A Novel Biomimetic Nanocomposite Exhibiting Petal Wetting Phenomenon: Fabrication and Experimental Investigations

A functional composite material that simultaneously exhibits hydrophobicity and water droplet adhesion has monumental potential in controlling fluid flow, studying phase separation, and biological research. This article reports the fabrication of a petal wetting biomimetic Boron Nitride Nanotubes (BNNTs) -Polydimethylsiloxane (PDMS) nanocomposite achieved by drop casting. The petal effect was investigated by non-destructive techniques. The nanotubes were synthesized by chemical vapor deposition at 1150 °C and were characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The mean diameter of the nanotubes was found to be 70 nm. The nanocomposites had BNNT fillers ranging from 0.5 wt. % to 2 wt. %. Water contact angles for pure PDMS polymer was 94.7° and for the 2 wt. % BNNT-PDMS nanocomposite was 132.4°. The petal wetting nanocomposite displayed a characteristic trait of high contact angle hysteresis. The surface roughness parameters of the nanocomposites were determined by atomic force microscopy. Laser scanning confocal microscopy aided in analyzing the droplet penetration and in observing the trapped air between the water droplet and the nanocomposite surface. Based on surface observations, roughness parameters, and the extent of droplet penetration by the surface, we shed light on the Cassie impregnating wetting regime followed by the biomimetic nanocomposite. Such a surface would be beneficial in the study of the embryogenesis of cells and aid in moisture collection.

The wetting properties of frosted glass

Frosted glass is a common, low cost material. Its roughness can be used to control how it is wet by water. In this paper, the wetting properties of silicone oil and water are investigated. For the oil, wetting is total since the oleophilic character of the glass is enhanced by its roughness. Due to the remarkable optical properties of frosted glass, the spreading of oil droplets on its surface was recorded over three months. Frosted glass is a parahydrophilic surface because of its large contact angle hysteresis (up to 80° ). The behaviour of oil and water droplets was compared on a long piece of inclined frosted glass. The trajectories (and the spreading) of the droplets were studied and phenomenological laws were deduced to describe the dependence of the droplet speed on the initial volume of the droplet and the angle of inclination. Such dependences of speed at long travel distances (100 times the capillary length) were deduced and rationalised with a simple model that takes into account the thickness of the wake. Moreover, we analysed the flow inside the wake of water droplets sliding on inclined frosted glass. Suggestions are given on how to exploit drainage of the water droplet wake and the high hysteresis of water within the framework of open microfluidics.

Icephobic and Anticorrosion Coatings Deposited by Electrospinning on Aluminum Alloys for Aerospace Applications

Anti-icing or passive strategies have undergone a remarkable growth in importance as a complement for the de-icing approaches or active methods. As a result, many efforts for developing icephobic surfaces have been mostly dedicated to apply superhydrophobic coatings. Recently, a different type of ice-repellent structure based on slippery liquid-infused porous surfaces (SLIPS) has attracted increasing attention for being a simple and effective passive ice protection in a wide range of application areas, especially for the prevention of ice formation on aircrafts. In this work, the electrospinning technique has been used for the deposition of PVDF-HFP coatings on samples of the aeronautical alloy AA7075 by using a thickness control system based on the identification of the proper combination of process parameters such as the flow rate and applied voltage. In addition, the influence of the experimental conditions on the nanofiber properties is evaluated in terms of surface morphology, wettability, corrosion resistance, and optical transmittance. The experimental results showed an improvement in the micro/nanoscale structure, which optimizes the superhydrophobic and anticorrosive behavior due to the air trapped inside the nanotextured surface. In addition, once the best coating was selected, centrifugal ice adhesion tests (CAT) were carried out for two types of icing conditions (glaze and rime) simulated in an ice wind tunnel (IWT) on both as-deposited and liquid-infused coatings (SLIPs). The liquid-infused coatings showed a low water adhesion (low contact angle hysteresis) and low ice adhesion strength, reducing the ice adhesion four times with respect to PTFE (a well-known low-ice-adhesion material used as a reference).

Performance of Sprayed PVDF-Al2O3 Composite Coating for Industrial and Civil Applications

Because of their great water repellency, Superhydrophobic coatings have a major impact on a variety of industrial applications. The current study’s key originality is the development of low-cost, stable, superhydrophobic, and corrosion-resistant composite coatings. In the present work, polyvinylidene fluoride (PVDF)/Al2O3 composite coatings were produced using the spray technique to investigate the wettability and corrosion behavior of the coated materials for industrial and civil applications. PVDF was mixed with various concentrations of Al2O3 nanoparticles, and the mixture was sprayed onto steel, aluminum, and glass substrates. The wettability and morphology of the coated surfaces were investigated using the sessile droplet method and scanning electron microscopy, respectively. The corrosion resistance of bare substrates was compared to that of those coated with PVDF alone and those coated with PVDF/Al2O3 nanoparticles using Tafel polarization techniques. The force of adhesion between the coat and the substrates was measured in pounds per square inch. A nanoindentation test was also used to measure the hardness of the coating layer. The PVDF/Al2O3 coated steel showed a significantly higher water contact angle and lower contact angle hysteresis, reaching 157 ± 2° and 7 ± 1°, respectively, compared to the coated aluminum and glass substrates. Corrosion test results showed that the superhydrophobic PVDF/Al2O3 coatings had a much higher corrosion protection efficiency for steel and aluminum than that of the PVDF ones. The PVDF/Al2O3 coated substrates showed moderate but still acceptable adhesion between the coating layer and the substrates. Moreover, the PVDF/Al2O3 coatings had much better mechanical properties than the PVDF only coatings. Such type of coating could be a promising candidate for possible industrial and civil applications.

Durable Superhydrophobic Coating for Efficient Microplastic Removal

The pollution caused by microplastics around the world is an increasingly significant issue that has to be tackled with different methods and technologies. Here, we report a straightforward and rapid process combining electrodeposition and electrophoresis to produce a durable superhydrophobic coating on an aluminum substrate (UNS A91070) that has a static contact angle (153°), sliding angle (1°), and contact angle hysteresis (1°). Field emission scanning electron microscopy and high-resolution transmission electron microscopy showed the presence of a hierarchical structure with nanolayers that were 70 nm thick. The chemical composition was also analyzed using attenuated total reflectance-Fourier transform infrared spectroscopy and high-resolution X-ray photoelectron spectroscopy, which revealed that the hierarchical structure was composed of zinc laurate (Zn(C11H20COO)2) that decreased the surface free energy of the system. Moreover, the coating showed high durability against abrasion caused by the P1200 SiC paper due to the presence of TiO2 particles in the upper layers as well as the homogeneous chemical composition of the hierarchical structure. Finally, taking advantage of the superoleophilic properties of superhydrophobic surfaces, the ability of the coating to remove high-density polyethylene microplastics from water was studied.

A Wind Tunnel Experimental Study of Icing on NACA0012 Aircraft Airfoil with Silicon Compounds Modified Polyurethane Coatings

Ice formation on the aerodynamic surfaces of an aircraft is regarded as a major problem in the aerospace industry. Ice accumulation may damage parts, sensors and controllers and alter the aerodynamics of the airplane, leading to a range of undesired consequences, including flight delays, emergency landings, damaged parts and increased energy consumption. There are various approaches to reducing ice accretion, one of them being the application of icephobic coatings. In this work, commercially available polyurethane-based coatings were modified and deposited on NACA 0012 aircraft airfoils. A hybrid modification of polyurethane (PUR) topcoats was adopted by the addition of nanosilica and three-functional spherosilicates (a variety of silsesqioxane compound), which owe their unique properties to the presence of three different groups. The ice accretion on the manufactured nanocomposites was determined in an icing wind tunnel. The tests were performed under three different icing conditions: glaze ice, rime ice and mixed ice. Furthermore, the surface topography and wetting behavior (static contact angle and contact angle hysteresis) were investigated. It was found that the anti-icing properties of polyurethane nanocomposite coatings strongly depend on the icing conditions under which they are tested. Moreover, the addition of nanosilica and spherosilicates enabled the reduction of accreted ice by 65% in comparison to the neat topcoat.