrinogen to fibrin. In this study, an ANSYS simulation is carried out to determine the prothrombin time (PT) of blood, the chemical changes that occur during coagulation and the anticoagulation factor. The addition of deionized water to the microchannels before the addition of blood and reagents results in a two-phase flow. The evaluation of this two-phase flow is necessary, and dynamic simulations are required to determine the PT. The chemical rate constant and order of the chemical reaction are derived from the actual prothrombin time. Moreover, the genetic algorithms in PYTHON and ANSYS are used to estimate chemical reaction parameters for a 20 s PT. The blood and anticoagulant exhibit increased dynamic behavior in the microchannel. In addition, particles are added to the microchannel and the dynamic mesh method is used to simulate the flow behaviors of the red and white blood cells in the microchannel. The PTs for different volumes of blood are also reported.

The modeling of transfer processes is a step in the generalization and interpretation of experimental data on heat transfer. The developed two-dimensional model is based on a homogeneous mixture model for boiling water flow in a microchannel with a new evaporation submodel. The outcome of the simulation is the distribution of velocity, void fraction and temperature profiles in the microchannel. The predicted temperature profile is consistent with the experimental literature data.

We report a microfluidic chaotic micromixer with staggered herringbone grooves having a geometry optimized for fast mobile-phase modification at the interface of a two-dimensional liquid chromatography system. The volume of the 300 mm mixers is 1.6 microliters and they provide mixing within 26 s at a flow rate of 4 mL min(-1) and 0.09 s at a flow rate of 1000 mL min(-1). Complete mixing is achieved within a distance of 3 cm along the 5 cm-long microchannel over the whole range of flow rates. The mixers can be used to mix aqueous phosphate-buffered saline solutions with methanol or acetonitrile at different ratios (1 : 2, 1 : 5 and 1 : 10). We also describe in detail an improved fabrication protocol for these mixers using a two-step soft photolithographic procedure. Mixers are made by replication in poly(dimethylsiloxane).

International audience; Amorphous silicon based microchannel plates are being developed to overcome performance limits of conventional microchannel plates. They offer a new flexibility and ease of fabrication. A comprehensive AMCP model is being constructed to analyze the performances of AMCPs. It includes Monte Carlo simulation of secondary electron emission distribution as a function of energy and angles and finite element analysis multiphysics software to compute electron trajectories. The paper presents the results of Monte Carlo simulations of secondary emission functions in silicon and the high secondary emissive material Al 2 O 3. We discuss the gain and potential performance as a function of geometry of such devices. The validity of the Eberhardt model for the analysis of AMCPs is also addressed.

The quality of micro-features in various technologies is mostly affected by the choice of the micro-fabrication technique, which in turn results in several limitations with regard to materials, productivity, and cost. Laser beam micro-machining has a distinct edge over other non-traditional methods in terms of material choices, precision, shape complexity, and surface integrity. This study investigates the effect of laser fluence and pulse overlap while developing microchannels in alumina ceramic using an neodymium-doped yttrium aluminum garnet (Nd:YAG) laser. Microchannels 200 &micro, m wide with different depths were machined using different laser peak fluence and pulse overlap (percentage of overlap between successive laser pulses) values. It was found that high pulse overlaps and fluences should be avoided as they give rise to V-shaped microchannels i.e., 100% bottom width errors. The optimal peak fluence range was found to be around 125&ndash, 130 J/cm2 corresponding to 3&ndash, 5 &micro, m depth per scan. In addition, channels fabricated with moderate pulse overlap were found to be of good quality compared to low pulse overlaps.

The pulsating nature of a closed loop pulsating heat pipe (CLPHP) makes the prediction of its thermal-hydraulic performance extremely challenging. Unfortunately, the design of CLPHPs is still quite an empirical process, requiring testing of many prototypes by cold plate fabricators and thus inhibiting their possibilities for application. This means that a reliable and robust CLPHP simulation code is needed to do the design and prediction of its thermal performance. In this study, an updated code based on a 1-D model that traces out the entire loop is presented, starting from its first version developed in 2015. The present code is based on the principles highlighted in the Three-Zone Model for flow boiling in a micro-channel (i.e. thin liquid film zone, liquid slug zone and dry-out zone), but applied to the oscillating flow of CLPHPs in both evaporating and condensing processes. The heat transfer processes between the wall and the fluid are modelled for both the liquid slugs (convective) and vapor plugs (film evaporation and condensation). The wall receives heat from the electronics to be cooled based on the boundary conditions applied locally and the wall conducts heat axially through a 1-D conduction scheme in addition to the two-phase pulsating mechanism. The present code can be classified as an engineering tool, being pragmatic and thus achieving accurate predictions with reasonably low computational times, as opposed to a research tool. More specifically, the kinetics of the flow are solved exclusively on the liquid slugs as the dynamics of the vapor and liquid film were assumed negligible, except for their changing lengths. The code also includes mechanisms related to bubble creation (new vapor plug formation by nucleation) and collapse (when two liquid slugs coalesce due to the vapor plug in between collapses).

The flow rate of a pressure-driven liquid through a microchannel may be enhanced by texturing its no-slip boundaries with grooves aligned with the flow. In such cases, the grooves may contain vapour and/or an inert gas and the liquid is trapped in the Cassie state, resulting in (apparent) slip. The flow-rate enhancement is of benefit to different applications including the increase of throughput of a liquid in a lab-on-a-chip, and the reduction of thermal resistance associated with liquid metal cooling of microelectronics. At any given cross-section, the meniscus takes the approximate shape of a circular arc whose curvature is determined by the pressure difference across it. Hence, it typically protrudes into the grooves near the inlet of a microchannel and is gradually drawn into the microchannel as it is traversed and the liquid pressure decreases. For sufficiently large Reynolds numbers, the variation of the meniscus shape and hence the flow geometry necessitates the inclusion of inertial (non-parallel) flow effects. We capture them for a slender microchannel, where our small parameter is the ratio of ridge pitch-to-microchannel height, and order-one Reynolds numbers. This is done by using a hybrid analytical–numerical method to resolve the nonlinear three-dimensional (3-D) problem as a sequence of two-dimensional (2-D) linear ones in the microchannel cross-section, allied with non-local conditions that determine the slowly varying pressure distribution at leading and first orders. When the pressure difference across the microchannel is constrained by the advancing contact angle of the liquid on the ridges and its surface tension (which is high for liquid metals), inertial effects can significantly reduce the flow rate for realistic parameter values. For example, when the solid fraction of the ridges is 0.1, the microchannel height-to-(half) ridge pitch ratio is 6, the Reynolds number of the flow is 1 and the small parameter is 0.1, they reduce the flow rate of a liquid metal (Galinstan) by approximately 50 %. Conversely, for sufficiently large microchannel heights, they enhance it. Physical explanations of both of these phenomena are given.

Measuring fluid pressure in microchannels is difficult and constitutes a challenge to even the most experienced of experimentalists. Currently, to the best of the authors&rsquo, knowledge, no optimal solution are being used for the design of pressure taps, nor guidelines concerning their shape and its relation with the accuracy of the readings. In an attempt to address this issue, a parametric study was devised to evaluate the performance of different pressure tap designs, 18 in total. These were obtained by combining three shape parameters: sub-channel width (w) and sub-channel&ndash, tap radius (R) or angle (&alpha, ), while having the sub-channel length kept constant. For each configuration, pressure drop measurements were carried out along several lengths of a straight microfluidic rectangular channel and later compared to an analytical solution. The microchannels were fabricated out of PDMS using standard soft-lithography techniques, pressure drop was measured with differential pressure sensors, the test fluid was DI water and the flow conditions varied from creeping flow up to R e c &sim, 100. Pressure taps, having smooth contours (characterised by the radius R) and a sub-channel width (w) of 108 &mu, m , performed the best with results from that of radius R = 50 &mu, m only falling short of the theory by a mere &sim, 5 % .

Fabrication of precise micro-features in bioceramic materials is still a challenging task. This is because of the inherent properties of bioceramics, such as low fracture toughness, high hardness, and brittleness. This paper places an emphasis on investigating the multi-objective optimization of fabrication of microchannels in alumina (Al2O3) bioceramics by using rotary ultrasonic machining (RUM). The influence of five major input parameters, namely vibration frequency, vibration amplitude, spindle speed, depth of cut, and feed rate on the surface quality, edge chipping, and dimensional accuracy of the milled microchannels was analyzed. Surface morphology and microstructure of the machined microchannels were also evaluated and analyzed. Unlike in previous studies, the effect of vibration frequency on the surface morphology and roughness is discussed in detail. A set of designed experiments based on central composite design (CCD) method was carried out. Main effect plots and surface plots were analyzed to detect the significance of RUM input parameters on the outputs. Later, a multi-objective genetic algorithm (MOGA) was employed to determine the optimal parametric conditions for minimizing the surface roughness, edge chipping, and dimensional errors of the machined microchannels. The optimized values of the surface roughness (Ra and Rt), side edge chipping (SEC), bed edge chipping (BEC), depth error (DE), and width error (WE) achieved through the multi-objective optimization were 0.27 &mu, m, 2.7 &mu, m, 8.7 &mu, m, 8 &mu, m, 5%, and 5.2%, respectively.

Laser micromachining has emerged as a promising technique for mass production of microfluidic devices. However, control and optimization of process parameters, and design of substrate materials are still ongoing challenges for the widespread application of laser micromachining. This article reports a systematic study on the effect of laser system parameters and thermo-physical properties of substrate materials on laser micromachining. Three dimensional transient heat conduction equation with a Gaussian laser heat source was solved using finite element based Multiphysics software COMSOL 5.2a. Large heat convection coefficients were used to consider the rapid phase transition of the material during the laser treatment. The depth of the laser cut was measured by removing material at a pre-set temperature. The grid independent analysis was performed for ensuring the accuracy of the model. The results show that laser power and scanning speed have a strong effect on the channel depth, while the level of focus of the laser beam contributes in determining both the depth and width of the channel. Higher thermal conductivity results deeper in cuts, in contrast the higher specific heat produces shallower channels for a given condition. These findings can help in designing and optimizing process parameters for laser micromachining of microfluidic devices.

The entrance region constitutes a considerable fraction of the channel length in miniaturized devices. Laminar slip flow in microchannel plate fin heat sinks under hydrodynamically developing conditions is investigated semi-analytically and numerically in this paper. The semi-analytical model for the pressure drop of microchannel plate fin heat sinks is obtained by solving the momentum equation with the first-order velocity slip boundary conditions at the channel walls. The simple pressure drop model utilizes fundamental solutions from fluid dynamics to predict its constitutive components. The accuracy of the model is examined using computational fluid dynamics (CFD) simulations and the experimental and numerical data available in the literature. The model can be applied to either apparent liquid slip over hydrophobic and superhydrophobic surfaces or gas slip flow in microchannel heat sinks. The developed model has an accuracy of 92 percent for slip flow in microchannel plate fin heat sinks. The developed model may be used to predict the pressure drop of slip flow in microchannel plate fin heat sinks for minimizing the effort and expense of experiments, especially in the design and optimization of microchannel plate fin heat sinks.

Pool boiling has a high heat removal capability and is considered as one of the most effective cooling methods due to utilization of latent heat of vaporization. Surface wettability plays a key role in boiling heat transfer since it controls the contact line between liquid, gas, and solid phases. Here, surfaces with mixed wettability (biphilic) were fabricated for assessing the effect of biphilic surfaces on bubble dynamics and boiling heat transfer as well as for the determination of an optimum hydrophobic area to the total surface area (A* = A(Hydrophobic)/A(total)) to achieve the best heat transfer performance. Pool boiling experiments were conducted on biphilic surfaces with A* ranging from 0.19% to 95%. It was shown that biphilic surfaces directly affected both the critical heat flux (CHF) and boiling heat transfer. According to the experimental results, the surface with A* of 38.46% delivered the highest CHF enhancement (197 W/cm(2), and maximum boiling heat transfer enhancement of 103%) among the tested biphilic surfaces. To represent a better understanding of related heat transfer mechanisms, bubble dynamics was obtained using a high-speed camera system. Visualization results revealed that bubble formation took place sooner on biphilic surfaces with A* of higher than 38.46%, thereby triggering the generation of vapor blanket on the surfaces and CHF occurrence at lower heat fluxes. (C) 2019 Elsevier Ltd. All rights reserved.

Porous hydrogel scaffolds are ideal candidates for mimicking cellular microenvironments, regarding both structural and mechanical aspects. We present a novel strategy to use uniquely designed ceramic networks as templates for generating hydrogels with a network of interconnected pores in the form of microchannels. The advantages of this new approach are the high and guaranteed interconnectivity of the microchannels, as well as the possibility to produce channels with diameters smaller than 7 μm. Neither of these assets can be ensured with other established techniques. Experiments using the polyacrylamide substrates produced with our approach have shown that the migration of human pathogenic Acanthamoeba castellanii trophozoites is manipulated by the microchannel structure in the hydrogels. The parasites can even be captured inside the microchannel network and removed from their incubation medium by the porous polyacrylamide, indicating the huge potential of our new technique for medical, pharmaceutical, and tissue engineering applications.

A numerical investigation is carried out on the effects of flow maldistribution on the temperature uniformity and overall thermal resistance in double-layered microchannel heat sinks. Different flow maldistribution models accounting for the effects of some typical header designs are considered together with different combinations of the average inlet velocity in the two layers of microchannels for a given total mass flow rate. The numerical simulations are carried out using an in-house FEM procedure previously developed by the authors for the analysis of cross-flow microchannel heat exchangers.

The present work aims at developing a numerical study on the drug diffusion in the bloodstream in a coronary artery with drug-eluting stent implanted. The blood was modeled as a single-phase, incompressible and Newtonian fluid and the Navier–Stokes equation was approximated according to the Finite Element Method (FEM). The dynamics of drug-eluting concentration in bloodstream was investigated using four drug-eluting stents with different mass diffusivities in microchannels with variable cross sections, including a real coronary artery geometry with atherosclerosis. The results reveal complex drug concentration patterns and accumulation in the vicinity of the fat buildup.

The increasing demands of computational power have accelerated the development of 3D circuits in the semiconductor industry. To resolve the accompanying thermal issues, two-phase microchannel heat exchangers using have emerged as one of the promising solutions for cooling purposes. However, the direct boiling in microchannels and rapid bubble growth give rise to highly unstable heat flux on the channel walls. In this regard, it is hence desired to control the supply of vapor bubbles for the elimination of the instability. In this research, we investigate a controllable bubble generation technique, which is capable of periodically producing bubble seeds at the sub-micron scale. These nanobubbles were generated in a solid-state nanopore filled with a highly concentrated electrolyte solution. As an external electric field was applied, the localized Joule heating inside the nanopore initiated the homogeneous bubble nucleation. The bubble dynamics was analyzed by measuring the ionic current variation through the nanopore during the bubble nucleation and growth. Meanwhile, we theoretically examined the bubble growth and collapse inside the nanopore by a moving boundary model. In both approaches, we demonstrated that by altering the pore size, the available sensible heat for the bubble growth can be manipulated, thereby offering the controllability of the bubble size. This unique characteristic renders nanopores suitable as a nanobubble emitter for microchannel heat exchangers, paving the way for the next generation microelectronic cooling applications.

In this paper, the effect of the scalability of small-scale devices on the separation of Co(II) from a binary Co(II)/Ni(II) mixture in a nitric acid solution by an organic Cyanex 272/TBP/kerosene (Exxsol D80) phase is studied. In particular, circular channels with diameters of 1, 2, and 3.2 mm are considered. The results were compared against those from a confined impinging-jets (CIJ) cell with a main channel diameter of 3.2 mm. The effects of total flowrate, residence time, Cyanex 272 concentration, and flowrate ratio on the mass transfer performance were investigated. It was found that at increased channel size, the throughputs were also increased but the extraction percentages remained the same. Higher extraction percentages were obtained by using the CIJ configuration at short residence times. However, for longer residence times, the mass transfer coefficients were similar and capillary channels should be preferred over the CIJ because of the ease of separation of the two phases at the end of the unit. The overall mass transfer coefficients ranged between 0.02 and 0.14 s&minus, 1 for the capillary channels during plug flow and between 0.05 and 0.45 s&minus, 1 for the CIJ cells during dispersed flow.

We investigate the effect of microfluidic flow on the microstructure and dynamics of a model surfactant mixture, combining synchrotron Small Angle X-ray Scattering (SAXS), microscopy and rheology. A system comprising a single-chain cationic surfactant, hexadecyl trimethyl ammonium chloride (C16TAC), a short-chain alcohol (1-pentanol) and water was selected for the study due to its flow responsiveness and industrial relevance. Model flow fields, including sequential contraction-expansion (extensional) and rotational flows, were investigated and the fluid response in terms of the lamellar d-spacing, orientation and birefringence was monitored in situ, as well as the recovery processes after cessation of flow. Extensional flows are found to result in considerable d-spacing increase (from approx 59 Å to 65 Å). However, under continuous flow, swelling decreases with increasing flow velocity, eventually approaching the equilibrium values at velocities ≃2 cm s(-1). Through individual constrictions we observe the alignment of lamellae along the flow velocity, accompanied by increasing birefringence, followed by an orientation flip whereby lamellae exit perpendicularly to the flow direction. The resulting microstructures are mapped quantitatively onto the flow field in 2D with 200 μm spatial resolution. Rotational flows alone do not result in appreciable changes in lamellar spacing and flow type and magnitude evidently impact the fluid microstructure under flow, as well as upon relaxation. The findings are correlated with rheological properties measured ex situ to provide a mechanistic understanding of the effect of flow imposed by tubular processing units in the phase behavior and performance of a model surfactant system with ubiquitous applications in personal care and coating industries.

Microchannels embedded polymer composites were fabricated by resin infusion process using carbon fabric, epoxy resin and hollow glass tubes (HGTs). The effect of a range of low-velocity impact (LVI) and mode-I delamination (M1D) damage on the flexural strength of microchanneled carbon- epoxy composites was studied. A self-healing approach was also employed to recover their lost flexural strength due to these damages. Moreover, influence of LVI, M1D damage and healing on the failure behavior of microchanneled carbon- epoxy composites was also investigated. The results of flex- ural after impact (FAI) and flexural after delamination (FAD) showed that LVI has more deleterious effect on the flexural strength of carbon- epoxy composites than M1D damage. The loss in flexural strength increased linearly with increase in both impact (by higher impact energies) and delamination damage (by longer delamination lengths). Scanning electron microscopic (SEM) study revealed that self-healing agent (SHA), stored in HGTs placed within carbon- epoxy composites, effectively healed both LVI and M1D damage with excellent healing efficiencies.

In this paper, two new analytical models have been developed to calculate two-phase slug flow pressure drop in microchannels through a sudden contraction. Even though many studies have been reported on two-phase flow in microchannels, considerable discrepancies still exist, mainly due to the difficulties in experimental setup and measurements. Numerical simulations were performed to support the new analytical models and to explore in more detail the physics of the flow in microchannels with a sudden contraction. Both analytical and numerical results were compared to the available experimental data and other empirical correlations. Results show that models, which were developed based on the slug and semi-slug assumptions, agree well with experiments in microchannels. Moreover, in contrast to the previous empirical correlations which were tuned for a specific geometry, the new analytical models are capable of taking geometrical parameters as well as flow conditions into account.