L. B. Boinovich

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.

A wetting experiment as a tool to study the physicochemical processes accompanying the contact of hydrophobic and superhydrophobic materials with aqueous media.

In most fields of technological application of superhydrophobic materials, such as protection against corrosion, icing, and capillary condensation, or micro fluidics applications, a superhydrophobic surface has to operate in contact with aggressive aqueous media. Therefore, the peculiarities of behaviour of hydrophobic and superhydrophobic surfaces on prolonged contact with water and the mechanisms of possible degradation of superhydrophobicity need to be discussed. In this study, a consideration of the physicochemical processes accompanying the contact of hydrophobic and superhydrophobic materials with water, acid, alkaline and saline aqueous solutions is presented on the basis of experimental data on three-phase equilibrium obtained by the sessile drop method. It is shown that simultaneous analysis of the contact angle and contact diameter of the sessile drop and liquid/vapour surface tension allows one to attribute degradation of the superhydrophobic and hydrophobic state to reversible and irreversible processes such as hydrolysis of hydrophobic molecules, growth of an oxide layer and so on. A method for estimating both the portion of wetted area and the intrinsic wettability state (hydrophobic versus hydrophilic) of texture elements for a heterogeneous wetting regime is proposed and discussed.

Principles of design of superhydrophobic coatings by deposition from dispersions.

The analysis demonstrating the abilities of various types of ordered textures, constructed from spherical particles, to provide high water contact angles on the surface of the hydrophobic material is presented. For several ordered three-dimensional surface structures we have derived the equations, relating the contact angles, formed by water droplets on a textured coating, with the geometric parameters of the texture and the chemical properties of the coating. The conditions providing the thermodynamic stability of heterogeneous wetting regime on textured surfaces have been discussed. It was demonstrated that multimodal surface roughness facilitates essentially the attainment of the superhydrophobic state and the surface textures exhibiting the stable superhydrophobic properties may be effectively manufactured by controlled aggregation of particles. Based on the analysis of forces acting between the nanoparticles both in the bulk dispersions and in the deposited film, we discuss the correlation of the properties of dispersion with the surface textures formed.

Synergistic Effect of Superhydrophobicity and Oxidized Layers on Corrosion Resistance of Aluminum Alloy Surface Textured by Nanosecond Laser Treatment

We report a new efficient method for fabricating a superhydrophobic oxidized surface of aluminum alloys with enhanced resistance to pitting corrosion in sodium chloride solutions. The developed coatings are considered very prospective materials for the automotive industry, shipbuilding, aviation, construction, and medicine. The method is based on nanosecond laser treatment of the surface followed by chemisorption of a hydrophobic agent to achieve the superhydrophobic state of the alloy surface. We have shown that the surface texturing used to fabricate multimodal roughness of the surface may be simultaneously used for modifying the physicochemical properties of the thick surface layer of the substrate itself. Electrochemical and wetting experiments demonstrated that the superhydrophobic state of the metal surface inhibits corrosion processes in chloride solutions for a few days. However, during long-term contact of a superhydrophobic coating with a solution, the wetted area of the coating is subjected to corrosion processes due to the formation of defects. In contrast, the combination of an oxide layer with good barrier properties and the superhydrophobic state of the coating provides remarkable corrosion resistance. The mechanisms for enhancing corrosion protective properties are discussed.

The Use of Digital Processing of Video Images for Determining Parameters of Sessile and Pendant Droplets

Methods used at different stages of digital processing of video images in studies of surface phenomena are considered. A segmentation method making it possible to detect several objects in an image field with a nonuniform illuminance is proposed; this method is based on dynamic thresholding. Various methods of determining the characteristics of the capillary system under consideration by fitting the parameters of Laplaces equation are discussed. The factors affecting the accuracy of parameter determination for sessile and pendant droplets are analyzed. On the basis of model calculations, the deviations (due to digitization of the interface profile during its digital processing) of the resultant surface tension and contact angle from exact values are estimated. Some experimental applications of the techniques described in the paper are discussed.

Wetting and surface forces

In this review we discuss the fundamental role of surface forces, with a particular emphasis on the effect of the disjoining pressure, in establishing the wetting regime in the three phase systems with both plane and curved geometry. The special attention is given to the conditions of the formation of wetting/adsorption liquid films on the surface of poorly wetted substrate and the possibility of their thermodynamic equilibrium with bulk liquid. The calculations of contact angles on the basis of the isotherms of disjoining pressure and the difference in wettability of flat and highly curved surfaces are discussed. Mechanisms of wetting hysteresis, related to the action of surface forces, are considered.

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.