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To date, the theoretical studies on dry abrasion on surfaces with microscopic roughness are based on JKR or DMT contact theory. Superhydrophobic nanocomposite coatings with low surface energy and microscopic surface roughness have extremely low real contact area and adhesion to the abrasive when subjected to contact and friction, making the above theories inapplicable. To date, there lack of theoretical studies on the dry abrasion characteristics of superhydrophobic nanocomposite coating surfaces. In response to this problem, a numerical understanding of the frictional force on superhydrophobic nanocomposite coatings is proposed based on Hertz contact theory, and the abrasion mechanisms of superhydrophobic nanocomposite coatings and its influencing factors are analyzed from the perspective of microscopic torque balance. The abrasion process is divided into three stages, and there exists a minimum value of bonding strength between the microstructure on the coating surface and the polymer matrix of the coating that determines whether abrasion will occur. This minimum bonding force is determined by Young's modulus of the polymer matrix, the radius of the microstructure, and the applied load. The conclusions can be a guide for the improvement of the anti-dry-abrasion property of superhydrophobic nanocomposite coatings from the perspective of coating preparation. [ABSTRACT FROM AUTHOR]

Under natural application conditions, water in the forms of condensates and high-speed raindrops tend to form Wenzel contact with the surface of superhydrophobic coatings, and in this case, the effect of the standing water on the abrasion mechanisms of superhydrophobic coatings is not clear. To investigate the issue, comparative tests of abrasion-induced aging on the surface of the superhydrophobic coating under dry, and water infiltration conditions are conducted. A theoretical prediction model of the friction magnitude on the surface of the superhydrophobic coating under the above two conditions is established, and the experimental phenomenon is analyzed based on the theoretical results of the model. The results show that once water wets superhydrophobic surfaces in Wenzel mode, the microscopic rough structures on the surfaces are more likely to fall off under external friction due to the presence of Laplace pressure, which means that the presence of water makes the superhydrophobic surfaces more prone to abrasion-induced aging in this condition. [Display omitted] [ABSTRACT FROM AUTHOR]

The droplet rolling angle is one of the important indicators to measure the coating's hydrophobic performance, but the specific factors affecting the droplet rolling angle on the micro-nanostructured superhydrophobic coating surface are not yet known. Based on the rolling mechanism of droplets on rough surfaces, and from the perspective of coating microscopic energy conservation, this paper points out that the micron-scale morphology and the nanoscale morphology can comprehensively affect the droplet rolling angle. From the above perspective, a mathematical model of the droplet rolling angle on the micro-nanostructure superhydrophobic coating surface was established. The model shows that the droplet rolling angle is positively correlated with the ratio of nano-sized pillar width to spacing, the ratio of micron-sized papilla radius to spacing, and the liquid–gas interfacial tension, and is negatively correlated to the droplet intrinsic contact angle, droplet volume and droplet density. The droplet rolling angle calculated by the presented model is in good agreement with the experimentally tested results. This model can provide good accuracy in predicting the droplet rolling angle on the micro-nanostructured superhydrophobic coating surface. [ABSTRACT FROM AUTHOR]

Methyl-silica superhydrophobic coatings have the capability of providing anti-pollution flashover protection for outdoor electrical insulators. However, in practical applications, the deposition and dissolution of atmospheric pollutants on the coating generates acidic or alkaline ions and leads to chemical degradation of the coating's surface property, which in turn causes a decrease in the anti-pollution flashover performance of the coating. Currently, there is a lack of research on the hydrophobic degradation characteristics of superhydrophobic methyl-silica coatings induced by chemical corrosion. Meanwhile, it is not clear how chemical corrosion affects the anti-pollution flashover performance of methyl-silica superhydrophobic coatings. To address the above issues, the static water contact angle and sliding angle on the corroded methyl-silica superhydrophobic coatings were tested to reflect the surface hydrophobicity degradation, and the results were theoretically interpreted; the surface leakage current amplitudes of the coatings aged to different degrees were measured under dirty and wet conditions to characterize the anti-pollution flashover performance of the corroded coatings. The surface leakage current amplitudes of the aged methyl-silica superhydrophobic coating under different corrosion environment temperatures, different applied electric field strengths, and different wetting times of surface pollutants are measured to characterize the anti-pollution flashover performance of the corrosion-aged coating, and the influences of the above factors on the anti-pollution flashover performance of the methyl-silica superhydrophobic coating are analyzed. [Display omitted] [ABSTRACT FROM AUTHOR]

Superhydrophobic (SH) and superamphiphobic (SAP) coatings have been viewed as an alternative to conventional hydrophobic room temperature vulcanizing (RTV) silicone rubber coatings in anti-pollution flashover application. However, being one of the most important properties in RTV-based anti-flashover materials and devices, the hydrophobicity transfer property was rarely documented for SH and SAP coatings, which restricts their practical applications in environments with heavy contamination. Herein, we propose for the first time a superhydrophobicity transfer effect in superwetting (SW) coatings (e.g., SAP and SH coatings) containing low-surface-energy molecules (e.g., polydimethylsiloxane (PDMS)) as a migratory reagent. When the surface is stained by artificial simulated pollutants, PDMS molecules can migrate from the coating to the contamination layer, followed by a surface modification. Through this process, the surface wettability of the contaminant layer gradually evolves from superhydrophilicity to hydrophobicity, and finally to SH state. This evolution is investigated in relation to the amount and species of migratory reagents, migration time, pollution grade, and pollutant species. The superhydrophobicity transfer effect offers the coated glass insulators an outstanding anti-pollution flashover strength. This work can provide a novel and universal protocol to realize a superhydrophobicity transfer property in SW coatings, which may stimulate further investigation and applications in not only anti-pollution flashover but also other fields such as anti-fouling and anti-icing. • Superhydrophobicity transfer effect is proposed for the first time. • Superwetting coatings with superhydrophobicity transfer property are obtained. • The mechanism behind the superhydrophobicity transfer effect is proposed. • Superhydrophobicity transfer effect improves the anti-pollution flashover strength of insulator. [ABSTRACT FROM AUTHOR]

Using micro-nanostructure superhydrophobic coating with the unique self-cleaning phenomenon is a feasible method to further reduce the frequency of insulator pollution flashover. However, the research on the anti-pollution flashover performance of superhydrophobic coating is still insufficient. This paper analyzes the various types of forces on the polluted particles accumulated on the superhydrophobic coating surface, and calculates that the vertical force or the horizontal force on the nano-scale and micro-scale particles is positive when they are in contact with most types of droplets. After being fully wetted, these particles are easily absorbed or pulled by the droplets and leave the coating surface. So it is necessary to establish a new DC pollution flashover mechanism model of superhydrophobic coatings surface from the electric field perspective. Based on this model, the relationship between the pollution flashover voltage and the dry flashover voltage is clarified in the state of extremely small leakage current. Furthermore, the anti-pollution flashover performance experiment was carried out on superhydrophobic coating, RTV coating, and uncoated surface to test the leakage current under different conditions. The experimental results show that at any pollution degree, the leakage current of superhydrophobic coating surface always remains about 16.3μA after being completely wetted. [Display omitted] [ABSTRACT FROM AUTHOR]

The application of the superhydrophobic coating is expected to be an effective method to further improve the pollution flashover voltage of insulators. However, so far, the relationship between electric field strength, droplet volume, and sliding angle in strong electric fields is not clear. This is not conducive to the evaluation of the practical application effect of superhydrophobic insulators. In this paper, the self-cleaning property of superhydrophobic coating and the critical motion condition of droplets in the electric field were analyzed theoretically. Then an experimental platform was set up to observe the movement of the droplet driven by the electric field, and the critical electric field strength for droplet motion was measured. Experimental and theoretical analysis results show that the critical electric field strength for positive droplet motion is less than that for the negative droplet. The droplet driven by the electric field can also remove water-insoluble pollution particles. And the maximum pollution concentration that droplets can remove by means of self-cleaning performance is calculated theoretically. The theoretical calculation method of the sliding angle of superhydrophobic coating in uniform DC electric field is obtained, which is in good agreement with the experimental results. This paper provides theoretical support for evaluating the practical performance of the superhydrophobic insulator in strong electric fields. [Display omitted] [ABSTRACT FROM AUTHOR]

If the apparent contact angle (ACA) of superhydrophobic materials could be significantly affected by drop volume, it would not be reliable to use ACA as an important index to evaluate the wettability of superhydrophobic materials. In order to explore the effect of drop volume on the ACA of hierarchical structured superhydrophobic surfaces, two kinds of hierarchical structured superhydrophobic surfaces, natural lotus leaf surface and superhydrophobic coating, were used for experimental analysis. In addition, a physical model of ACA considering drop volume was established. And the relationship curve between drop volume and ACA was calculated. The experiment and simulation results show that for each kind of drop, the volume has no significant effect on the ACA of hierarchical structured superhydrophobic surfaces. And the ACA can reflect the wettability of the superhydrophobic material effectively. [ABSTRACT FROM AUTHOR]