Liquid-Gas Surface Tension Voltage Dependence During Electrowetting on Dielectric of 5-90 nm Gold Nanofluids

This article investigates the effective liquid-gas surface tension changes of water and 5-90nm gold nanofluids measured during electrowetting on dielectric experiments. The Young-Laplace equation for sessile droplets in air was solved to fit the experimental droplet shape and determine the effective liquid-gas surface tension at each applied voltage. A good agreement between experimental droplet shapes and the predictions was observed for all the liquids investigated in applied range of 0-30V. The measured liquid-gas effective surface tensions of water and gold nanofluid decreased with voltage. At a given voltage, the effective liquid-gas surface tension of gold nanofluids initially decreased as the size of gold nanoparticles increased from 5 nm to 50 nm. Then, for 70nm and 90nm particle gold nanofluids, the effective liquid-gas surface tension started increasing too. The size of nanoparticles, and the applied voltage have a significant effect on variation of the effective liquid-gas surface tension with variations as much as 93% induced by voltage at a given particle size and 80% induced by particle size at a given voltage

Surface microstructuring to modify wettability for 3D printing of nano-filled inks

This paper investigates the effect of surface wettability on the cross-sectional profiles of printed nanofluid inks which can have a significant role on conductivity of printed lines that are used in the production of printed electronics. Glass substrates were coated with heptadecafluorodecyltrichlorosilane, nonafluorohexyltrimethoxysilane and methyltrimethoxysilane using a dipping method to enhance the wettability of the nanofluid silver ink. Inkjet printing techniques were also applied to develop micro-structural textures on the surface of the glass substrate and thereby modify the wettability of the substrate. The glass substrate, coated with heptadecafluorodecyltrichlorosilane was micro-structured using a UV curable ink to enhance the wettability for the silver nanoparticle ink. Using inkjet printing techniques to micro-structure the substrate allows modification of the wettability of the substrate whilst simultaneously printing on to the substrate. This enables the potential of increasing the performance of such printed lines, essentially permitting additional particulate material to be deposited thus increasing conductivity. The cross-sectional profile of the printed line was predicted numerically and analytically and compared to experimental data where agreement was observed. In addition, three analytical expressions for printed lines on the substrate were developed by writing the force balance equations in the x, y and z directions on a slice of printed line between z and z+dz

Modification of the Young-Laplace equation and prediction of bubble interface in the presence of nanoparticles

Bubbles are fundamental to our daily life and have wide applications such as in the chemical and petrochemical industry, pharmaceutical engineering, mineral processing and colloids engineering. This paper reviews the existing theoretical and experimental bubble studies, with a special focus on the dynamics of triple line and the influence of nanoparticles on the bubble growth and departure process. Nanoparticles are found to influence significantly the effective interfacial properties and the dynamics of triple line, whose effects are dependent on the particle morphology and their interaction with the substrate. While the Young–Laplace equation is widely applied to predict the bubble shape, its application is limited under highly non-equilibrium conditions. Using gold nanoparticle as an example, new experimental study is conducted to reveal the particle concentration influence on the behaviour of triple line and bubble dynamics. A new method is developed to predict the bubble shape when the interfacial equilibrium conditions cannot be met, such as during the oscillation period. The method is used to calculate the pressure difference between the gas and liquid phases, which is shown to oscillate across the liquid–gas interface and is responsible for the interface fluctuation. The comparison of the theoretical study with the experimental data shows a very good agreement, which suggests its potential application to predict bubble shape during non-equilibrium conditions

Manufacturing a TiO2-Based Semiconductor Film with Nanofluid Pool Boiling and Sintering Processes toward Solar-Cell Applications

For the first time, nanofluid boiling was applied as a process for the creation of a semiconductor TiO2 nanoparticle film that can be deposited onto a conductive substrate (F-doped SnO2 glass: FTO). A steel-base device designed for pool boiling was used to deposit a TiO2-based nanofluid consisting of nanoparticles with an average size of about 20 nm. The boiling of the nanofluid directly on the FTO glass substrate allowed for the deposition of the nanoparticles onto the FTO surface. In principle, the surface responsible for transferring heat to the fluid can be covered with these nanoparticles when the nanofluid boils. Using the as-deposited films, crystal growth of the TiO2 nanoparticle was controlled by varying the strategies of the post-sintering profile. The maximum temperatures, periods, and ramping rates for the obtained samples were systematically changed. Scanning electron microscopy (SEM) revealed that a densely packed TiO2-nanoparticle layer was obtained for the as-deposited substrate via pool boiling. For the maximum temperature at 550 °C, the TiO2 grain sizes became larger (~50 nm) and more round-shaped TiO2 nanostructures were identified. Notably, we have demonstrated for the first time how the sintering of TiO2 nanoparticles proceeds for the nanoporous TiO2 films using high-resolution transmission electron microscopy (TEM) measurements. We found that the TiO2 nanoparticles fused with each other and crystal growth occurred through neighboring 2–4 nanoparticles for the 550 °C sample, which was proved by the TEM analysis that continuous lattice fringes corresponding to the (101) anatase phase were clearly observed through the entire area of some nanoparticles aligned horizontally. In addition, the loss of the TiO2 nanofluid (precursor solution) was completely avoided in our TiO2 deposition. Unlike the commonly used spin-coating method, nanofluid pool boiling would provide an alternative cost-effective approach to manufacture semiconductor layers for various applications, such as solar cells

Spreading of triple line and dynamics of bubble growth inside nanoparticle dispersions on top of a substrate plate

his work investigates the feasibility of engineering surface wettability by using different nanoparticles. As an illustration, detailed formation of gas bubbles on top of a stainless steel substrate plate in a quiescent pool of aqueous gold and alumina nanofluids is studied. The presence of nanoparticles is shown to be able to modify the dynamics of triple line and bubble growth significantly. An early pinning of the bubble triple line is observed and a larger bubble contact angle is found for bubbles growing in a gold nanofluid, whereas an opposite phenomenon is observed for bubbles growing in an alumina nanofluid compared to those of pure water. Other bubble parameters such as departure volume, bubble frequency, and waiting time of bubble formation are also affected by the presence of nanoparticles. The variation of solid surface tensions due to the existence of nanoparticles and the resultant force at the triple line should be responsible for such differences. Such results illustrate the big potential of nanoparticle in engineering surface wettability of a solid-liquid-gas system

Convective heat transfer of aqueous alumina nanosuspensions in a horizontal mini-channel

This work reports an experimental study of convective heat transfer of aqueous alumina nanofluids in a horizontal mini-channel under laminar flow condition 40 < Re < 1,000. The variation of local heat transfer coefficients, in both entrance and developed flow regimes, was obtained as a function of axial distance. The heat transfer coefficient of nanofluids was found to be dependent on not only nanoparticle concentration but also mass flow rate. Different to the behavior in conventional-sized channels, the major heat transfer coefficient enhancement is shown in the fully developed regime in the minichannel where up to 40% increase is observed. Discussions of the results suggest that apart from the need of a careful assessment of different thermo-physical properties of nanofluids, i.e., viscosity, specific heat and thermal conductivity, the heterogeneous nature of nanoparticle flow should be considered especially under high flow rate conditions

Controlled Assembly of Nanorod TiO2 Crystals via a Sintering Process: Photoanode Properties in Dye-Sensitized Solar Cells

We present for the first time a synthetic method of obtaining 1D TiO2 nanorods with sintering methods using bundle-shaped 3D rutile TiO2 particles (3D BR-TiO2) with the dimensions of around 100 nm. The purpose of this research is (i) to control crystallization of the mixture of two kinds of TiO2 semiconductor nanocrystals, that is, 3D BR-TiO2 and spherical anatase TiO2 (SA-TiO2) on FTO substrate via sintering process and (ii) to establish a new method to create photoanodes in dye-sensitized solar cells (DSSCs). In addition, we focus on the preparation of low-cost and environmentally friendly titania electrode by adopting the “water-based” nanofluids. Our results provide useful guidance on how to improve the photovoltaic performance by reshaping the numerous 3D TiO2 particles to 1D TiO2-based electrodes with sintering technique

Bubble growth rate from stainless steel substrate and needle nozzles

The formation of bubbles from a substrate nozzle immersed in water is studied experimentally and compared with bubble formation from needle nozzles under the same conditions. Nozzles with different internal diameter sizes in the range of 0.11-0.84 mm and low gas flow rates from 0.015 to 0.85 ml/min are used. The bubble formation is recorded by a high speed video camera and detailed characteristics of bubble formation are obtained. Interestingly, it is realized that the bubble volume expansion rate follows a cyclic behavior for the substrate nozzles while it shows a smooth decrease after an initial increase for the needle nozzles. Force balance analysis suggests that fluctuations in the substrate are related to the rapid increase of the attaching inertial force at the initial stages of bubble formation. In contrast, the inertial force in the case of the needle is smaller and takes longer to acquire its maximum value. The results also show that bubbles emerge earlier on top of the substrate than on the needle but the initial bubble volume is smaller in the former case. However, the bubble expansion rate is larger for the substrate than for the needle and as a result the final bubble volume is similar for both cases. By using experimentally captured values of bubble height and radius of contact line, the Young-Laplace equation predicts well the bubble evolution until just before the bubble detachment. Other bubble characteristics follow similar trends for both inlets with time and volume

Nanofluid Thermal Conductivity and Effective Parameters

Due to the more powerful and miniaturized nature of modern devices, conventional heat-transfer working fluids are not capable of meeting the cooling needs of these systems. Therefore, it is necessary to improve the heat-transfer abilities of commonly used cooling fluids. Recently, nanoparticles with different characteristics have been introduced to base liquids to enhance the overall thermal conductivity. This paper studies the influence of various parameters, including base liquid, temperature, nanoparticle concentration, nanoparticle size, nanoparticle shape, nanoparticle material, and the addition of surfactant, on nanofluid thermal conductivity. The mechanisms of thermal conductivity enhancement by different parameters are discussed. The impact of nanoparticles on the enhanced thermal conductivity of nanofluids is clearly shown through plotting the thermal conductivities of nanofluids as a function of temperature and/or nanoparticle concentration on the same graphs as their respective base liquids. Additionally, the thermal conductivity of hybrid nanofluids, and the effects of the addition of carbon nanotubes on nanofluid thermal conductivity, are studied. Finally, modeling of nanofluid thermal conductivity is briefly reviewed