Experimental Studies of the Hydrodynamics of Liquid Droplet Generation and Transport in Microchannels

Droplet microfluidics is a promising field since it overcomes many of the limitations of single phase microfluidic systems. The improved mixing time scale, the increase of number of samples and the isolation of droplets are some of its virtues. The core of droplet microfluidics is a two-phase flow condition that is subjected to scaling of the confining geometry. With the scaling the complexities of the flow phenomena arise. For that reason both the processes of droplet generation and transport are not fully understood for various flow and fluid conditions. The work in this thesis aims to experimentally examine droplet generation and transport in microchannels for flow and fluid conditions that are experimentally challenging to perform. Examination of droplet generation in a T-junction microchannel design was performed with a quantitative velocity field approach known as micro particle image velocimetry (μPIV). The studies on droplet generation focused on very fast generation regimes, namely transition and dripping that have not been studied for a T-junction design. This achievement was accomplished because of the development of a fast optical detection and triggering system that allowed for acquiring images of different identical droplets at the same position. μPIV results indicate that the quantitative velocity field patterns of different regimes share some similarities. The filling stage in the transition and dripping regimes had some resemblance in their velocity patterns. The velocity patterns for the start of droplet pinch-off were alike for the squeezing and transition regimes. Furthermore, the presence of a surfactant in the droplet phase above the critical micelle concentration (CMC) did not have an effect on the general velocity patterns as long as the capillary number Ca was matched with the no-surfactant condition. The studies of hydrodynamic properties of droplet transport were performed in hard materials to avoid cumulative error sources, such as material pressure compliance and swelling effects. The project had several parts: designing a microchannel network that allowed studying the hydrodynamic properties of small droplets, surface treatments of the channel material for stable droplet generation and examining the hydrodynamics of small liquid droplets with sizes that have not been reported in the literature. The studies examined effects of changing the interfacial tension, viscosity, and flow conditions on the transport of droplets. The experimental results from the hydrodynamic transport studies indicated that for the droplet sizes that were examined the pressure drop of droplets was affected by the capillary number Ca and length of the droplet Ld. Also, the presence of surfactants altered the hydrodynamic properties of droplets. At a high concentration of surfactants the droplets pressure drop was reduced significantly. Moreover, the type of surfactant affected the magnitude of the pressure drop. Experimental results indicate that if the concentration of surfactants was very low (below CMC) it did not have an effect on the droplet excess pressure. These findings are important to consider in designing droplet microfluidic systems with complex channel networks that involve droplet sorting, splitting, and merging for droplets that contain surfactants.4 month

Electroosmotic Flow Characterization and Enhancement in PDMS Microchannels

Electroosmotic flow is widely used as a solution pumping method in numerous microfluidic applications. This type of flow has several advantages over other pumping techniques, such as the fast response time, the ease of control and integration in different microchannel designs. The flow utilizes the scaling of channel dimensions, which enhances the effects of the electrostatic forces to create flow in microchannels under an electrical body force. However, the electrostatic properties of the solution/wall material pairings are unique and must be experimentally measured. As a consequence, accurate knowledge about the electrostatic properties of the solution and wall material pairings is important for the optimal design of microfluidic devices using electroosmotic flow. Moreover, the introduction of new solutions and new channel materials for different applications is common in the microfluidics area. Therefore, any improvement on the experimental techniques used to examine the electrostatic properties of microchannels is beneficial to the research community. In this work, an improvement to the current-monitoring technique for studying the electrokinetic properties of microchannels is achieved by replacing the conventional straight channel design with a new Y-channel design. The errors from both the undesired pressure driven flow and solution electrolysis were addressed and significantly reduced. The new design offers high accuracy in finding the electrokinetic properties of microchannels. The experimental outcome from the new channel design is better compared to the outcomes of the straight channel, which helps in distinguishing the important electroosmotic pumping regions from the current-time plot. Moreover the time effectiveness in performing the experiments with the new channel design is better compared to that for the straight channel design. A modified analysis approach is also presented and validated for finding the electrokinetic properties from the outcomes of the current-monitoring technique, which is called the current-slope method. This approach is validated by comparing its findings with the results of the conventional length method. It was found for most situations that the discrepancy between the two methods, the current-slope and total length method, are within the uncertainty of the experimental measurements, thus validating the new analysis approach. In situations where it is hard to distinguish the start and end of solution replacement from the current-time plot of the current-monitoring technique, the current-slope method is advised. With the new design, different parametric studies of electroosmotic flow in PDMS based microchannels are estimated. At first the zeta potential of biological buffers are studied. Moreover the effect of continuous electroosmotic pumping, the chip substrate structure, and temperature on the average zeta potential of microchannels are examined. It was found that for air plasma treated PDMS microchannels the chip substrate material does not have an effect on the average zeta potential of the microchannels. The following chemical treatments are attempted with the aim of improving the surface and electrostatic properties of PDMS based microchannels: prepolymer additive with acrylic acid, extraction of PDMS, and both heat and plasma induced HEMA (Hydroxyethyl methacrylate) grafting on the surface of PDMS. Extensive characterization is performed with different experimental methods. The stability of the artificial hydrophilic properties of the PDMS microchannels with time was improved with both the extraction and HEMA grafting techniques. On the other hand, there was no evidence of any improvement in the zeta potential of microchannels with the surface treatments

Temperature dependence on the mass susceptibility and mass magnetization of superparamagnetic Mn–Zn–ferrite nanoparticles as contrast agents for magnetic imaging of oil and gas reservoirs

The mass susceptibility (χmass) and mass magnetization (Mmass) were determined for a series of ternary manganese and zinc ferrite nanoparticles (Mn–Zn ferrite NPs, MnxZn1−xFe2O4) with different Mn:Zn ratios (0.08 ≤ x ≤ 4.67), prepared by the thermal decomposition reaction of the appropriate metal acetylacetonate complexes, and for the binary homologs (MxFe3−xO4, where M = Mn or Zn). Alteration of the Mn:Zn ratio in Mn–Zn ferrite NPs does not significantly affect the particle size. At room temperature and low applied field strength the mass susceptibility increases sharply as the Mn:Zn ratio increases, but above a ratio of 0.4 further increase in the amount of manganese results in the mass susceptibility decreasing slightly, reaching a plateau above Mn:Zn ≈ 2. The compositional dependence of the mass magnetization shows less of a variation at room temperature and high applied fields. The temperature dependence of the mass magnetization of Mn–Zn ferrite NPs is significantly less for Mn-rich compositions making them more suitable for downhole imaging at higher temperatures (>100 °C). For non-shale reservoirs, replacement of nMag by Mn-rich Mn–Zn ferrites will allow for significant signal-to-noise enhancement of 6.5× over NP magnetite

Impulse Pressure-Assisted Diffusion Bonding (IPADB): Review and Outlook

Diffusion bonding is a solid-state welding technique used to join similar and dissimilar materials. Relatively long processing times, usually in the order of several hours as well as fine polished surfaces make it challenging to integrate diffusion bonding in other production processes and mitigate widespread use of the technology. Several studies indicate that varying pressure during diffusion bonding in contrast to the traditionally applied constant load may reduce overall processing- and bonding times. Such processes are referred to as impulse pressure-assisted diffusion bonding (IPADB) and they are, for the first time, reviewed in this work using the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) methodology. Results of the review indicate that varying pressure can indeed reduce bonding times in diffusion bonding and reduce the requirements for pre-bond surface preparation. Additional research is required and should go beyond small and simple sample geometries to concentrate on making IPADB accessible to industrial applications

CFD studies of the effect of holes and angles of upstream duct of horizontal axis wind turbines

This work presents an approach to the analysis of duct-augmented wind turbines using Computational Fluid Dynamics (CFD). The main objective is to find the optimum duct shape and design that gives maximum boost to the performance of wind turbines. A duct surrounding the rotor is able to increase the power coefficient above the Betz limit, so it has attracted great attention for many years. In this work, an extensive analysis of the performance of duct augmented wind turbines is presented, considering the influence of various duct angles and axial holes in the diffuser on the efficiency, in which a new formulation for the far-wake velocity is proposed. This study consists of two main parts. The first part compares the experimental performance of a conventional wind turbine with the identical turbine modeled and solved using CFD. Once the CFD results are validated, the second part presents an extensive parametric study by integrating a convergent duct with different parametric designs into the wind turbine model. The study reveals that the CFD results are in close agreement with the experimental results. It is found that the presence of holes in the duct has a detrimental effect on performance. However, the increase in the angle of the duct enhanced the performance, and there was an average increase in power output by 96% with a duct angle of 20°

Processing of Single-Walled Carbon Nanotubes with Femtosecond Laser Pulses

There are continued efforts to process and join single wall carbon nanotubes (SWCNTs) in order to exploit their exceptional functional properties for real-world applications. In this work, we report experimental observations of femtosecond laser irradiation on SWCNTs, in order to process and join them through an efficient and cost-effective technique. The nanotubes were deagglomerated in ethanol by an ultrasonicator and thin slurries of SWCNTs were spread evenly on glass substrates. A laser micromachining workstation for laboratory FemtoLAB (workshop of photonics) has been employed to irradiate the different SWCNTs film samples. The effect of laser parameters, such as pulse wavelength, laser power, etc., were systematically tuned to see the possibility of joining the SWCNTs ropes. Several experiments have been performed to optimize the parameters on different samples of SWCNTs. In general, the nanotubes were mostly damaged by the infrared (1st harmonics femtosecond laser) irradiation on the focal plane. However, the less damaging effect was observed for second harmonics (green wavelength) irradiation. The results suggest some joining of nanotubes along the sides of the focus plane, as well as on the center at the brink of nanotubes. The joining is considered to be established within the region of the high field intensity of the exposed femtosecond laser beam

Evaluation of dynamic properties of nano oil palm empty fruit bunch filler/epoxy composites

Cured epoxy resins pretense a constraint for variety of advanced applications due to its notably poor thermal and dynamic (viscoelastic) properties, hence required to minimize their properties prior to their usage. The aim of the present study is to evaluate the effect of nano oil palm empty fruit bunch (OPEFB) fibers at different loadings (1%, 3% and 5%) on the dynamic mechanical properties through dynamic mechanical analysis (DMA) in terms of storage modulus E′, loss modulus E″ and glass transition temperature (Tg) of epoxy composites. Results explored that dynamic properties of the epoxy composites get improved remarkably by the incorporation of nano OPEFB to epoxy composites, while 3% loading displays marked decrease in damping factor with relative to pure epoxy composites and the rest. Overall we perceived that the 3% loading of nano OPEFB filler is the best and optimal to enhance dynamic mechanical properties and to modify the damping factor of the epoxy composites resulting in most promising light weight and thermally stable composite structural materials. Keywords: Oil palm fibers, Epoxy matrix, Nanocomposites, Glass transition temperature, Dynamic mechanical properties, Damping facto

Synthesis of Co(OH)2/CNTs nanocomposite with superior rate capability and cyclic stability for energy storage applications

The good rate capability and longer cyclic performance are the two key features electrochemical capacitors that are highly dependent on the electrochemical stability, structure, electrical conductivity, composition, and nature of the charge storing-mechanism involved by its electrodes. Herein, we fabricated layered Co(OH) _2 and their nanocomposite with carbon nanotubes (CNTs, 5%) via a two-step approach for electrochemical applications. The as-prepared nanocomposite based electrode displays good specific capacitance (Cs), negligible capacity fade, and promising rate capability on electrochemical tests via a three-electrode configuration. More precisely, the nanocomposite based electrode showed Cs of 802 Fg ^−1 at 0.5 Ag ^−1 and loss just 3.8% of its initial capacitance (at 1st cycle) after 5000 cyclic tests. Furthermore, the nanocomposite electrode lost around 14% of its initial capacitance on increasing the current density from 0.5 to 5 Ag ^−1 that reveals its novel rate capability. The observed superior electrochemical aptitude of the fabricated nanocomposite is credited to the layered nanoarchitecture of the Co(OH) _2 and CNTs matrix. The CNTs-matrix, because of their lower properties, performs multiple roles to improve the supercapacitive performance of the whole composite. Firstly, they accelerate the charge transfer within the nanocomposite matrix due to its higher electrical conductivity. Secondly, they facilitate mass transport due to its hollow structure. Thirdly, they sandwich between the layers of Co(OH) _2 and suppress the stacking process. Fourthly, the added CNTs itself act as a capacitive supplement and further improve the specific capacitance of the nanocomposite. Finally, CNTs buffers the whole nanocomposite against the volume expansion during the continuous cyclic tests. The electrochemical and structural stability of Co(OH) _2 /CNTs sample was also evaluated by EIS and PXRD characterizations after electrochemical tests. The acquired result showed that fabricated nanocomposite has great potential for advanced energy storage applications

Concentrated solar power integration with refinery process heaters

Crude oil heating is considered an energy-intensive process in the oil industry that requires a huge amount of heat to process the crude oil. There is scarcity of a thorough research that deals with the techno-economic viability of introducing renewable energy solutions to the refinery industry including its environmental benefits. Therefore, a renewable energy solution i.e. a parabolic trough system is reviewed and examined to support minimizing the burning of natural gas in crude heaters and relying on thermal heat from sun radiation to increase crude oil temperature prior to going into the fractionation column. The system is designed to support refinery operation during day time whereas system design and analysis were done from thermal and financial points of view. Furthermore, benefits such as natural gas savings, reduction in CO2 emissions, and total payback period are presented in the paper to reflect the feasibility of constructing such a solution. Moreover, a MATLAB simulation was carried out to define the design points for the solar field and related heat exchanger components. This is to assure that the system can operate during winter and summer seasons given that the direct normal irradiance (DNI) is typically variant throughout the year. It has been concluded that integrating a parabolic trough collector into the operation of an oil refinery, i.e. crude oil heater, can potentially result in natural gas savings of 555,515 MMBtu and can prevent 30,020 tons of CO2 emissions annually. Moreover, the system is anticipated to result in cost savings of approximately 1.65 M $ per year