Modification of wetting properties of laser-textured surfaces by depositing triboelectrically charged Teflon particles

Hydrophilic laser-textured silicon wafers with natural oxide surfaces were rendered hydrophobic by depositing electrostatically charged submicrometer Teflon particles, a process termed as triboelectric Teflon adhesion. Silicon surfaces were micro-textured (～5 μm) by laser ablation using a nanosecond pulsed UV laser. By varying laser fluence, micro-texture morphology of the wafers could be reproduced and well controlled. Wetting properties of the triboelectrically charged Teflon-deposited surfaces were studied by measuring apparent static water contact angles and water contact angle hysteresis as a function of substrate roughness and the amount of Teflon deposited. A similar study was also performed on various micro-textured silicon carbide surfaces (sandpapers). If the average substrate roughness is between 15 and 60 μm, superhydrophobic surfaces can be easily formed by Teflon deposition with water contact angle hysteresis less than 8°. This environmentally benign solvent-free process is a highly efficient, rapid, and inexpensive way to render contact-charged rough surfaces hydrophobic or superhydrophobic.     

Creation of low hysteresis superhydrophobic paper by deposition of hydrophilic diamond-like carbon films

The creation of low hysteresis superhydrophobic paper is reported using a combination of oxygen plasma etching and plasma deposition of an 80 nm non-fluorinated, hydrophilic diamond-like carbon (DLC) coating. The DLC has an equilibrium (flat surface) contact angle (θ _e ) of 68.2° ± 1.5°, which is well below the 90° contact angle that is typically believed to be a prerequisite for superhydrophobicity. Coating of paper substrates with the DLC film yields an advancing contact angle of 124.3° ± 4.1°, but the surface remains highly adhesive, with a receding contact angle <10°. After 60 min of plasma etching and DLC coating, a low hysteresis, superhydrophobic surface is formed with an advancing contact angle of 162.0° ± 6.3° and hysteresis of 8.7° ± 1.9°. To understand the increase in contact angle and decrease in hysteresis, atomic force microscopy and optical profilometry studies were performed. The data demonstrates that while little additional nanoscale roughness is imparted beyond the first 5 min of etching, the roughness at the microscale continually increases. The hierarchical structure provides the appropriate roughness to create low hysteresis superhydrophobic paper from a hydrophilic coating.     

Balance of polyacrylate-fluorosilicone block copolymers as icephobic coatings

Polyacrylate-fluorosilicone block copolymers, namely, polyacrylate-b-polydimethylsiloxane and polyacrylate-bpolymethyltrifluoropropylsiloxane were synthesized for fabricating icephobic coatings. The surface morphology and chemical composition of the block copolymers were characterized by atomic force microscopy and X-ray photoelectron spectroscopy, suggesting that the fluorosilicone blocks aggregated on the top of the copolymer surfaces. Results of water contact angles and ice shear strength demonstrated a certain amount adding of methacryloisobutyl polyhedral oligomeric silsesquioxane could lead to the decrease of contact angle hysteresis and increase of surface roughness, consequently resulting in significant reduction of the ice adhesion strength. Therefore, the block copolymers with the combined advantages of silicone and fluoropolymers could be potentially applied as icephobic coatings.     

Predictive model for ice formation on superhydrophobic surfaces

The prevention and control of ice accumulation has important applications in aviation, building construction, and energy conversion devices. One area of active research concerns the use of superhydrophobic surfaces for preventing ice formation. The present work develops a physics-based modeling framework to predict ice formation on cooled superhydrophobic surfaces resulting from the impact of supercooled water droplets. This modeling approach analyzes the multiple phenomena influencing ice formation on superhydrophobic surfaces through the development of submodels describing droplet impact dynamics, heat transfer, and heterogeneous ice nucleation. These models are then integrated together to achieve a comprehensive understanding of ice formation upon impact of liquid droplets at freezing conditions. The accuracy of this model is validated by its successful prediction of the experimental findings that demonstrate that superhydrophobic surfaces can fully prevent the freezing of impacting water droplets down to surface temperatures of as low as -20 to -25 °C. The model can be used to study the influence of surface morphology, surface chemistry, and fluid and thermal properties on dynamic ice formation and identify parameters critical to achieving icephobic surfaces. The framework of the present work is the first detailed modeling tool developed for the design and analysis of surfaces for various ice prevention/reduction strategies.     

超疏水涂层防覆冰机理及制备

结构物表面严重覆冰有时会威胁人们的生命财产安全,影响经济社会的平稳运行。超疏水涂层具有超大接触角、低滚动角的特性,在防覆冰领域有很好的应用前景。本文重点总结了超疏水涂层防覆冰机理的相关研究结果,并简单综述了聚合物-微粒复合材料类、特殊表面结构类两种典型超疏水型防覆冰涂层的研究进展,指出了目前在超疏水型防覆冰涂层研究中存在的不足,并对其发展趋势进行了展望。     

Condensation-induced wetting state and contact angle hysteresis on superhydrophobic lotus leaves

In this paper, we demonstrate how condensed moisture droplets wet classical superhydrophobic lotus leaf surfaces and analyze the mechanism that causes the increase of contact angle hysteresis. Superhydrophobic lotus leaves in nature show amazing self-cleaning property with high water contact angle (>150°) and low contact angle hysteresis (usually <10°), causing droplets to roll off at low inclination angles, in accordance with classical Cassie–Baxter wetting state. However, when superhydrophobic lotus leaves are wetted with condensation, the condensed water droplets are sticky and exhibit higher contact angle hysteresis (40–50°). Compared with a fully wetted sessile droplet (classical Wenzel state) on the lotus leaves, the condensed water droplet still has relatively large contact angle (>145°), suggesting that the wetting state deviates from a fully wetted Wenzel state. When the condensed water droplets are subjected to evaporation at room conditions, a thin water film is observed bridging over the micropillar structures of the lotus leaves. This causes the dew to stick to the surface. This result suggests that the condensed moisture does not uniformly wet the superhydrophobic lotus leaf surfaces. Instead, there occurs a mixed wetting state, between classical Cassie–Baxter and Wenzel states that causes a distinct increase of contact angle hysteresis. It is also observed that the mixed Cassie–Baxter/Wenzel state can be restored to the original Cassie–Baxter state by applying ultrasonic vibration which supplies energy to overcome the energy barrier for the wetting transition. In contrast, when the surface is fully wetted (classical Wenzel state), such restoration is not observed with ultrasonic vibration. The results reveal that although the superhydrophobic lotus leaves are susceptible to being wetted by condensing moisture, the configured wetting state is intermediate between the classical Cassie–Baxter and Wenzel states.     

Antiicing performance of superhydrophobic coatings on aluminum and stainless steel

The mechanisms determining the antiicing protection imparted by superhydrophobic coatings to the aluminum and steel surfaces of structural parts and control equipment of aircrafts are discussed. The results of testing at negative surface temperatures and high velocities of air—vapor flow demonstrated that application of superhydrophobic coatings produced by various methods provides a substantial mitigation of ice accretion compared to uncoated metal surfaces. The superhydrophobic coatings on aluminum surfaces completely prevent the formation of ice at moderate flow velocities of up to 40 m s^?1. At higher flow rates under conditions of ice formation on the test desk, the effect of periodic self-cleaning of the superhydrophobic surface was detected. Superhydrophobic coatings on stainless-steel parts of air pressure sensor also demonstrate explicit antiicing effect associated with the reduced heating power required to completely prevent ice formation on the sensor surface.     

Contact line and contact angle dynamics in superhydrophobic channels

The dynamics of the wetting and movement of a three-phase contact line confined between two superhydrophobic surfaces were studied using a mean-field free-energy lattice Boltzmann model. Principle features of superhydrophobic surfaces, such as trapped vapor/air between rough microstructures, high contact angles, reduced contact angle hysteresis, and low resistance to fluid flow, were all observed. Movement of the three-phase contact line over a well-patterned superhydrophobic surface displays a periodic stick-jump-slip behavior, while the dynamic contact angle changes accordingly from maximum to minimum. Two regimes were found for the flow velocity as a function of surface roughness and can be related directly to the balance between driving force and flow resistance. This work provides a better understanding of dynamic wetting and fluid flow behaviors over superhydrophobic surfaces and hence could be useful in related applications.     

Preparation of Cu-Coated Stainless Steel Surfaces for Superhydrophobic Investigation

Cu-coated stainless steel surfaces containing micro- and nanoscale binary structures with different surface roughness were successfully fabricated by means of a facile one-step electroless plating technology. The resulting surfaces were modified by the low free energy material HFTHTMS (HFTHTMS = (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trimethoxysilane). The experimental results of wettability exhibit that such unmodified surfaces have a strong adhesive force to water droplets, and their contact angles increase with increasing surface roughness, whereas the modified surfaces by HFTHTMS show the superhydrophobic characteristic with contact angles higher than 150° and sliding angles lower than 5°.     

Formation of superhydrophobic surfaces by biomimetic silicification and fluorination

The amazing water repellency of many biological surfaces, exemplified by lotus leaves, has recently received a great deal of interest. These surfaces, called superhydrophobic surfaces, exhibit water contact angles larger than 150 degrees and a low contact angle hysteresis because of both their low surface energy and heterogeneously rough structures. In this paper, we suggest a biomimetic method, "biosilicification", for generating heterogeneously rough structures and fabricating superhydrophobic surfaces. The superhydrophobic surface was prepared by a combination of the formation of heterogeneously rough, nanosphere-like silica structures through biosilicification and the formation of self-assembled monolayers of fluorosilane on the surface. The resulting surface exhibited the water contact angle of 160.1 degrees and the very low water contact angle hysteresis of only 2.3 degrees, which are definite characteristics of superhydrophobic surfaces. The superhydrophobic property of our system probably resulted from the air trapped in the rough surface. The wetting behavior on the surface was in the heterogeneous regime, which was totally supported by Cassie-Baxter equation.     

Electrophoretic deposition of unstable colloidal suspensions for superhydrophobic surfaces

A novel method to fabricate superhydrophobic surfaces using electrophoretic deposition (EPD) is presented. EPD presents a readily scalable, customizable, and potentially low cost surface manufacturing process. Low surface energy materials with high surface roughness are achieved using EPD of unstable hydrophobic SiO(2) particle suspensions. The effect of suspension stability on surface roughness is quantitatively explored with optical absorbance measurements (to determine suspension stability) and atomic force microscopy (to measure surface roughness). Varying suspension pH modulates suspension stability. Contrary to most applications of EPD, we show that superhydrophobic surfaces favor mildly unstable suspensions since they result in high surface roughness. Particle agglomerates formed in unstable suspensions lead to highly irregular films after EPD. After only 1 min of EPD, we obtain surfaces with low contact angle hysteresis and static contact angles exceeding 160°. We also present a technique to enhance the mechanical durability of the superhydrophobic surfaces by adding a polymeric binder to the suspension prior to EPD.     

Preparation of Cu-Coated Stainless Steel Surfaces for Surperhydrophobic Investigation

Cu-coated stainless steel surfaces containing micro- and nanoscale binary structures having different surface roughness were successfully fabricated by means of a facile one-step electroless plating technology, and the resulting surfaces were modified by the low free energy material HFTHTMS (HFTHTMS = (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trimethoxysilane). The experimental results of wettability exhibit that such unmodified surfaces have a strong adhesive force to water droplets, and their contact angles increase with increasing surface roughness, whereas the modified surfaces by HFTHTMS show the superhydrophobic characteristic with contact angles higher than 150° and sliding angles lower than 5°.     

Superhydrophobicity on two-tier rough surfaces fabricated by controlled growth of aligned carbon nanotube arrays coated with fluorocarbon

Considerable effort has been expended on theoretical studies of superhydrophobic surfaces with two-tier (micro and nano) roughness, but experimental studies are few due to the difficulties in fabricating such surfaces in a controllable way. The objective of this work is to experimentally study the wetting and hydrophobicity of water droplets on two-tier rough surfaces for comparison with theoretical analyses. To compare wetting on micropatterned silicon surfaces with wetting on nanoscale roughness surfaces, two model systems are fabricated: carbon nanotube arrays on silicon wafers and carbon nanotube arrays on carbon nanotube films. All surfaces are coated with 20 nm thick fluorocarbon films to obtain low surface energies. The results show that the microstructural characteristics must be optimized to achieve stable superhydrophobicity on microscale rough surfaces. However, the presence of nanoscale roughness allows a much broader range of surface design criteria, decreases the contact angle hysteresis to less than 1 degrees , and establishes stable and robust superhydrophobicity, although nanoscale roughness could not increase the apparent contact angle significantly if the microscale roughness dominates.     

Hysteresis controlled water droplet splitting on superhydrophobic paper

Fabrication of surfaces with heterogeneous contact angle hysteresis enables extraction of droplet samples from bulk liquid volumes. These surfaces are created by printing high hysteresis wax islands onto low hysteresis superhydrophobic paper. The volume of the sampled droplets depends on the hysteresis of the printed islands, which can be controlled through both physical and chemical means. Physically, hysteresis is modified through the addition of surface roughness. Chemical hysteresis is tuned by changing the active chemical groups present on the wax surface. The observed control of the volume of sampled droplets, which is necessary for quantitative biochemical or chemical assays, extends to scenarios in which multiple droplet samples are extracted simultaneously from a single bulk droplet. Demonstration of the capacity of this technique to perform colorimetric glucose immunoassays is described. The ability to obtain well-defined microliter sample volumes and to extract several samples simultaneously from the same source enables the development of two-dimensional paper-based microfluidic devices for biomedical testing.     

含氟硅丙烯酸酯乳液超疏水膜中疏水基团的梯度分布与表面微观结构研究

利用含氟疏水基团的梯度分布,结合草莓形纳米SiO2粒子提供的双重粗糙表面,制备了具有类"荷叶效应"的超疏水涂膜,水接触角达(174.2±2)°,滞后角几乎接近0°.通过原子力显微镜、扫描电镜和水接触角的测试对膜表面形貌及疏水性能进行了表征;探讨了其表面微观结构与表面疏水性能的关系.草莓形复合粒子在膜表面的无规则排列赋予涂膜表面不同等级的粗糙度,使水滴与涂膜表面接触时能够形成高的空气捕捉率,这种微观结构与疏水基团的梯度分布相结合,赋予了含氟硅丙烯酸酯乳液涂膜表面超疏水性能.     

A study on superhydrophobic coating in anti-icing of glass/porcelain insulator

This paper focuses on the preparation of a superhydrophobic coating on glass/porcelain insulators which possess anti-icing property below freezing temperature. Inspired by lotus-effect, the fabrication of a superhydrophobic coating has two steps: the first step is to construct a hierarchical SiO₂ coating on the substrate surface, and the second step is the chemical modification of the SiO₂ coating with 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PDTS). The precursor for the hierarchical SiO₂ coating is a suspension of SiO₂ sol particles and dispersible SiO₂ powder particles. According to the TEM testing, SiO₂ sol particles prepared by sol–gel method has an average particle size about 2–5 nm, while the size of the dispersible SiO₂ particles is ca. 20 nm. The precursor was sprayed on glass/porcelain insulators, and then dried at ambient condition, finally heat-treated at 773 K for 2 h. The morphology of the superhydrophobic coating was characterized by TEM and AFM, and experimental results indicated that the coating featured [hierarchical structure consisting of both large bumps with micron-sized height (0.8 μm) and tiny papillae with the size about 30 nm] micron-sized roughness (0.8 μm) combined with nano-sized roughness (about 2 nm). Moreover, the scratch test showed that the coating tightly adhered to the surface of the glass/porcelain insulators. The superhydrophobic property of the coating was examined by a contact angle measurement, and the results demonstrated that the static water contact angle is high up to 163.6°, and the sliding angle is 1.4°. The superhydrophobic property of the coating was also confirmed by the outdoor tests in winter, and it was found that the superhydrophobic coating had the function in anti-icing, based on which the anti-icing mechanism underlying was discussed in terms of the interaction between impacting droplets and superhydrophobic surface.     

Contact angle hysteresis on regular pillar-like hydrophobic surfaces

A series of pillar-like patterned silicon wafers with different pillar sizes and spacing are fabricated by photolithography and further modified by a self-assembled fluorosilanated monolayer. The dynamic contact angles of water on these surfaces are carefully measured and found to be consistent with the theoretical predictions of the Cassie model and the Wenzel model. When a water drop is at the Wenzel state, its contact angle hysteresis increases along with an increase in the surface roughness. While the surface roughness is further raised beyond its transition roughness (from the Wenzel state to the Cassie state), the contact angle hysteresis (or receding contact angle) discontinuously drops (or jumps) to a lower (or higher) value. When a water drop is at the Cassie state, its contact angle hysteresis strongly depends on the solid fraction and has nothing to do with the surface roughness. Even for a superhydrophobic surface, the contact angle hysteresis may still exhibit a value as high as 41 degrees for the solid fraction of 0.563.     

Creation of “Rose Petal” and “Lotus Leaf” Effects on Alumina by Surface Functionalization and Metal‐Ion Coordination

Functional differences between superhydrophobic surfaces, such as lotus leaf and rose petals, are due to the subtle architectural features created by nature. Mimicry of these surfaces with synthetic molecules continues to be fascinating as well as challenging. Herein, we demonstrate how inherently hydrophilic alumina surface can be modified to give two distinct superhydrophobic behaviors. Functionalization of alumina with an organic ligand resulted in a rose‐petal‐like surface (water pinning) with a contact angle of 145° and a high contact angle hysteresis (±69°). Subsequent interaction of the ligand with Zn²⁺ resulted in a lotus‐leaf‐like surface with water rolling behavior owing to high contact angle (165°) and low‐contact‐angle‐hysteresis (±2°). In both cases, coating of an aromatic bis‐aldehyde with alkoxy chain substituents was necessary to emulate the nanowaxy cuticular feature of natural superhydrophobic materials.     

UV and thermally stable superhydrophobic coatings from sol-gel processing

A method for the preparation of inorganic superhydrophobic silica coatings using sol-gel processing with tetramethoxysilane and isobutyltrimethoxysilane as precursors is described. Incorporation of isobutyltrimethoxysilane into silica layers resulted in the existence of hydrophobic isobutyl surface groups, thereby generating surface hydrophobicity. When combined with the surface roughness that resulted from sol-gel processing, a superhydrophobic surface was achieved. This surface showed improved UV and thermal stability compared to superhydrophobic surfaces generated from polybutadiene by plasma etching. Under prolonged UV tests (ASTM D 4329), these surfaces gradually lost superhydrophobic character. However, when the as-prepared superhydrophobic surface was treated at 500 degrees C to remove the organic moieties and covered with a fluoroalkyl layer by a perfluorooctylsilane treatment, the surface regained superhydrophobicity. The UV and thermal stability of these surfaces was maintained upon exposure to temperatures up to 400 degrees C and UV testing times of 5500 h. Contact angles remained >160 degrees with contact angle hysteresis approximately 2 degrees.     

Multiscale roughness and stability of superhydrophobic biomimetic interfaces

The stability of a composite interface of roughness-induced superhydrophobic surfaces is studied. To have high contact angle and low contact angle hysteresis, superhydrophobic surfaces should be able to form a composite interface with air pockets in the valleys between asperities (pillars). However, the composite interface may be unstable and can be irreversibly transformed into a homogeneous interface. We formulate a stability criterion and analyze the stability of the composite interface for several typical roughness profiles. To resist destabilizing mechanisms, multiscale (hierarchical) roughness is required. Such multiscale roughness is found in natural and artificial superhydrophobic surfaces.