Characteristics of Oil-in-Oil Emulsions under AC Electric Fields.

Emulsions have been applied in a number of industries such as pharmaceutics, cosmetics, and food, which are also of great scientific interest. Although aqueous emulsions are commonly used in our daily life, oil-in-oil (o/o) emulsions also play an irreplaceable role in view of their unique physics and complementary applications. In this paper, we investigate typical behaviors of organic droplets surrounded by organic medium (o/o emulsions) with different functional groups controlled by the AC electric field. Droplet behaviors can be catalogued into five types: namely, "no effect", "movement", "deformation", "interface rupture", and "disorder". We identify the key dimensionless number Wee·Ca, combined with the channel geometry, for characterizing the typical behaviors in silicon oil/1,6-hexanediol diacrylate and mineral oil/1,6-hexanediol diacrylate emulsions. Unlike aqueous emulsion, the Maxwell-Wagner relaxation inhibits the electric effect and leads to an effective frequency, ranging from 0.5 to 3 kHz. The increasing viscosity of the droplet facilitates the escalation by promoting the shearing effect under the same flow conditions. Ethylene glycol droplets primarily show the efficient coalescence even at a low Wee·Ca, which is attributed to the attraction of free charges induced by the increasing conductivity. In 1,6-hexanediol diacrylate/silicon oil emulsion, the droplet tends to form a liquid film that expands into the entire channel due to the affinity of the droplet to the channel wall. A variety of elongated columns are observed to oscillate between the electrodes at high voltages. These findings can contribute to understanding the electrohydrodynamic physics in o/o emulsion and controlling droplet behaviors in a fast response, programmable, and high-throughput way. We expect that this droplet manipulation technology can be widely adopted in a broad range of chemical synthesis and biological and material science.

Compound Droplet Impact on a Thin Hydrophobic Cylinder.

The impact of compound droplets on solid surfaces is a ubiquitous phenomenon that pervades both the natural and technological fields. A comprehensive understanding of the dynamics of the droplet impact on solid surfaces is therefore of paramount importance for a broad range of applications. In this study, we investigate the impact of a water-in-oil compound droplet on a thin hydrophobic cylindrical surface, with regard to the Weber number and cylinder dimensions. Owing to the prewetting effect of the oil, the droplet completely engulfs the cylinder during impact. The ensuing breakups of oil and water engender various unique impact outcomes, which are depicted via a phase map. The phase boundaries are described by analyzing the gravitational and drag forces exerted by the cylinder. A threshold value of the Weber number is found beyond which its effect on the azimuthal spreading process becomes less obvious. The distinctive axial spreading processes of oil and water are illustrated through high-speed imaging from both front and side perspectives, revealing that droplet oscillation is critically influenced by the Weber number. Our work elucidates the impact dynamics of compound droplets on curved surfaces, providing pivotal insights into related thermal management, droplet printing, and coating fabrication applications.

AC-electric-field-controlled multi-component droplet coalescence at microscale.

Droplet coalescence with fast response, high controllability and monodispersity has been widely investigated in industrial production and bioengineering. Especially for droplets with multiple components, programmable manipulation of such droplets is crucial for practical applications. However, precise control of the dynamics can be challenging, owing to the complex boundaries and the interfacial and fluidic properties. AC electric fields, with their fast response and high flexibility, have attracted our interest. We design and fabricate an improved flow-focusing microchannel configuration together with a non-contact type of electrode featuring asymmetric geometries, based on which we conduct systematic investigations of the AC-electric-field-controlled coalescence of multi-component droplets at the microscale. Parameters such as flow rates, component ratio, surface tension, electric permittivity and conductivity were given our attention. The results show that droplet coalescence in different flow parameters can be achieved in milliseconds by adjusting the electrical conditions, which shows high controllability. Specifically, both the coalescence region and reaction time can be adjusted by a combination of applied voltage and frequency, and unique merging phenomena have appeared. One is contact coalescence with the approach of paired droplets, while the other is squeezing coalescence, which occurs in the start position and promotes the merging process. The fluid properties, such as the electric permittivity, conductivity and surface tension, present a significant influence on merging behavior. The increasing relative dielectric constant leads to a dramatic reduction of the start merging voltage from the original 250 V to 30 V. The range of effective voltage for coalescence decreases with the addition of surfactant, offering a stricter and yet higher selectivity on electrical conditions, about 1500 V. The conductivity presents a negative correlation with the start merging voltage due to the reduction of the dielectric stress, from 400 V to 1500 V. Finally, we achieve the precise fabrication process of the Janus droplet via implementation of the proposed method, where the components of the droplets and the coalescence conditions are well controlled. Our results can serve as a potent methodology to decipher the physics of multi-component droplet electro-coalescence and contribute to applications in chemical synthesis, bioassay and material synthesis.

Design and Manufacture of Millimeter-Scale 3D Transformers for RF-IC.

The development of radio-frequency integrated circuits (RF-IC) necessitates higher requirements for the size of microtransformers. This paper describes millimeter-scale 3D transformers in millimeter-scale, solenoidal, and toroidal transformers manufactured using Micro-electromechanical Systems (MEMS). Two through-silicon via (TSV) copper coils with a high aspect ratio are precisely interleaved on a reserved air core (magnet core cavity) with a vertical height of over 1 mm because of the thickness of the substrate, which increases the performance while reducing the footprint. The effects of the wire width, coil turns, magnetic core, and substrate on the performance of the two transformers are discussed through numerical simulations. When an air core is present, solenoidal transformers are better than toroidal transformers in terms of performance and footprint; however, the gap decreases when the size is reduced. Additionally, the magnetic core significantly improves the performance of the toroidal transformer compared to that of the solenoid. Thus, the toroidal transformer has a higher potential for further size reduction. The two types of transformers were then manufactured completely using MEMS and electroplating. This paper discusses the influence of various parameters on millimeter-scale 3D transformers and realizes processing in silicon, which provides the foundation for integrating transformers in a chip.

Conjugate heat transfer study of various cooling structures and sensitivity analysis of overall cooling effectiveness.

The conjugate heat transfer of a turbine blade is influenced by several factors. To analyze the influence of each factor, the published one-dimensional conjugate heat transfer model was improved through theoretical analysis in this study. An overall cooling effectiveness equation containing three dimensionless parameters (adiabatic film cooling effectiveness η, Biot number on the mainstream side Big, and ratio between the heat transfer coefficients of the external and internal walls hg/hi) was obtained. The sensitivity of the overall cooling effectiveness ϕ to these three parameters was obtained through a multi-parameter sensitivity analysis. The results showed that increasing η could improve ϕ the most effectively. The interactions between the dimensionless parameters were analyzed by developing sensitivity charts. The results showed that increasing η from 0.4 to 0.5 could reduce the sensitivity of ϕ to the other two parameters by approximately 15%, whereas increasing Big had little effect on the sensitivity of ϕ to each dimensionless parameter. Increasing hg/hi could improve the sensitivity to η. The above conclusions could also be applied to the plate film hole and plate impingement effusion structures. The effects of different internal cooling structures and film hole structures on the three dimensionless parameters were studied by performing numerical simulations, which verified the accuracy of the one-dimensional conjugate heat transfer model in this study. The results showed that the internal cooling structures had little effect on the distribution of η and Big. The heat transfer coefficient on the coolant side could be effectively improved by installing film holes. The film hole structures mainly affected ϕ by influencing the distribution of η.

Dynamics of alternating current electric field-assisted non-Newtonian droplet formation with geometry confinement.

Recently, microfluidic techniques have been widely applied for biomaterial droplet manipulations due to their precision and efficiency. Many biosamples such as deoxyribonucleic acid and blood samples are non-Newtonian fluids with complex rheology, which brings challenges in control over them. The electric field is characterized by fast response and excellent adaptation to control microscale fluid flow. Here, we systematically investigate the alternating current electric field-assisted formation of non-Newtonian droplet in a flow-focusing microchannel with different sizes of channel orifice. The dependencies of flow conditions, microchannel geometries and electric parameters on the dynamics of non-Newtonian droplet formation are thus elucidated. An effective capacitance electric model is developed to reveal and predict the interaction between the fluid flow and the electric field. Furthermore, the flow field of non-Newtonian droplet formation is captured via the high-speed microparticle image velocimetry system. The characteristics of the regimes of droplet formation and the influences of the channel orifice are revealed quantitatively. Our work offers elaborate references to the control of non-Newtonian droplet formation, which benefits a wide range of applications in biology and chemistry.

Triple-layered encapsulation through direct droplet impact.

HYPOTHESIS: Multilayer capsules not only provide better protection for the core, but also enable multifunctionalities. However, their fabrication is challenging. Rapid encapsulation by the impact of the droplet is a simplified approach to form the compound droplet with a single shell layer. Therefore, it is worth exploring the potential for multilayer capsule formation with the approach. EXPERIMENTS: We investigate the impact of an aqueous core droplet through a layered liquid pool to form compound droplet and use ultraviolet polymerization to solidify its outer layer. The critical conditions to form the compound droplet are studied. We then explore the protection features of the capsule. FINDINGS: We succeed in fabricating defect-free capsules that featuring a triple-layered structure with a solid outer shell. The corresponding formation dynamics is revealed. We illustrate that the capsule provides reliable protection to the core through fluorescent intensity monitoring, pH level indication and bacteria revival test. Our method can also be adapted to tailor the functionality of the capsule, which is demonstrated by fabricating the magnetically steerable capsule. Our proposed approach offers great potentials for protecting sensitive ingredients and allows great flexibility in customizing capsule functionalities.

Broadband Lumped-Element Parameter Extraction Method of Two-Port 3D MEMS In-Chip Solenoid Inductors Based on a Physics-Based Equivalent Circuit Model.

Integrated 2D spiral inductors possess low inductance per unit area, which limits their application range. However, the state of investigation into the lumped-element parameter extraction method for integrated 3D in-chip multi-turn solenoid inductors, which possess higher inductance per unit area, is inadequate. This type of inductor can thus not be incorporated into fast computer-aided design (CAD)-assisted circuit design. In this study, we propose a broadband two-port physics-based equivalent circuit model for 3D microelectromechanical system (MEMS) in-chip solenoid inductors that are embedded in silicon substrates. The circuit model was composed of lumped elements with specific physical meanings and incorporated complicated parasitics resulting from eddy currents, skin effects, and proximity effects. Based on this model, we presented a lumped-element parameter extraction method using the electronic design automation software package, Agilent Advanced Design System (ADS). This method proved to be consistent with the results of two-port testing at low to self-resonant frequencies and could thus be used in CAD-assisted circuit design. The lumped element value variations were analyzed based on the physical meaning of the elements with respect to variations in structures and the substrate resistivity of inductors. This provided a novel perspective in terms of the design of integrated in-chip solenoid inductors.

Numerical Investigation on the Optimum Thermal Design of the Shape and Geometric Parameters of Microchannel Heat Exchangers with Cavities.

Due to the large surface-area-to-volume ratio, microchannel heat exchangers have a higher heat transfer rate compared with traditional scale heat exchangers. In this study, the optimum microchannel cavity with high heat transfer and low flow resistance is designed to further improve microchannel exchangers' thermal performance. A three-dimensional laminar flow model, consisting of Navier-Stokes equations and an energy conservation equation is solved and the conjugate heat transfer between the silicon basement and deionized water is taken into consideration. The impact of the shape, aspect ratio, size and spacing of the cavity on the thermal performance of microchannel exchangers are numerically investigated, respectively. The results indicated that the cavity on the sidewall can enhance heat transfer and reduce flow resistance simultaneously, and cavities with a relatively small expansion angle and streamlined edge could enhance thermal performance the most. Based on the conclusions, a new cavity shape is proposed, and the simulation results verify its excellent thermal performance as expected. Furthermore, investigation is performed to figure out the optimum design of the new cavity and the optimal geometric parameters of the cavity under different flow conditions have been obtained in principle for microchannel exchangers' design.

Experimental Investigation of the Flow and Heat Transfer Characteristics in Microchannel Heat Exchangers with Reentrant Cavities.

The application of microchannel heat exchangers is of great significance in industrial fields due to their advantages of miniaturized scale, large surface-area-to-volume ratio, and high heat transfer rate. In this study, microchannel heat exchangers with and without fan-shaped reentrant cavities were designed and manufactured, and experiments were conducted to investigate the flow and heat-transfer characteristics. The impact rising from the radius of reentrant cavities, as well as the Reynolds number on the heat transfer and the pressure drop, is also analyzed. The results indicate that, compared with straight microchannels, microchannels with reentrant cavities could enhance the heat transfer and, more importantly, reduce the pressure drop at the same time. For the ranges of parameters studied, increasing the radius of reentrant cavities could augment the effect of pressure-drop reduction, while the corresponding variation of heat transfer is complicated. It is considered that adding reentrant cavities in microchannel heat exchangers is an ideal approach to improve performance.

Powder Filling and Sintering of 3D In-chip Solenoid Coils with High Aspect Ratio Structure.

In this study, a 3D coil embedded in a silicon substrate including densely distributed through-silicon vias (TSVs) was fabricated via a rapid metal powder sintering process. The filling and sintering methods for microdevices were evaluated, and the effects of powder types were compared. The parameters influencing the properties and processing speed were analyzed. The results showed that the pre-alloyed powder exhibited the best uniformity and stability when the experiment used two or more types of powders to avoid the segregation effect. The smaller the particle diameter, the better the inductive performance will be. The entire structure can be sintered near the melting point of the alloy, and increasing the temperature increases strength, while resulting in low resistivity. Finally, an 800-µm-high coil was fabricated. This process does not need surface metallization and seed layer formation. The forming process involves only sintering instead of slowly growing copper with a tiny current. Therefore, this process has advantages, such as a process time of 7 h, corresponding to an 84% reduction compared to current electroplating processes (45 h), and a 543% efficiency improvement. Thus, this process is more efficient, controllable, stable, and suitable for mass production of devices with flexible dimensions.

Flow Characteristics of the Entrance Region with Roughness Effect within Rectangular Microchannels.

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-Stokes equations were solved using finite volume method (FVM). In the simulation, hydraulic diameters (Dh) ranging from 50 to 200 µm 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.

Thermal Drift Investigation of an SOI-Based MEMS Capacitive Sensor with an Asymmetric Structure.

High-precision, low-temperature-sensitive microelectromechanical system (MEMS) capacitive accelerometers are widely used in aerospace, automotive, and navigation systems. An analytical study of the temperature drift of bias (TDB) and temperature drift of scale factor (TDSF) for an asymmetric comb capacitive accelerometer is presented in this paper. A five-layer model is established for the equivalent expansion ratio in the TDB and TDSF formulas, and the results calculated by the weighted average of thickness and elasticity modulus method are closest to the results of the numerical simulation. The analytical formulas of TDB and TDSF for an asymmetric structure are obtained. For an asymmetric structure, TDB is only related to thermal deformation and fabrication error. Additionally, half of the fixed electrode distance is not included in the expressions of Δ d and Δ D for asymmetric structures, thus resulting in the TDSF of the asymmetric structure being smaller compared to a symmetric structure with the same structural parameters. The TDSF of the symmetric structure is [-200.2 ppm/°C, -261.6 ppm/°C], while the results of the asymmetric structure are [-11.004 ppm/°C, -72.404 ppm/°C] under the same set of parameters. The parameters of the optimal asymmetric structure are obtained for fabrication guidance using numerical methods. In the experiment, the TDSF and TDB of the packaged structure and the non-packaged structure are compared, and a significant effect of the package on the signal output is found. The TDB is reduced from 3000 to 60 µg/°C, while the TDSF is reduced from 3000 to 140 ppm/°C.

Experimental and Numerical Investigations on the Flow Characteristics within Hydrodynamic Entrance Regions in Microchannels.

Flow characteristics within entrance regions in microchannels are important due to their effect on heat and mass transfer. However, relevant research is limited and some conclusions are controversial. In order to reveal flow characteristics within entrance regions and to provide empiric correlation estimating hydrodynamic entrance length, experimental and numerical investigations were conducted in microchannels with square cross-sections. The inlet configuration was elaborately designed in a more common pattern for microdevices to diminish errors caused by separation flow near the inlet and fabrication faults so that conclusions which were more applicable to microchannels could be drawn. Three different microchannels with hydraulic diameters of 100 µm, 150 µm, and 200 µm were investigated with Reynolds (Re) number ranging from 0.5 to 50. For the experiment, deionized water was chosen as the working fluid and microscopic particle image velocimetry (micro-PIV) was adopted to record and analyze velocity profiles. For numerical simulation, the test-sections were modeled and incompressible laminar Navier-Stokes equations were solved with commercial software. Strong agreement was achieved between the experimental data and the simulated data. According to the results of both the experiments and the simulations, new correlations were proposed to estimate entrance length. Re numbers ranging from 12.5 to 15 was considered as the transition region where the relationship between entrance length and Re number converted. For the microchannels and the Reynolds number range investigated compared with correlations for conventional channels, noticeable deviation was observed for lower Re numbers (Re < 12.5) and strong agreement was found for higher Re numbers (Re > 15).

Manufacturing of a Compact Micro Air Bearing Device for Power Micro Electro Mechanical System (MEMS) Applications Using Silica Film Assisted Processing.

The focus of this study is on the manufacturing of micro air bearings (MABs) using silica film assisted processing. Structure of the three-layer micro air bearing is described in detail and the salient process flow of etching and bonding is illustrated. The main manufacturing challenges and the methods adopted to overcome them are also presented. The uniformity of wet etching for nozzles with 20 µm in diameter to silica film is improved by adopting an ultrasound assisted method. Particular attention is given to the novel fabrication procedures for the second layer of MAB (with three depths on aft side). This paper develops new applications of silica film in Micro Electro Mechanical System (MEMS) processing for MABs to realize the etching of multi-depth on the same side and efficient three-layer bonding with increased bonding areas. A silica etch mask is proven to achieve a higher accuracy in surface topography when compared to a photoresist mask for multi-depth etching, resulting in precise depth and vertical control. The bonding rate of three-layer direct bonding for MAB is increased by 50% from 0.05 to 0.1 with the novel silica film protection method.

Anti-Reflectance Optimization of Secondary Nanostructured Black Silicon Grown on Micro-Structured Arrays.

Owing to its extremely low light absorption, black silicon has been widely investigated and reported in recent years, and simultaneously applied to various disciplines. Black silicon is, in general, fabricated on flat surfaces based on the silicon substrate. However, with three normal fabrication methods-plasma dry etching, metal-assisted wet etching, and femtosecond laser pulse etching-black silicon cannot perform easily due to its lowest absorption and thus some studies remained in the laboratory stage. This paper puts forward a novel secondary nanostructured black silicon, which uses the dry-wet hybrid fabrication method to achieve secondary nanostructures. In consideration of the influence of the structure's size, this paper fabricated different sizes of secondary nanostructured black silicon and compared their absorptions with each other. A total of 0.5% reflectance and 98% absorption efficiency of the pit sample were achieved with a diameter of 117.1 µm and a depth of 72.6 µm. In addition, the variation tendency of the absorption efficiency is not solely monotone increasing or monotone decreasing, but firstly increasing and then decreasing. By using a statistical image processing method, nanostructures with diameters between 20 and 30 nm are the majority and nanostructures with a diameter between 10 and 40 nm account for 81% of the diameters.

Fabrication and Optimization of High Aspect Ratio Through-Silicon-Vias Electroplating for 3D Inductor.

In this study, the filling process of high aspect ratio through-silicon-vias (TSVs) under dense conditions using the electroplating method was efficiently achieved and optimized. Pulsed power was used as the experimental power source and the electroplating solution was prepared with various additive concentrations. Designed control variable experiments were conducted to determine the optimized method. In the control variable experiments, the relationship of multiple experimental variables, including current density (0.25⁻2 A/dm²), additive concentration (0.5⁻2 mL/L), and different shapes of TSVs (circle, oral, and square), were systematically analyzed. Considering the electroplating speed and quality, the influence of different factors on experimental results and the optimized parameters were determined. The results showed that increasing current density improved the electroplating speed but decreased the quality. Additives worked well, whereas their concentrations were controlled within a suitable range. The TSV shape also influenced the electroplating result. When the current density was 1.5 A/dm² and the additive concentration was 1 mL/L, the TSV filling was relatively better. With the optimized parameters, 500-µm-deep TSVs with a high aspect ratio of 10:1 were fully filled in 20 h, and the via density reached 70/mm². Finally, optimized parameters were adopted, and the electroplating of 1000-µm-deep TSVs with a diameter of 100 µm was completed in 45 h, which is the deepest and smallest through which a three-dimensional inductor has ever been successfully fabricated.

Genome-Wide Association Studies of Photosynthetic Traits Related to Phosphorus Efficiency in Soybean.

Photosynthesis is the basis of plant growth and development, and is seriously affected by low phosphorus (P) stress. However, few studies have reported for the genetic foundation of photosynthetic response to low P stress in soybean. To address this issue, 219 soybean accessions were genotyped by 292,035 high-quality single nucleotide polymorphisms (SNPs) and phenotyped under normal and low P conditions in 2015 and 2016. These datasets were used to identify quantitative trait nucleotides (QTNs) for photosynthesis-related traits using mrMLM, ISIS EM-BLASSO, pLARmEB, FASTmrMLM, FASTmrEMMA, and pKWmEB methods. As a result, 159 QTNs within 31 genomic regions were found to be associated with four photosynthesis-related traits under different P stress conditions. Among the 31 associated regions, five (q7-2, q8-1, q9, q13-1, and q20-2) were detected commonly under both normal and low P conditions, indicating the insensitivity of these candidate genes to low P stress; five were detected only under normal P condition, indicating the sensitivity of these candidate genes to low P stress; six were detected only under low P condition, indicating the tolerantness of these candidate genes to low P stress; 20 were reported in previous studies. Around the 159 QTNs, 52 candidate genes were mined. These results provide the important information for marker-assisted breeding in soybean and further reveal the basis for the application of P tolerance to photosynthetic capacity.

Anti-reflectance investigation of a micro-nano hybrid structure fabricated by dry/wet etching methods.

Black silicon fabrication and manipulation have been well reported by institutes around the world and are quite useful for solar absorption and photovoltaic conversion. In this study, silicon micro-nano hybrid structures were fabricated, and the morphologies of the hybrid structures were analyzed. This paper studied nanostructures formed on tips, pits and a flat surface using a dry etching method and a wet etching method. In terms of nanostructure morphology, nanostructures etched by the wet etching method (13 µm) were taller than those etched by the dry etching method (1 µm), but the wet etched morphology was less organized. After the nanostructures were grown, six samples with nano sturctures and three samples with micro sturctures were measured by a photometer for reflectivity testing. The nine samples were compared and analyzed using the integral of reflectivity and solar emissivity at the earth's surface. The results show that the nanostructures grown on a tip surface using the wet etching method had the minimum reflectivity in the wavelength range of 300 nm-1100 nm, in consideration of the forbidden energy gap of silicon.