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This paper aims to simulate an immiscible two-phase flow in two-dimensional T-shaped, modified T-shaped, cross-shaped, and modified cross-shaped micromixers numerically. The effects of various parameters, including Capillary number, phase flow rate ratio, and channel width ratio, on the generation of droplets were studied. Also, modification of T-shaped and cross-shaped micromixers was proposed. The results reveal that equidistant tiny droplets are generated in the modified micromixers compared to ordinary geometry under the same conditions. Three types of squeezing regime, laminar regime, and jetting regime were observed in different values of the capillary number. It was also observed that the droplet size decreases with the increase in capillary number. Moreover, this study demonstrates that the multiphase flows in micro-devices are very sensitive to even small changes in the channel geometry.

We designed a new type of $T$-type electroosmotic micromixer with variable modules (EMVMs). By applying 7V alternating current (AC) voltage to the EMVM, the mixing efficiency at different Reynolds number (Re) is discussed by changing the rotation angles of modules at different positions in different directions. We found that the mixing efficiency of the EMVM can reach 99% when the first block is rotated along the xz-axis. Then, we change the torsion angle of the same variable module and find that the mixing efficiency can reach 99.3% when the torsion is $0^{\circ }$. Finally, we change the voltage value and find that the mixing efficiency of the EMVM increases with the increase of voltage values.

With fast development of microfluidic systems, fluid micro-mixing becomes a very important issue. In this paper, recent developments on various micromixers and their working mechanisms are reviewed, including the external agitation methods applied in active mixing and the channel geometries adopted in passive mixer design. The chaotic mixing and the influences of Re would be mainly discussed. At moderate and high Re, the fluid inertial effects usually facilitate the chaotic mixing. At low Re, generation of chaotic advection becomes more difficult but can still be achieved through fluid manipulations such as stretching and folding. Chaotic mixers can be characterized using dynamical system techniques, such as Poincaré plot, and Lyapunov exponent.

This paper investigates the effects of external excitation on the mixing performance of the micromixer with curved bluff-body structure. The micromixer was fabricated by the MEMS process of polydimethylsiloxane (PDMS). The mixing process and mixing efficiency were evaluated with a high speed camera. Results showed that the finger-spiked type flow patterns were generated in the mixing chamber under an excitation frequency of 5Hz. It turns out that the mixing efficiency as high as 85.6% is achieved at Re=0.25 with a single bluff-body structure. It demonstrates that the new design can be used to achieve complete mixing within ultra short length at mciroscale.

In this paper, we have studied the effect of variable-angle grooves and baffles on the mixing efficiency of the micromixer. In order to explore the influence on the micromixer with different types of grooves and baffles, we designed grooves and baffles with different geometric parameters and placed them in T-channels to interfere with fluid flow. We studied VAM30^∘ (variable-angle grooves and baffles micromixer with an angle of 30^∘) directions and distributions as well as their different groove depths and baffle heights affect the mixing performance. We tried to divide the grooves and baffles into five groups, and discussed the effects of staggered depth and height on mixing efficiency. The mixing efficiencies of micromixer in the Re (Reynolds number) range of 0.1–100 were calculated, and the fluid flow in the microchannel was analyzed. The simulation results show that VAM30^∘ is more favorable for solution mixing. The mixing efficiency of the micromixer could reach 98.9% with the change of different geometric parameters. This is because when the structure changes, the flow state of the fluid is improved, which is conducive to lengthening the residence time of the fluid in the channel. With the increase of Re, it is also conducive to enhancing the chaotic convection and improving the mixing efficiency.

In this paper, we mainly study the mixing performance of the micromixer with quartic Koch curve fractal (MQKCF) by numerical simulation. Changing the structure of the microchannel based on the fractal principle can significantly improve the fluid flow state in the microchannel and improve the mixing efficiency of the micromixer. This paper discussed the effects of different fractal deflection angles, microchannel heights and different fractal times on the mixing efficiency under four different Reynolds numbers (Re). It is found that changing the deflection angle of the fractal can bring extremely high benefits, which makes the fluid deflect and fold in the microchannel, enhancing the chaotic convection in the microchannel, and improve the mixing efficiency of the fluid. Under the reasonable arrangement of the quartic Koch curve fractal principle, it can give the micro-mixture more than 99% mixing efficiency. Based on the excellent mixing performance of MQKCF, it also has extremely high application value in the biochemical neighborhood.

In this paper, the influence of the parameters of CO₂ laser cutting system on the rhombus microchannel is studied. In the experiment, we changed the laser processing speed, laser processing power, and laser processing number of CO₂ laser cutting system, and the depth and width of microchannel are studied by changing these three parameters. It is found that the depth and width of microchannel increase with the increase of laser processing number and laser processing power. For example, when the speed is 6mm/s and the number of laser processing is three times, the width of microchannel increases from 0.51mm to 0.64mm and the depth increases from 0.75mm to 1.60mm with the increase of power. However, the depth and width of the microchannel decrease with the increase of laser processing speed. For example, when the power is 8W and the number of laser processing is three times, the width of the microchannel decreases from 0.51mm to 0.39mm, and the depth of the microchannel decreases from 0.75mm to 0.34mm.

We mainly studied the mixing performance of three new T-type electroosmotic micromixers (T-EMs). The three T-EMs are designed as rhomboids with three different shapes of obstructions inside. The rhomboid micromixers utilize the principle of convergence and divergence to improve mixing efficiency. By changing the Reynolds number (Re), the change of mixing efficiency under direct current (DC) voltage 20V and alternating current (AC) voltage 2mV was observed. We changed the shape of the intermediate obstacles, discussed the shape of the internal barrier on the mixing efficiency of T-EM, and increased the mixing efficiency by increasing the voltage value. The mixing efficiency of T-EM with 150$\mu$m channel width can reach 97.5% under DC condition and 97.6% with circular barrier under AC condition.

Passive micromixers utilize no external energy input. It mainly depends on the structure change of microchannel to produce chaotic convection effect, which increases the contact surface and contact time between the fluid. The mixing efficiency of stagger non-abreast baffles (SNB) micromixer and stagger abreast baffles (SAB) micromixer is compared at five kinds of Reynolds (Res). It may affect the mixing efficiency including the height ($H$) and number ($n$) of baffles, the distance between two adjacent baffles, non-abreast baffles, and abreast baffles. As the $\theta$ of the baffles changes, it contributes to enhance the chaotic convection of the micromixer. When Re $=1$, the worst mixing efficiency of the most efficient seven SABs ($\theta =15^{\circ }$, $H=0.09$ mm, $D=0.60$ mm, $n=7$) micromixer reaches an astonishing 98%. The concentration change along the microchannel is studied in this paper. At low Re, SAB makes fluid contact zigzag in microchannel and enhances molecular diffusion. At high Re, SAB causes chaotic convection in the microchannel and completes the mixing quickly.

This paper demonstrates the functionality of a simple and convenient microfluidic method in synthesizing a series of poly(vinylpyrrolidone) (PVP) stabilized nanoparticles (NPs) of various novel metals (Pt, Pd, Ru, Rh, Ag, and Au) with an average diameter of $<$2 nm. In this method, the use of microfluidic mixture provided a homogenous mixing of the metal precursors and reducing agent nearly at the molecular level, that yield monodispersed sub-nanosize NPs. Core diameters of the produced NPs determined by transmission electron microscopy (TEM), were $1.3\pm 0.3$, $1.6\pm 0.3$, $1.3\pm 0.2$, $1.7\pm 0.4$, $1.9\pm 0.4$ and $1.2\pm 0.2$nm for Pt, Pd, Ru, Rh, Ag and Au NPs, respectively. Of them, Pt NPs were detailed characterized. The obtained Pt NPs were found to have fcc crystal structure with 1.2 nm crystalline size which is very similar to the corresponding TEM result. The efficiency of the synthesis of NPs by micromixer was compared with batch/NaBH₄ reduction method for the Pt NPs. It was found that in batch method the as-prepared NPs decreased the reducing ability of NaBH₄ by catalytic degradation. In contrast, the micromixer could separate the produced metal NPs from the reaction system soon after the formation of NPs and enables feeding the fresh NaBH₄ solution throughout the synthesis. Fourier Transform Infrared (FTIR) spectrometry measurements of adsorbed ${}^{12}$CO molecules on Pt NPs showed that the NPs surface were negatively charged with a high population of edge and vertices atoms.

In this paper, endeavor has been made to design and analyze different Y-type micromixers by characterizing the mixing and flow behavior in ultra-low Reynolds number region. The effects of different geometric and flow parameters on the mixing and pressure drop are studied through computational fluid dynamics (CFD) simulations using COMSOL Multiphysics software. The parameters investigated are obstacle geometry, obstacle arrangement, obstacle depth, aspect ratio (AR), entrance angle ($E_{\theta })$, Reynolds number (Re) and obstacle packing factor (OPF). The simulation results reveal that rectangular shaped obstacles in staggered arrangement gives the best mixing. Increasing obstacle depth and OPF increases both mixing and pressure drop whereas increasing entrance angle enhances mixing but has negligible effect on pressure drop. Also both the mixing and pressure drop performance enhances with decreasing AR and lower Reynolds number gives better mixing in the lower laminar flow region.

In the present study, for the first time, the flow and mass transfer in the rotary micropump-micromixers were investigated by the SPH method. In fact, the present work shows the ability of the SPH method to model the mixing process due to pumping action. The incompressible SPH method applied for modeling is improved by the kernel gradient corrective tensor, a particle shifting algorithm, and an improved periodic boundary condition. SPH is a proper method for modeling the mixing process because there is no modeling for the convective terms and so, the false diffusion is not observed in the SPH modeling. In the present study, first, a viscous micropump comprising a microchannel in which a circular cylinder rotates with special eccentricity is modeled and validated. Then, the geometry is manipulated in order to achieve a desirable micromixer.

Based on the advantages of microfluidics in the field of nanoparticle synthesis, a controllable synthesis method for silver nanoparticles using a double-layer Y-shaped splitting and recombination (SAR) micromixer is proposed. First, the liquid phase synthesis mechanism of silver nanoparticles, the working principle of the double-layer Y-shaped SAR micromixer, and the mixing performance of micromixer at different Reynolds number (Re) are analyzed. Then, the micromixer is used to synthesize silver nanoparticles, and the effects of reductant concentration, polyvinylpyrrolidone (PVP) and inlet flow rate on the size, distribution and morphology of the synthesized silver nanoparticles are investigated comprehensively. The synthesized silver nanoparticles are characterized by UV-spectrometer and transmission electron microscopy (TEM). The experimental results show that the reductant concentration, PVP, and inlet flow rate have a direct impact on the size, distribution, monodispersity and morphology of the synthesized nanoparticles. The moderate reductant concentration makes the size of silver nanoparticles larger and the size uniformity is better. Adding PVP to the experimental reagent can prevent the aggregation of silver nanoparticles, consequently, the synthesized particles have a uniform distribution and a better morphology. The changes in inlet flow rate and Re directly affect the mixing efficiency, which in turn affect the formation of silver atoms and silver nanocrystal nuclei and have a greater impact on particle concentration. The proposed micromixer has excellent mixing performance and can be used in other fields such as controllable synthesis, biomedicine and microchemical systems.

Mixing control is an important issue in micro-fluid chip applications, such as μTAS (Micro-Total Analysis System) or LOC (Lab-on-Chip) because the flow at micro-scale is highly laminar. Several flow control schemes had been developed for complete mixing in the micro-channels in the past decades. However, most of the mixing control schemes are performed by utilizing specific excitation devices, such as electrokinetic, magnetic or pressure drivers. This paper investigates a new control scheme which is composed of a series of flow manipulation by changing the pressure at the two inlets of the micromixer as the external excitation. The fluids from two inlets are introduced to a square mixing chamber, which provides a space where the streamwise and transverse flow motions take place. The results show that the micromixer can be used to produce a large recirculation zone with series of small transverse fringes under external excitations. It is found that this new flow pattern enhances mixing processes at the micro-scale. A complete mixing can be achieved under appropriate flow control with the corresponding design.

This work describes a portable microsensor for analyzing the silicate concentration in water. Conventionally adopted silicate analysis methods involve bulky instrumentation that are limited in portability and immediateness. The proposed silicate microsensor consists of a microliquid core waveguide, passive spiral micromixer, and bubble traps that possess excellent signal enhancement properties. The microsensor size is 52 × 26 mm, while each measurement requires only 115 μl of a sample and reagents, thereby reducing the sample requirement for a considerable amount of time and work to collect expensive reagents. The spiral micromixer has a mixing capability superior to that of a premix mixture. Bubble traps have been developed to trap air bubbles formed in the microchannel in order to prevent gas bubbles from interfering with the measurements. As a linear function of silicate concentration, the absorbance response ranges from 0 to 250 nM. Additionally, the linearity is excellent with a linear R value of 0.9985 and the experimental detection limit is 8.9 nM. The proposed portable microsensor significantly contributes to aqueous inspection, subsequently creating a highly value-added technology for chemical sensors and microsystems.

To achieve high mixing efficiency at a micro scale, a passive micromixer with feedback channels for a recycle flow is devised. Through side channels, the mixed fluids are guided back into the main channel due to the Coanda effect, generating circular flows and thereby improving the mixing effect. Experimental results have revealed that the high reflux ratio benefits mixing efficiency. The objective of this paper is to investigate the relationship between geometric parameters and the reflux ratio. Geometric parameters, including inlet width, outlet width and channel depth, are numerically investigated through Computational Fluid Dynamics techniques. The simulations are performed at Reynolds numbers from 5 to 95 with an increment of 10. Our study shows that the reflux ratio is significantly affected by the Reynolds number, inlet width, and channel depth, but the outlet width has little effect on it between Reynolds numbers from 5 to 95.