P. Varshney

Superhydrophobic coatings for aluminium surfaces synthesized by chemical etching process

ABSTRACT In this paper, the superhydrophobic coatings on aluminium surfaces were prepared by two-step (chemical etching followed by coating) and one-step (chemical etching and coating in a single step) processes using potassium hydroxide and lauric acid. Besides, surface immersion time in solutions was varied in both processes. Wettability and surface morphologies of treated aluminium surfaces were characterized using contact angle measurement technique and scanning electron microscopy, respectively. Microstructures are formed on the treated aluminium surfaces which lead to increase in contact angle of the surface (>150°). Also on increasing immersion time, contact angle further increases due to increase in size and depth of microstructures. Additionally, these superhydrophobic coatings show excellent self-cleaning and corrosion-resistant behavior. Water jet impact, floatation on water surface, and low temperature condensation tests assert the excellent water-repellent nature of coatings. Further, coatings are to be found mechanically, thermally, and ultraviolet stable. Along with, these coatings are found to be excellent regeneration ability as verified experimentally. Although aforesaid both processes generate durable and regenerable superhydrophobic aluminium surfaces with excellent self-cleaning, corrosion-resistant, and water-repellent characteristics, but one-step process is proved more efficient and less time consuming than two-step process and promises to produce superhydrophobic coatings for industrial applications.

Fabrication of Mechanically Stable Superhydrophobic Aluminium Surface with Excellent Self-Cleaning and Anti-Fogging Properties

The development of a self-cleaning and anti-fogging superhydrophobic coating for aluminium surfaces that is durable in aggressive conditions has raised tremendous interest in materials science. In this work, a superhydrophobic Al surface was synthesized by employing chemical etching technique with a mixture of hydrochloric and nitric acids, followed by passivation with lauric acid. The surface morphology analysis revealed the presence of rough microstructures on the coated Al surface. Superhydrophobicity with water contact angle of 170 ± 3.9° and sliding angle of 4 ± 0.5° was achieved. The surface bounced off the high-speed water jet, indicating the excellent water-repellent nature of the coating. It also continuously floated on a water surface for four weeks, showing its excellent buoyancy. Additionally, the coating maintained its superhydrophobicity after undergoing 100 cycles of adhesive tape peeling test. Its superhydrophobic nature withstood 90° and 180° bending and repeated folding and de-folding. The coating exhibits an excellent self-cleaning property. In a low temperature condensation test, almost no accumulation of water drops on the surface showed the excellent anti-fogging property of the coating. This approach can be applied to any size and shape of Al surface, and hence has great industrial applications.

Role of water temperature in case of high mass flux spray cooling of a hot AISI 304 steel plate at different initial surface temperatures

ABSTRACT In case of spray evaporative cooling, the heat transfer rate is controlled by various factors such as droplet renewal rate, Leidenfrost effect, and the rate of heat extraction by each droplet. In the current work, in case of high mass flux spray cooling (~55 kg/m2s), the heat extraction rate is tried to enhance by increasing the water temperature. Furthermore, from different initial surface temperatures (300°C–800°C), cooling experiments were conducted at various water temperatures (10°C–50°C). The surface temperatures and heat fluxes are calculated using an inverse heat conduction software (INTEMP). The result reveals that with the increasing water temperature, the heat removal rate rises in both transition and nucleate boiling regimes due to the increment of latent heat extraction time during the residence period of the water droplet on the hot plate. The maximum percentage in the enhancement of initial heat flux, average heat flux (AHF), and critical heat flux (CHF) are achieved in the nucleate boiling regime (<600°C); however, the increment in the transition boiling regime is also significant. Abbreviations: , hfg: Latent heat of vaporisation of water, J/kg; , Cp: Specific heat of water, J/kg oC; : Specific heat of water vapour, J/kg oC; : Average impingement density, kg/m2s; : Local impingement density at ith location, kg/m2s; : Surface temperature of plate, saturation temperature of water and injection temperature of water, °C; : Velocity of the droplet, m/s; : Density of water vapour, kg/m3; , : Density of water, kg/m3; ∆T: Temperature difference between the plate and the water droplet, °C; AHF, : Average surface heat flux, MW/m2; CHF,: Critical surface heat flux, MW/m2; DAQ: Data acquisition system; Dd: Diameter of droplet, µm; Fw: Flow rate of water, m3/s; g: Acceleration due to gravity, m/s2; IHF: Initial surface heat flux, MW/m2; k: Thermal conductivity of steel plate, W/m°C; m: Mass of water droplet, kg; OES: Optical Emission Spectrophotometer; P: Spray pressure, MPa; q: Heat flux, MW/m2; S1, S2 and S3: Surface heat flux zones; t: Time, s; T: Transient temperature of steel plate, °C; T1, T2, T3, and T4: Water temperatures, °C; TC1, TC2, and TC3: Thermocouple locations; We: Weber number; x, X, y, Y, Z: Distance along the length, breadth, and thickness of the plate, mm; z: Characteristic length, m; λ: Vapour film wavelength, m; ρ: Density of steel plate, kg/m3; σ: Surface tension of water, N/m; : Wetting front length, m; : Number of locations at which local impingement densities were measured; : Cooling efficiency

Single step method to fabricate durable superliquiphobic coating on aluminum surface with self-cleaning and anti-fogging properties

The development of self-cleaning and anti-fogging durable superliquiphobic coatings for aluminum surfaces has raised tremendous interest in materials science. In this study, a superliquiphobic coating is fabricated on an aluminum surface by a single-step dip-coating method using 1H,1H,2H,2H-Perfluorooctyltrichlorosilane-modified SiO2 nanoparticles. The successful implementation of the aforesaid coating in different applications requires extensive investigations of its characteristics and stability. To understand the properties of the coating, surface morphology, contact angle, self-cleaning, anti-fogging, and water repellency were investigated under perturbation conditions. Additionally, the dynamics of water and oil on the coated sample also were studied. Furthermore, the durability of the coating also was examined by performing thermal, chemical, and mechanical stability tests. It was found that the coating is superliquiphobic for water, ethylene glycol, glycerol and hexadecane, and shows thermal, chemical, and mechanical stability. Further, it exhibits self-cleaning and anti-fogging properties. This approach can be applied to any size and shape aluminum surface; thus, it has great industrial applications.

A facile modification of steel mesh for oil–water separation

The development of a superhydrophobic and superoleophilic steel mesh surface, which is durable and regenerable under aggressive conditions, has raised tremendous interest in oil–water separation applications. In this work, via a facile chemical etching method using a mixture of hydrochloric acid and nitric acid followed by treatment with lauric acid, a superhydrophobic and superoleophilic steel mesh surface was synthesized. The surface morphology analysis shows the presence of rough microstructures on the coated steel mesh surface. The coated mesh exhibited superhydrophobicity, with a water contact angle of 171 ± 4.5° and a sliding angle of 4 ± 0.5°, and superoleophilicity, with an oil static contact angle of about 0°, that caused water to run off the mesh while allowing oil to permeate through it. Petroleum ether–water and benzene–water mixtures were successfully separated via a simple filtering method using the coated mesh with a separation efficiency of more than 99%. Additionally, the coating was found to be mechanically, thermally and chemically stable and regenerable. Furthermore, the water-drop impact dynamics for the coated mesh surface were also studied. The aforementioned properties of the durable coated steel mesh show that it is a good candidate for facile, fast, and repeatable oil–water separation applications.

Development of liquid repellent coating on cotton fabric by simple binary silanization with excellent self-cleaning and oil-water separation properties

This paper aims to develop a facile and single step method for the fabrication of superhydrophobic coating on cotton fabric. The coating has been prepared by using two silane trichloro(octadecyl)silane and (pentaflurophenyl)triethoxy silane by solution immersion technique. The wettability, surface topography and chemical compostion of the cotton fabric before and after treatment were charecterized by contact angle measurement, scanning electron microscope, and energy dispersive X-ray spectrum, respectively. Additionally, the functional group present in coating was analysed by FT-IR spectra. The coated fabric shows a contact angle of 172.9±3°, 169±3° and 167±3° for water, ethylene glycol and glycerol, respectively. The chemical stability of the coated sample has been evaluated by immersion of the sample in different pH solutions and different solvents, showing the excellent chemical stability of coating. Ultrasonication with water, detergent and petroleum ether, and water jet impact test reveals the mechanical stability of coating. The thermal stability of the coated fabric has been examined by annealing the sample at different temperature. Additionally, it shows resistance to stain and UV irradiation. Furthermore, the coated cotton fabric exhibits excellent self-cleaning and oil-water separation properties, which makes it suitable for industrial applications.

The diminishment of specific heat and surface tension of coolant droplet in a dropwise evaporation process: A novel methodology to enhance the heat transfer rate

ABSTRACT The rate of dropwise evaporation is significantly altered by additives, such as benzene, n-hexane and acetone in water. These additives change some of the thermal and physical properties of the coolants, which have significant impact on various parameters that controls the droplet evaporative cooling, such as sensible, heat extraction period, droplet momentum and contact area. The open literature does not reveal the effects of the aforesaid additives on the dropwise evaporation. Therefore, in the current work, an attempt has been made to investigate the effects of above-mentioned additives on dropwise evaporation rate and reveal the mechanism involved. The droplet evaporative cooling experiments are conducted on a 2 mm thick AISI 304 steel plate (10 × 10 mm). The result shows that with increment in benzene and n-hexane concentration in water, the evaporation time significantly reduces. This is attributed to the decreasing surface tension, specific heat and contact angle. However, in case of acetone, the reduction in evaporation time is achieved only up to a concentration of 300 ppm, beyond which the evaporation time increases. This is because of the significant consumption of time in recoiling of the droplet. In addition to the above, the mechanism for the aforesaid enhancement process is tried to reveal by developing the models. For the validation of the developed equations, experimental results are compared with the numerically computed data. The comparison discloses that the developed model is quite accurate and shows insignificant variation from the experimental results. R2 and RMSE are also calculated for both the developed models and based on minimum recommended RMSE; the best model is also suggested.

Fabrication of durable and renegerable superhydrophobic coatings on metallic surfaces for potential industrial applications

Fabrication of anti-corrosion and self-cleaning superhydrophobic coatings for metallic surfaces which are regenerable and durable in the aggressive conditions has shown tremendous interest in materials science. In this work, the superhydrophobic coatings on metallic surfaces (aluminum, steel, copper) were prepared chemical etching process. Thermal, mechanical, ultra-violet stability of these coatings was also tested. Along with this, regeneration of coatings and self-cleaning, corrosion resistance and water repelling characteristics were also studied. The surface morphology shows the presence of rough microstructures on the treated surfaces and the contact angle measurements confirm the superhydrophobic nature. Superhydrophobic surfaces show the excellent self-cleaning and anti-corrosion behaviour. Water jet impact, floatation on water surface, and low temperature condensation tests proves the excellent water-repellent nature of the coatings. Additionally, these coatings are found to be thermal, mechanical and ultra-violet stable. Further, these surfaces are reproducible. These durable superhydrophobic metallic surfaces have potential industrial applications.