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In recent years, many studies have been done to fabricate superhydrophobic surfaces. These surfaces have slip condition which cause self-cleaning property and also drag reduction. The hierarchical micro/nanostructures which are coated with a low surface energy material are needed to fabricate high static contact angle superhydrophobic surfaces. In order to have thermal stability, chemical resistance and low surface energy Polytetrafluoroethylene (Teflon) is used in this research. To produce the superhydrophobic surface, an appropriate layer of Teflon is coated on the aluminum substrate and the micron sized aluminum particles are deposited on the Teflon layer by fluidizing method. Then to reduce surface energy, the second Teflon layer is sprayed on the top of the aluminum particles. At the end using sprayed method the hydrophobic nano-particles of silica are deposited on the surface as a final hydrophobic layer. The effect of Teflon thickness, size of micro-particles and adding hydrophobic nano-particles are investigated. The scanning electron microscopy (SEM) images of the cured surfaces show that application of micro-particles, prevent surface to be smooth after curing, create appropriate micro-scale structures and also cause micro-scale cracks compared to smooth Teflon surfaces. The creation of these micro-structures leads to increasing in static contact angle and decreasing in dynamic angle of surfaces. By modifying the surface structures with aluminum micro-particles, Teflon layer coat and subsequent deposition of hydrophobic silica nano-particles, static contact angle of 165±3° and dynamic angle of less than 7 degrees are achieved.

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Superhydrophobic surfaces are the surfaces with self-cleaning behavior due to surface slip condition. This property is applicable to produce drag reducing, anti-corrosive, and anti-fouling surfaces. Superhydrophobic coatings have been vigorously researched through numerous physical and chemical approaches, including lithography, self-assembly, electrospinning, chemical vapor deposition, plasma or chemical etching, and sol−gel techniques, and so forth. The large-scale fabrication of these surfaces is a challenging issue that restricts employment of these surfaces in industrial applications. Hydrophobic coating of micro/nano particles and deposition of the particles on the surface is a solution that facilitates large-scale fabrication of superhydrophobic surfaces. In this study, rotational vapor phase deposition and immersion method are used to fabricate hydrophobic aluminum flakes. Two reaction times are investigated and the results of two coating method and two particle sizes are presented. The results show that vapor phase deposition method is efficient as well as the immersion method while the latter is not cost effective. Stability test of the prepared samples showed that particle sizes are important in the vapor phase coating and the reaction time of 6 h is better than the 12 h.

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Superhydrophobic surfaces received many applications in various industries such as desalinization, heat exchanger, anti-fog and self-cleaning surface production. In this study a wet etching process were used to produce superhydrophobic copper surfaces. The specimens were etched by multiple ferric chloride and deionized water solutions to create micro-nano structures on their surfaces. The electronic scanning electron microscopy (SEM) images of the resulted surfaces show a formation of micro-nano structures with specific templates. Contact and sliding angle measurement of surfaces after etching process show that contact angles of specimens improve to nearly 140o while sliding angle of all samples were 180o , which is the same as a rose petal property. In the next step, to promote hydrophobicity of surfaces, increase contact angle and decrease sliding angle, specimens were immersed in an ethanol and stearic acid solution with a specific concentration. As well as, effects of etching time and etchant concentration on the sliding and contact angles with/without stearic acid modification were investigated. Results show that contact angles increased and sliding angles decreased remarkably so that it reduced to lower than 10o in some cases and lotus effect were achieved.

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Due to low surface energy and hierarchical roughness, fluids on superhydrophobic surfaces are mobile. The slip velocity on these surfaces is formulated using Navier’s slip length. On regular surfaces, slip length is only a few nano-meters. On superhydrophobic surfaces, slip length can be as large as 500 µm. Literature studies usually make the entire surface superhydrophobic which may not be the optimum situation. To find the desirable regions, the problem should be analyzed numerically. Most of the numerical studies are for flat plates. On curved surfaces (e.g. foils), due to the adverse pressure gradient and possibility of separation, analysis is more complicated. Here, the effect of using superhydrophobic surface for a SD7003 hydrofoil is studied numerically and at different Reynolds numbers and slip lengths. The flow pattern is considered laminar, incompressible and isothermal and a hydrofoil made of aluminum with a chord length of 10cm is selected. Results of the shear stress, pressure coefficient and the drag coefficient on the typical boundary condition were compared with the case of slip boundary condition. It was found that by increasing the slip length, the drag coefficient decreases. It was also found that the effectiveness of using superhydrophobic surfaces in decreasing the drag coefficient improves at higher Reynolds numbers. By increasing the Reynolds number from 4.5×〖10〗^4 to 7.5×〖10〗^4 and at the slip length of 50 µm, the drag coefficient reduction increases from 0.7% to 7%.

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In the present study, thermal performance of a microchannel heat sink with superhydrophobic walls is studied for different ratios of the wall convergence. To this end, three-dimensional Navier-Stokes equations and energy equation subject to the slip boundary conditions, viz. velocity slip and temperature jump, are numerically solved using the finite volume method. Then, the variations of thermal resistance of the heat sink with the number of channels, width- and height-tapered ratios, are studied for a fixed pumping power. The results show that by utilizing the superhydrophobic walls, the optimum width-tapered ratio of the channel is higher than that of the hydrophilic walls. The accentuated effect of the number of channels on thermal performance in the presence of liquid-solid interfacial slip weakens the effect of converging the width of the channel. It is also revealed that the optimum number of channels also increases to give prominence to the effect of interfacial slip by diminishing the smallest dimension of the channel. Finally, it is shown that for a pumping power of 0.05 W, using a heat sink with converging microchannels and superhydrophobic walls, reduces the overall thermal resistance by 28 percent, compared to that with conventional microchannels. In fact, the increase in fluid flow rate resulting from the use of converging microchannels with superhydrophobic walls outweighs the undesirable effect of temperature jump on heat transfer, in a sense that the heat sink performance augments considerably.

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Superhydrophobic surfaces have gained significant attention as a promising approach for drag reduction of submerged objects. Accurate evaluation and prediction of drag reduction induced by these surfaces require expensive experimental measurements, numerical simulations, or the development of reliable models and correlations. In this paper, a model is proposed for calculating the skin friction coefficient and drag reduction of superhydrophobic flat surfaces. Utilizing previous data on the skin friction coefficient of flat surfaces under no-slip boundary conditions, a model is developed to estimate the skin friction reduction and skin friction coefficient of these surfaces after applying superhydrophobic coatings. The validity of the model is verified by comparing its results with those of computational fluid dynamics (CFD) simulations of flow over a flat plate at different velocities. The results of the model and simulations indicate that for inlet velocities of 1, 5, and 25 m/s and a slip length of 50 μm, drag reductions of 15%, 41%, and 77%, respectively, are expected. Additionally, the skin friction reduction increases with increasing flow Reynolds number. The developed model is validated for flat surfaces and its ability to accurately estimate the skin friction coefficient and drag force of these surfaces is thoroughly examined. However, further investigations are required to assess the model's validity for curved surfaces and variable slip lengths.