Microfluidics and Nanofluidics最新文献

Nitrocellulose microspheres have garnered extensive use in propellants and launching agents due to their inherent safety, robust flowability, and high explosive power. However, conventional preparation methods for these microspheres are often hampered by complex processes, low safety factor and poor sphericity. This study explores an innovative approach to nitrocellulose microsphere fabrication utilizing microfluidic technology. We designed and assembled two high-throughput preparation devices—a coaxial and a centrifugal device—employing 3D printing technology. Our findings demonstrate an 18-fold increase in efficiency over traditional single-pass microfluidic techniques. Additionally, we examined the impact of these devices on the microspheres’ size distribution. The proposed device showcases significant advantages, including reduced cost, enhanced efficiency, and shorter production cycles, indicating promising potential for wide-scale application in nitrocellulose microsphere preparation.

The performance of hollow fiber membrane contactor for CO₂ removal using MEA-based nanofluid was experimentally evaluated. Different types of nanoparticles, including Al₂O₃, Fe₃O₄, and functionalized MWCNT in this study. The influence of various operating conditions including gas and absorbent flow rates, absorbent concentration, and nanofluid characteristics on separation performance was thoroughly examined. The results showed that compared to conventional amine solvents, the nanofluid absorbents significantly enhance CO₂ absorption performance. In comparison to the base fluid, the mass transfer coefficient was raised by 320, 120, and 40% for 0.15 wt% MWCNT, Al₂O₃ and Fe₂O₃, respectively. The MWCNT showed much more compliance with amine solvents due to its carboxyl functional groups and higher surface area which make it more stable in a strong polar mixture. The study underscores the importance of stability, viscosity, and shear stress of nanofluids as key parameters affecting CO₂ absorption performance.

In the present study, we explore the dynamics of bubble formation in a flow-focusing device designed for gas-non-Newtonian liquid two-phase flow. The flow-focusing device with a cross-section of a square (300 μm × 300 μm) is constructed on polydimethylsiloxane using lithographic techniques and subsequently sealed with polymethylmethacrylate. A high-speed camera is employed to document the process of bubble formation during the experiment, complemented by computational fluid dynamics methods for an in-depth analysis. The gas is nitrogen, and the liquid is sodium carboxymethyl cellulose solutions with mass fractions of 0.1, 0.2, and 0.3%, respectively. The inlet flow rates of gas and liquid are set at 1–2 ml/min in the simulation and the experiment, and the observed flow patterns are all slug flows. Experimental findings suggest that the duration of bubble formation can be bifurcated into two distinct parts. The first part is predominantly influenced by the velocity of the inlet gas, and the correlation coefficient between velocity and time is −0.56, while the second part is impacted by the shear-thinning properties of the liquid, which are correlated with the flow index and viscosity coefficient of the non-Newtonian liquids, and the correlation coefficients are −0.47 and 0.48, respectively. The computational fluid dynamics results of gas-non-Newtonian liquid two-phase flow with gas and liquid flow rates of 2 ml/min corroborate that the manifestation of the aforementioned time segmentation phenomenon primarily depends on the vortex intensity at the bubble’s head and the orientation of pressure gradients. When the bubble neck size approaches 0, the viscosity of the surrounding liquid decreases rapidly, and alterations in the velocity field near the bubble neck trigger fluctuations in the viscosity of the non-Newtonian liquid, thereby influencing the bubble formation process.

The Molecular Tagging (MT) technique is a promising methodology for locally measuring velocity and temperature fields in rarefied gas flows. Recently, Molecular Tagging Velocimetry (MTV) has been successfully applied to gas flows in mini-channels in the continuum regime at high pressure and early slip-flow regime at lower pressure. As the operating pressure decreases, diffusion effects become more pronounced, and in MTV, they hinder the extraction of the correct velocity profile by simply dividing the displacement profile of the tagged molecular line by time of flight. To address this issue, a reconstruction method that considers Taylor dispersion was previously developed to extract the velocity profile, considering the diffusion effects of the tracer molecules within the carrier gas. This reconstruction method successfully extracted the correct velocity profile in the continuum flow regime. However, the method still faces challenges in the slip-flow regime. Since there is currently no consensus in the literature regarding the kinetic diameter value of acetone vapor, the diffusion coefficient estimation is uncertain especially at low pressures. This is why, in this study, we propose an original optical method to measure the diffusion coefficient of acetone vapor. This is achieved by linking the temporal evolution of the spatial photoluminescence distribution of acetone vapor to the diffusion coefficient via the Chapman-Enskog theory. Our research provides measurements of these parameters for a wide range of pressures (0.5–10 kPa) at ambient temperature.

The formation of double-emulsions or core/shell microdroplets in microchannels, essential for various chemical applications, traditionally relies on costly and time-consuming laboratory methods. In this regard, computational fluid dynamics (CFD) and artificial neural network (ANN) techniques were employed. The present study developed ANN models to predict the relationship between shell thickness and double-emulsion size in a double-T microchannel, using two datasets comprising 180 experimental and CFD data points. Assessing this relationship involved analyzing various input factors, including the Capillary, Weber (case A), and Reynolds numbers (case B) of the core, shell, and continuous phases. Among twelve training algorithms and four activation functions, the Levenberg–Marquardt (LM) algorithm with sigmoidal activation functions (Tansig and Logsig), in contrast to the linear activation functions (Poslin and Purelin), achieved the highest predictive accuracy. Additionally, the predictive accuracy of ANN models was found to be significantly improved when trained using a combination of capillary and Weber numbers, as opposed to models trained only using capillary, Weber, and Reynolds numbers. The optimal neural network architectures were [10 5] neurons for case A (tansig and logsig) and [8] neurons for case B (tansig), yielding coefficients of determination (R²) of 0.99 and 0.98, respectively. These models demonstrated high precision and effective generalization, evidenced by statistical measures such as R², MSE, RMSE, AAD, %AARD, and computational time. Moreover, their ability to generalize within the training dataset further substantiates their predictive capacity.

A compelling solution to the issue of high heat flux generated by flexible electronic devices has been found in liquid-based microfluidic cooling devices. It has been earlier realized that the varying microchannel hydrodynamics influences the overall heat transfer in these devices. However, microfluidic cooling devices that incorporate intricate microchannels have not been explored to their full potential. In this study, we investigate the use of 3-D intricate microchannel geometries in microfluidic heat sinks, their generated hydrodynamics, and their profound impact on the overall heat transfer process. Utilizing 3D-printed scaffold removal technology, three distinct microfluidic devices were fabricated, each distinguishable by its cross-sectional shape of the microchannel designs (coil, square, and triangle). These microfluidic devices, based on Polydimethylsiloxane-Graphene oxide (PDMS-GO) as substrate material, have been examined experimentally and numerically for their heat dissipation capacities under constant temperature heat source of 358 K at flow rates ranging from 40 to 400 μL/min. Experimental observation illustrates that the coil-microchannel configuration exhibited superior heat dissipation capabilities, outperforming both the square and triangle microchannels across all flow settings. Furthermore, numerical simulations corroborated this experimental finding by providing insights into through-plane temperature distribution, heat transfer coefficient, pressure drop, and channel hydrodynamics. Our study intends to advance the understanding of microchannel cooling, as well as emphasizes the importance of geometrical configuration towards optimal electronic hotspot cooling.

Cylindrical column with packed stationary phase is the workhorse of liquid chromatography systems. These stationary phases are commonly classified on the basis of different form factors namely, beads and monoliths for protein chromatography. Monolithic rods are one of the important geometries derived from polymers through complex polymerization schemes with additional requirements such as cross-linkers and specific reaction conditions. To address these practical difficulties and enable ease of fabrication at laboratory scale, acrylic copolymers are hypothesized to perform as a monolithic stationary phase suitable for protein chromatography. The present work proposes a rapid fabrication technique to obtain monolithic rods that could be reconditioned without any of the above additional steps. It is characterized with monolith diameter that could be controlled using acrylic copolymer concentration. Formation of the copolymeric stationary phase inside microchannel led to annular geometry and in turn, demonstrated fabrication of moon-shaped channels (MSCs) for the first time in literature. An online monitoring system facilitated tracer breakthrough analysis with MSCs to report sharp peak front and an estimate of channel void volume. Breakthrough curves with single protein validated the selection of blue dextran as tracer and indicated retention of proteins due to electrostatic interactions on the functional copolymer surface.

We developed a surface-enhanced Raman scattering (SERS)-active plasmonic core-satellite nanostructure and incorporated it into a membrane filter-integrated microfluidic device for continuous monitoring of molecules in solution. The core-satellite nanostructures were fabricated by immobilizing a high number density of gold nanoparticles (AuNPs) on silica beads.to create many nanogaps among the AuNPs. The sizes of the nanogaps were fine-tuned by adding a silver (Ag) shell to optimize the SERS activity. In addition, citrate molecule, the capping agent of the nanoparticles, was displaced by alkali halides. The displacement not only reduced the SERS signals of citrate but also enhanced the adsorption of target molecules. The alkali halide-treated core-satellite nanostructures were accumulated onto a membrane filter integrated into a microfluidic device, serving as a uniform and sensitive SERS substrate. By increasing the volume of the sample solution flowing through the membrane filter, we increased the number of molecules adsorbed to the nanostructures, amplifying the intensities of their characteristic Raman peaks. Our microfluidic SERS device demonstrated continuous SERS detection of malachite green at a concentration as low as 500 fM. In summary, while various core-satellite nanostructures and microfluidic SERS devices have been reported, the integration of the membrane filter-containing microfluidic device with the core-satellite nanostructures facilitated sensitive and continuous molecule detection in our study.

Interface tracking simulations of gas–liquid Taylor flow in horizontal square microchannels were carried out to understand the relation between the pressure drop in the bubble part and the curvatures at the nose and tail of a bubble. Numerical conditions ranged for 0.00159 ≤ Ca_T ≤ 0.0989, 0.0817 ≤ We_T ≤ 25.4, and 8.33 ≤ Re_T ≤ 791, where Ca_T, We_T, and Re_T are the capillary, Weber, and Reynolds numbers based on the total volumetric flux. The dimensionless pressure drop in the bubble part increased with increasing the capillary number and the Weber number. The curvature at the nose of a bubble increased and that at the tail of a bubble decreased as the capillary number increased. The variation of the curvature at the tail of a bubble was more remarkable than that at the nose of a bubble due to the increase in the Weber number, which was the main cause of large pressure drop in the bubble part at the same capillary number. The relation between the bubble velocity and the total volumetric flux was also discussed. The distribution parameter of the drift-flux model without inertial effects showed a simple relation with the capillary number. A correlation of the distribution parameter, which is expressed in terms of the capillary number and the Weber number, was developed and was confirmed to give good predictions of the bubble velocity.

Microfluidic devices have been widely used to measure the diffusion coefficients and hydrodynamic radii of various molecules, especially proteins. The existing devices that use diffusion-based gradient generation apply obstacles such as membranes or hydrogels to avoid additional fluid flow affecting the evolution of concentration distribution and precise measurement. Here, a free-diffusion based microfluidic device was developed which is capable of measuring the diffusion coefficients of various, different-sized proteins and dyes without using any obstacles by minimizing pressure differences due to its symmetrical geometry. The fluorescent detection and the ease of application of the device enable accelerated measurements and interpretation of results. Time-lapse pictures of 30 s were taken of the diffusion profiles and a custom-made self-written Python program was used to fit the profiles to the theoretical functions and calculate the diffusion coefficients. Diffusion coefficients of bovine serum albumin, immunoglobulin G and rhodamine B were determined with this method and compared to their theoretical and experimental values.