Andrei S. Pashinin

Durable Icephobic Coating for Stainless Steel

In this work, we present a modification of a stainless steel surface to impart superhydrophobic properties to it that are robust with respect to mechanical stresses associated with cyclic icing/deicing treatment, as well as to long-term contact with aqueous media and high humidity. The durability of the superhydrophobic state is ensured by the texture with multimodal roughness stable against mechanical stresses and a 2D polymer network of fluorooxysilane chemically bound to the texture elements. The designed superhydrophobic coating is characterized by contact angles exceeding 155° and a maximum rolling angle of 42° after 100 icing/deicing cycles.

Analysis of long-term durability of superhydrophobic properties under continuous contact with water.

In view of more and more extending areas of application of hydrophobic and superhydrophobic materials and coatings, the problem of degradation of superhydrophobic state becomes extremely important. In this paper, the results of studies of time evolution of water contact angle on some siloxane-based hydrophobic and superhydrophobic surfaces in the conditions of long-term continuous contact with water are presented. Different mechanisms responsible for reversible and nonreversible deterioration of contact angle values were discussed and experimentally verified. We noted that evolution of water contact angle in time in the conditions of continuous contact with water needs to be considered when studying the durability of superhydrophobic coating because of its high sensitivity to the state of the coating and the details of its chemical structure.

Origins of Thermodynamically Stable Superhydrophobicity of Boron Nitride Nanotubes Coatings

Superhydrophobic surfaces are attractive as self-cleaning protective coatings in harsh environments with extreme temperatures and pH levels. Hexagonal phase boron nitride (h-BN) films are promising protective coatings due to their extraordinary chemical and thermal stability. However, their high surface energy makes them hydrophilic and thus not applicable as water repelling coatings. Our recent discovery on the superhydrophobicity of boron nitride nanotubes (BNNTs) is thus contradicting with the fact that BN materials would not be hydrophobic. To resolve this contradiction, we have investigated BNNT coatings by time-dependent contact angle measurement, thermogravimetry, IR spectroscopy, and electron microscopy. We found that the wettability of BNNTs is determined by the packing density, orientation, length of nanotubes, and the environmental condition. The origins of superhydrophobicity of these BNNT coatings are identified as (1) surface morphology and (2) hydrocarbon adsorbates on BNNTs. Hydrocarbon molecules adsorb spontaneously on the curved surfaces of nanotubes more intensively than on flat surfaces of BN films. This means the surface energy of BNNTs was enhanced by their large curvatures and thus increased the affinity of BNNTs to adsorb airborne molecules, which in turn would reduce the surface energy of BNNTs and make them hydrophobic. Our study revealed that both high-temperature and UV-ozone treatments can remove these adsorbates and lead to restitution of hydrophilic BN surface. However, nanotubes have a unique capability in building a hydrophobic layer of adsorbates after a few hours of exposure to ambient air.

Effect of wettability on sessile drop freezing: when superhydrophobicity stimulates an extreme freezing delay.

An increasing number of studies directed at supercooling water droplets on surfaces with different wettabilities have appeared in recent years. This activity has been stimulated by the recognition that water supercooling phenomena can be effectively used to develop methods for protecting outdoor equipment and infrastructure elements against icing and snow accretion. In this article, we discuss the nucleation kinetics of supercooled sessile water droplets on hydrophilic, hydrophobic, and superhydrophobic surfaces under isothermal conditions at temperatures of -8, -10, and -15 °C and a saturated water vapor atmosphere. The statistics of nucleation events for the ensembles of freezing sessile droplets is completed by the detailed analysis of the contact angle temperature dependence and freezing of individual droplets in a saturated vapor atmosphere. We have demonstrated that the most essential freezing delay is characteristic of the superhydrophobic coating on aluminum, with the texture resistant to contact with ice and water. This delay can reach many hours at T = -8 °C and a few minutes at -23 °C. The observed behavior is analyzed on the basis of different nucleation mechanisms. The dissimilarity in the total nucleation rate, detected for two superhydrophobic substrates having the same apparent contact angle of the water drop but different resistivities of surface texture to the contact with water/ice, is associated with the contribution of heterogeneous nucleation on external centers located at the water droplet/air interface.

Femtosecond Laser Treatment for the Design of Electro-insulating Superhydrophobic Coatings with Enhanced Wear Resistance on Glass

Femtosecond laser treatment of a glass surface was used to fabricate a multimodal roughness having regular surface ripples with a period of a few micrometers decorated by aggregates of nearly spherical nanoparticles. UV-ozone treatment followed by chemisorption of the appropriate functional fluorosilanes onto the textured surface makes it possible to fabricate a superhydrophobic coating with a specific surface resistance on the order of petaohms on a glass surface. The main advantage of the fabricated coating under severe operating conditions with abrasion loads is the significant durability of its electro-insulating properties. The longevity of the high surface resistivity, even on long-term contact with a water vapor-saturated atmosphere, is directly related to the peculiarities of the surface texture and ripple structure.

Interaction between Hydrophobic and Superhydrophobic Materials with Aqueous Media

Interactions between hydrophobic materials and aqueous media was studied using the analysis of specific surface resistance. Methods for measuring surface resistance are developed for both samples exposed in vapors and those in direct contact with liquid. As a result of the measurements performed, it is confirmed that water molecules penetrate the matrix of the material under prolonged contact with hydrophobic and superhydrophobic materials with aqueous media. The processes of formation on the surface of hydrophobic and superhydrophobic materials of the wetting/adsorption films are observed and the method of estimation their thickness is suggested.

Interactions of silicone rubbers designed for electrical engineering applications with aqueous media

Interactions of a number of electrical engineering silicone rubbers with water and aqueous salt, weakly acidic, and weakly basic solutions were studied by measuring contact angles. A mechanism for leakage currents along the surfaces of silicone insulators was proposed.

The development of coatings that give superhydrophobic properties to the surface of silicone rubber

Hydrophobizing agents on the basis of functional silanes with fluorohydrocarbon substituents have been synthesized. Using these agents and Aerosil nanoparticles, textured coatings were developed that give superhydrophobic properties to the surface of electrotechnical rubbers. Specific features of interaction between the hydrophobic surfaces and water were studied based on analysis of contact angles. For all the hydrophobic materials and coatings studied, even during short-term contact with water, the change in the state of the material surface is shown to be taken into account, with the proposed superhydrophobic coatings demonstrating the most long-term water resistance.

Superhydrophobic coatings based on boron nitride nanotubes: The mechanism of superhydrophobicity and self-regeneration of highly hydrophobic properties

The mechanism of superhydrophobicity is discussed for coatings based on boron nitride nanotubes, i.e., on a material with high surface energy. It is shown that the thermodynamic stability of the superhydrophobic state is related to a surface roughness determined by nanotube packing and a decrease in surface energy due to the adsorption of hydrocarbon contaminants. The phenomenon of self-regeneration of highly hydrophobic properties is observed after the removal of organic impurities from the surface using different methods. The possibility for the cyclic switching of wettability from superhydrophobicity to hydrophilicity and back is demonstrated by successive coating treatment in plasma at a variation of its composition from atomic hydrogen plasma to methane plasma.

Application of contact angle analysis and surface-assisted laser desorption/ionization mass spectrometry to studying the surface state of AMg-6 and Ad-0 aluminum alloys

Surface- and matrix-assisted laser desorption/ionization mass spectrometry and analysis of contact angles have been employed to examine the surfaces of structural materials (Ad-0 and AMg-6 aluminum alloys). The combination of methods used has been shown to enable one to compare the degrees of nonuniformity of different surfaces and determine the effect of the chemical composition of surface impurities on surface properties. Owing to the high sensitivity and informativity, the proposed combination of the methods can be used to determine the nonuniformity of surfaces subjected to the action of rocket fuel components.