Numerical simulation of geometry effect on mixing performance in L-shaped micromixers

Abstract

This study accomplishes a numerical analysis of mixing in a microchannel with repeating L-shaped units in order to research the effect of the extension of L-shaped units in a three-dimensional (3D) space and the angle of repeating units on the process of mixing. In the first part, Geometry 2 and Geometry 3 are designed by extending the units of Geometry 1 in a 3D space. In the second part, an L-shaped micromixer, a 90° V-shaped micromixer, and a 60° V-shaped micromixer are analyzed. It is observed that Geometry 1 and Geometry 2 perform better than Geometry 3 in terms of mixing due to the spiral path with 360° rotation of the flow. The L-shaped micromixer is more efficient than the 90° and 60° V-shaped micromixers. A maximum mixing index of about 88% is achieved in all serpentine microchannels at the following Reynolds numbers: Re = 150 and Re = 200.     

Micromixing-assisted preparation of TiO2 films from ammonium hexafluorotitanate and urea by liquid phase deposition based on simulation of mixing process in T-shaped micromixer

Preparation of well-ordered TiO₂ films from alkaline solution by liquid phase deposition (LPD) method is still challenging owing to the fact that, in traditional mixing process, several disadvantages exist in controllable homogenous chemical reactions. In this contribution, mixing process in a porous dispersed double T-junction micromixer was designed and investigated using numerical simulation. It is obtained that high mixing intensity can be achieved by micromixer with inlet velocity of 0.6?m?s^?1 and microfiltration meshes 10?μm in size. A micromixing-assisted platform consisted of two syringe pumps and a micromixer was manufactured to efficiently fabricate TiO₂ films via mixing of (NH₄)₂TiF₆ and CO(NH₂)₂ as reactants and fluorine doped tin oxide, F: SnO₂ (FTO), conducting glass substrates by LPD in alkaline solutions. The results suggest that surface morphology of TiO₂ films fabricated by micromixing can be easily controlled in comparison with that of specimens prepared by traditional stirring mixing process. The dense and well-ordered TiO₂ films derived from micromixing have enhanced adhesion strength, high hardness, good hydrophilic and excellent photoelectrochemical properties. Particularly, homogeneous reaction could be inhibited in micromixer and an ideal supersaturation solution would be formed via micromixing treatment, which significantly facilitates the formation of high quality TiO₂ films by heterogeneous nucleation on substrate surface. These obtained achievements are expected to promote further application of TiO₂ films in a variety of domains including photocatalysis and corrosion protection of metals.     

Electrically conducting polypyrrole-fuller's earth nanocomposites: Their preparation and characterisation

Cu(II)-exchanged fuller's earth was prepared by ion-exchanging Ca²⁺ ions which are present within the interlayer of fuller's earth with Cu(II)ions by the solution-phase ion-exchange process. Pyrrole was introduced into Cu(II)-exchanged fuller's earth to spontaneously polymerize to within the interlayer to result in a nanocomposite of Cu(I)-polypyrrole-fuller's earth where both Cu(I) and polypyrrole occupy within the interlayer spaces of fuller's earth. The nanomaterial [Cu(I)-PPY-FE] has been fully characterized with X-ray diffraction studies, FTIR spectroscopy, DC polarisation test with both blocking stainless steel and non-blocking copper electrodes. The material is found to be a mixed conductor whose ionic mobility is 1.5 times faster than electronic mobility. DC polarisation studies also clearly revealed that the mobile ionic species in this material to be cuprous ions. AC impedance studies have been carried out with blocking stainless steel electrodes at different applied potentials. The necessary theoretical background to explain AC impedance results is also provided and the results obtained agree very well with the corresponding data obtained by other mutually independent methods. The electronic conductivities are around 3.0 × 10⁻⁴ S cm⁻¹ and the ionic conductivities are around 9.0 × 10⁻³ S cm⁻¹. The material may find applications in semi-fuel cells such as air-metal batteries.     

Laminar Fluid Mixing in Micromixers: Description of Pull-Push Effects

A numerical study was performed on laminar mixing of fluids by means of embedded microbarriers against the flow direction in two three-dimensional T-micromixers. Two microchannels with different obstacle arrangements were considered, and the effect of volumetric flow rate Q on two-stream pull-push motion was investigated. With increasing Q and distance of the barriers to the sidewalls K, different vortices and fluid movement patterns develop in the two micromixers. Maximum mass transfer is obtained for the micromixer with smaller K value. With decreasing K, pull-push movement strongly affects fluid merging, and the quality of mixing increases. Moreover, the effect of fluid tortuosity was studied, and a direct relationship between hydrodynamic fluid tortuosity and mass transfer was found.     

Passive micromixer with baffles distributed on both sides of microchannels based on the Koch fractal principle

Since the beginning of the 21st Century, the development of microfluidic chip technology has been very rapid and has attracted the attention of more and more scholars. As an important part of the microfluidic chip, the performance of the micromixer is critical. The fractal structure in the microchannels helps to improve the mixing performance of the micromixer and improve the mixing efficiency of the micromixer. The research results of other scholars are of great significance to the research of the present paper, which mainly studies the effect of changing the baffle state on the mixing efficiency of the micromixer based on the Koch fractal principle. Through simulation analysis, it was found that the mixing efficiency of the baffles distributed on both sides of the microchannel was higher than the mixing efficiency of the baffles distributed on the microchannel side. When the distance between adjacent baffles was divided into 0.15, 0.25 and 0.35 mm, simulated data suggested that the baffle distance of 0.15 mm was best. Increasing the number of baffles from six to eight groups increased the mixing path of the fluid in the microchannel and improved mixing efficiency. A comparison of mixing efficiencies of the 0°, 15° and 30° baffle angles revealed that very significant improvement in mixing efficiency was obtained at 30°. © 2019 Society of Chemical Industry     

Optimal designs of staggered dean vortex micromixers

A novel parallel laminar micromixer with a two-dimensional staggered Dean Vortex micromixer is optimized and fabricated in our study. Dean vortices induced by centrifugal forces in curved rectangular channels cause fluids to produce secondary flows. The split-and-recombination (SAR) structures of the flow channels and the impinging effects result in the reduction of the diffusion distance of two fluids. Three different designs of a curved channel micromixer are introduced to evaluate the mixing performance of the designed micromixer. Mixing performances are demonstrated by means of a pH indicator using an optical microscope and fluorescent particles via a confocal microscope at different flow rates corresponding to Reynolds numbers (Re) ranging from 0.5 to 50. The comparison between the experimental data and numerical results shows a very reasonable agreement. At a Re of 50, the mixing length at the sixth segment, corresponding to the downstream distance of 21.0 mm, can be achieved in a distance 4 times shorter than when the Re equals 1. An optimization of this micromixer is performed with two geometric parameters. These are the angle between the lines from the center to two intersections of two consecutive curved channels, θ, and the angle between two lines of the centers of three consecutive curved channels, ϕ. It can be found that the maximal mixing index is related to the maximal value of the sum of θ and ϕ, which is equal to 139.82°.     

布置成涡结构微混合器内的流动与混合特性

基于成涡结构强化混合原理设计了一种改进型的平面被动式微混合器。采用商业软件CFD-ACE+对该结构微混合器的混合特性进行了三维数值模拟和结构优化,进一步揭示该微混合器结构对通道内流体流动与混合特性的影响。结合混合通道内流体的浓度和流型分布的数值和实验结果可知,该新型微混合器在布置成涡结构的弯曲通道内形成了扩展涡、分离涡和Dean涡,实现了涡系的叠加和强化,加大了流体间的扰动,从而增加了流体的接触面积强化混合;在综合考虑流体混合强度和压降分布等因素下,成涡结构正向布置且缝宽比W_d/W=1/4,厚宽比B/W=3/10,布置角度θ_a=120°的该微混合器在较广Re范围内的混合效果显著。     

Activated carbon‐polyethylenedioxythiophene composite electrodes for symmetrical supercapacitors

A symmetrical (p/p) supercapacitor has been fabricated by making use of activated carbon (AC)‐polyethylenedioxythiophene (PEDOT)‐composite electrodes for the first time. The composite electrodes have been prepared via electrochemical deposition of β‐napthalenesulphonate doped PEDOT onto AC electrodes. The characteristics of the electrodes and the fabricated supercapacitor have been investigated using cyclic voltammetry (CV) and AC impedance spectroscopy. The electrodes show a maximum specific capacitance of 158 Fg^?1 at a scan rate of 10 mV s^?1. This indicates that the in situ electro‐polymerization of ethylenedioxythiophene (EDOT) onto AC could improve the performance of carbon electrodes for use in supercapacitors. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Fabrication of carbon nanotube field effect transistors by AC dielectrophoresis method

Single wall carbon nanotubes (SWNTs) suspended in isopropyl alcohol have been placed between two electrodes by AC dielectrophoresis method. The number of SWNTs bridging the two electrodes is controlled by SWNT concentration of the suspension and deposition time. Through selectively burning off the metallic SWNTs by current induced oxidation, the back-gate carbon nanotube field effect transistors (CNTFETs) with a channel current on-off ratio of up to 7 × 10⁵ have been successfully fabricated. The success rate of the CNTFETs in 20 samples is 60%. These results suggest that AC dielectrophoresis placement method is an efficient technique to fabricate CNTFETs with some flexibilities of controlling CNT reconnection, length and orientation.     

The β‐δ‐Phase Transition and Thermal Expansion of Octahydro‐1,3,5,7‐Tetranitro‐1,3,5,7‐Tetrazocine

Phase behavior of octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) is investigated by X‐ray powder diffraction (XRD). The XRD patterns at elevated temperature show that there is a co‐existing temperature range of β‐ and δ‐phase during the phase transition process. Additionally, mechanical forces can catalyze the conversion from δ‐ back to β‐phase. Based on the diffraction patterns of β‐ and δ‐phase at different temperatures, we calculate the coefficients of thermal expansion by Rietveld refinement. For β‐HMX, the linear coefficients of thermal expansion of a‐axis and b‐axis are about 1.37×10⁻⁵ and 1.25×10⁻⁴ °C⁻¹. A slight decrease in c‐axis with temperature is also observed, and the value is about −0.63×10⁻⁵ °C⁻¹. The volume coefficient of thermal expansion is about 1.60×10⁻⁴ °C⁻¹, with a 2.2% change from 30 to 170 °C. For δ‐HMX, the linear coefficients of thermal expansion of a‐axis and c‐axis are found to be 5.39×10⁻⁵ and 2.38×10⁻⁵ °C⁻¹, respectively. The volume coefficient of thermal expansion is about 1.33×10⁻⁴ °C⁻¹, with a 2.6% change from 30 to 230 °C. The results indicate that β‐HMX has a similar volume coefficient of thermal expansion compared with δ‐HMX, and there is about 10.5% expansion from β‐HMX at 30 °C to δ‐HMX at 230 °C, of which about 7% may be attributed to the reconstructive transition.     

Electrical Conductivity of SiOCN Ceramics by the Powder‐Solution‐Composite Technique

SiOCN ceramics have been prepared by the polymer pyrolysis method. The preceramic polymers were synthesized from a polysiloxane cross‐linked with two different N‐containing compounds: a silazane or a ternary amine. The corresponding SiOCN ceramics were obtained by pyrolysis in nitrogen atmosphere at five different temperatures from 1000°C to 1400°C. The electrical conductivity of the powdered SiOCN ceramic samples was determined by the powder‐solution‐composite technique. The results show an increase in room temperature AC conductivity of three orders of magnitude, from ≈10^?5 (S/cm) to ≈10^?2 (S/cm), with increasing pyrolysis temperature from 1000°C to 1400°C. Furthermore, the electrical conductivity of the amine‐derived SiOCN is three to five times higher than that of the silazane‐derived ceramic at each pyrolysis temperature. The combined structural study by Raman spectroscopy and chemical analysis suggests that the increase of electrical conductivity with the pyrolysis temperature is due to the sp³‐to‐sp² transition of the amorphous carbon phase. The higher conductivity of the amine‐derived SiOCN is also discussed considering features like the volume% of the free‐carbon phase and its possible N‐doping.     

AC impedance studies on nickel oxide electrodes to determine self-discharge characteristics of Ni-H2 batteries

AC impedance measurements were made on nickel oxide electrodes in an alkaline medium. The electrodes were prepared by electrodeposition of nickel oxide on nickel foils from a nickel nitrate solution. The main purpose of the investigation was to determine the dependence of impedance parameters on the state of charge of the electrode. The results demonstrated that the current phase shift and impedance modulus vary linearly with state of charge at low frequencies. Hence, the state of charge can be estimated from the values of phase shift and modulus. The state of charge of nickel electrodes exposed to argon and hydrogen was determined at various time intervals. It was found that the electrode exposed to hydrogen discharged faster than that exposed to argon, indicating that hydrogen enhances the self-discharge rate of nickel oxide electrodes.     

Voltage relaxation measurements of the electron and hole mobilities in yttria-doped zirconia

The voltage relaxation of galvanic cells with zirconia electrolytes polarized between an inert silver electrode and either a Pt/air or a Fe/FeO electrode has been analysed to obtain the mobilities of both electronic minority charge carriers. At 900°C the mobility of the electrons is 2.4 × 10^?2 cm²/Vs, that of the holes is 2 orders of magnitude lower, 1.6 × 10^?4 cm²/Vs. The activation enthalpy is 0.55 eV for the electrons and 1.4 eV for the holes. From conductivity data, the concentrations of electrons and holes at 900°C and 1 atm oxygen partial pressure are calculated to be 3 × 10¹⁰ and 6 × 10¹⁷ cm^?3 respectively. With the use of platinum inert electrodes the formation of intermetallic Pt—Zr compounds appears at an oxygen partial pressure of 2.3 × 10^?22 atm at 900°C. From this the Gibbs formation energy of yttria-doped zirconia is calculated to be ?9.1 eV at 900°C.     

Memory effects of carbon nanotube-based field effect transistors

In this paper, we report the study on the non-volatile memory effects of carbon nanotube-based field effect transistors (CNTFETs), in which semiconducting single-wall carbon nanotubes (SWNTs) bridge the gold electrodes and the doped silicon substrate acts as the back gate. We find that our CNTFETs exhibit good performance with on/off ratio of more than 10⁴ and they also show strong memory effects. Hysteretic behaviors of the drain current as a function of the gate voltage are clearly observed at room temperature. The threshold voltage shift increases with increasing the sweeping range of the gate voltage. The CNTFET memory effects show good charge retention capability with the data storage time of around 7 days at ambient condition. Besides, the threshold voltage shift of the as-prepared CNTFETs is found to decrease with time and saturate after around 3 days. Water and alcohol molecules adsorbed on the carbon nanotube are suggested to be the origin of the phenomena. It is also observed that the threshold voltage shift in “top-contact” structures is larger than those in “bottom-contact” structures at the same gate voltage sweeping range.     

Electrochemical investigation of Li–Al anodes in oligo(ethylene glycol) dimethyl ether/LiPF<Subscript>6</Subscript>

1 M LiPF₆ dissolved in oligo(ethylene glycol) dimethyl ether with a molecular weight 500 g mol⁻¹ was investigated as a new electrolyte (OEGDME500, 1 M LiPF₆) for metal deposition and battery applications. At 25 °C a conductivity of 0.48 × 10⁻³ S cm⁻¹ was obtained and at 85 °C, 3.78 × 10⁻³ S cm⁻¹. The apparent activation barrier for ionic transport was evaluated to be 30.7 kJ mol⁻¹. OEGDME500, 1 M LiPF₆ allows operating temperature above 100 °C with very attractive conductivity. The electrolyte shows excellent performance at negative and positive potentials. With this investigation, we report experimental results obtained with aluminum electrodes using this electrolyte. At low current densities lithium ion reduction and re-oxidation can be achieved on aluminum electrodes at potentials about 280 mV more positive than on lithium electrodes. In situ X-ray diffraction measurements collected during electrochemical lithium deposition on aluminum electrodes show that the shift to positive potentials is due to the negative Gibbs free energy change of the Li–Al alloy formation reaction.     

Improvements in thermal conductivity and mechanical properties of HDPE/nano-SiC composites by the synergetic effect of extensional deformation and ISBS

A new melt blending method under synergy of extensional deformation and in-situ bubble stretching for high-density polyethylene (HDPE) thermally conductive composites filled by nano silicon carbide (nano-SiC) was reported. Effects of loadings and mixing time of azodicarbonamide (AC) foaming agent on the properties of the composites were experimentally studied. Scanning electron microscopy imaging showed that the nano-SiC particles dispersed uniformly in the HDPE matrix with the addition of AC. The complex viscosity and storage modulus increased with increasing AC content and decreased with increasing mixing time. The mechanical properties of the composites improved with the addition of AC and proper mixing times. The thermal conductivity of the composites increased from 0.2 to 0.7 W m⁻¹ K⁻¹ without any damage to the mechanical properties when the mixing time increased from 2 to 6 min. These results showed that the new mixing technique enables us to prepare particle-filled thermally conductive polymer composites. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47648.     

Densification of Y2O3 by flash sintering under an AC electric field

Frequency dependence of the densification behavior of undoped Y₂O₃ sintered by the AC-flash sintering was systematically investigated at 500 V·cm^?1 over a frequency range from 0.05 Hz to 1 kHz. The Y₂O₃ bodies sintered under an AC field showed a uniform microstructure, without an asymmetric grain size distribution between the electrodes. Almost fully-densified Y₂O₃ body was consolidated at 1 kHz exhibited a relative density greater than 99 % and an average grain size of 1.6 μm. The almost full densification probably resulted from the high input power at the relatively high onset temperature of 1300 °C at this frequency. The temperature dependence of the power dissipation during the AC-flash sintering experiments can be ascribed to the periodic fluctuations of the specimen temperature at low frequencies and to the phase shift between the applied field and the specimen current at high frequencies.     

Flexible supercapacitor-like actuator with carbide-derived carbon electrodes

Soft and flexible electric driven transducers based on carbide-derived carbon (CDC), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄), polyvinylidenefluoride-co-hexafluoropropylene (PVdF(HFP)) are proposed, and electroactive performances of these materials are reported. By its nature, the synthesized device has two features when voltage is applied. Firstly, it is a bending-type electrochemical actuator. Besides the external change of shape, this device is also an electrochemical capacitor, providing opportunity to store a considerable amount of charge. Laminated actuators can work in open air at low voltages (1–3 V). Their operating frequency is between 5 × 10⁻³ – 1 × 10¹ Hz (it is from 5 millihertz to 200 hertz) and 10 Hz and the maximum strain calculated from transducer displacement is 0.6%. The gravimetric capacitance of CDC in actuator electrodes was found to be 119 F g⁻¹ at 1 mV s⁻¹ sweep rate of the applied triangle voltage. The effects of synthesis temperature of CDC and associated changes in the porosity and surface area on the actuator displacement are discussed. The results of this study demonstrated a great potential of CDC as an active material for actuator electrodes, especially in these applications where the performance of the actuator has to be standardized and highly predictable.     

Tin oxide nanosensor fabrication using AC dielectrophoretic manipulation of nanobelts

Nanobelts are a new class of semiconducting metal oxide nanowires. The ribbon-like nanobelts are chemically pure and structurally uniform single crystals, with clean, sharp, smooth surfaces, and rectangular cross-sections. Positive and negative dielectrophoresis (DEP) was demonstrated for the first time on semiconducting oxide nanobelts. This effect was then used for the fabrication of a nanodevice, which consisted of SnO₂ nanobelts attached to castellated gold electrodes defined on a glass substrate, and covered by a microchannel. The SnO₂ nanobelts (width ∼ 100-300 nm, thickness ∼ 30-40 nm) were suspended in ethanol and introduced into the microchannel. An alternating (AC) voltage of ∼9.8 V peak to peak, with variable frequency, was applied between the electrodes (minimum electrode gap ∼ 20 μm), which corresponds to an average electric field strength of less than 2.5 × 10⁵ V/m. In the 10 Hz-1 kHz range, repulsion between the nanobelts and the electrodes occurred, while in the 1-10 MHz range, attraction was observed. Once the nanobelts touched the electrodes, those that were sufficiently long bridged the electrode gaps. The device was characterized and can potentially be used as a nanosensor.     

AN ELECTROSTATIC METHOD OF DRYING SALINE WATER

Times for drying and nucleation of saline solutions exposed to relatively high fluxes of unipolar air ions, produced by corona electrodes, were studied with a beta-ray gauge. Ion-treated solutions of NaCl of various concentrations dried about 3.2 times faster when exposed to either 2.54 x 10¹²;positive or 4.42 x 10¹² negative air ions cm_-2 s^-1 than the control samples. The nucleation site in a solution was observed to be directly below the emitting electrode; whereas nucleation appeared to be spatially random for untreated solutions. The solutions nucleated earlier when treated with the ion fluxes than the control. Electric wind caused by the ionic drag is proposed as the principal driving force for the enhancement of the drying process. The phenomenon of elctrostriction might have also aided in nucleating the solutions.