Properties Of Superhydrophobic Surfaces Research Articles

An outlook on tunable superhydrophobic nanostructural surfaces and their possible impact on ice mitigation

Abstract For decades, there has been a demand for the fabrication of surfaces that could prevent ice accumulation or remove ice easily. Ice buildup on the surfaces of infrastructural equipment such as airplanes, cars, ships, dams, wind turbines, and off-shore platforms reduces their efficiency and imposes safety risks. Traditional anti-icing and de-icing equipment is costly and is often inadequate in terms of performance. Some methods also have a negative impact on the environment. The last decade’s intensive investigation of the non-wetting properties of superhydrophobic (SHP) surfaces has raised the possibility of using these surfaces as icephobic surfaces, as well. In this review, the fundamental wetting properties of solid surfaces and SHP surfaces in particular are described first. SHP fabrication techniques and the effect of surface texture on the mechanism of water droplet interaction with the SHP surfaces are also discussed. Next, a comprehensive review of SHP surfaces’ performance in ice mitigation is presented, particularly the bouncing mechanism of super-cooled water droplets on substrates at subzero temperatures and the subsequent reduction of ice adhesion strength. The robustness of these surfaces, as well as the limitations of their performance in various environmental conditions, is also considered. It is believed that SHP surfaces’ ability to retain the Cassie state of wetting, even during impact by high velocity droplets and during frost formation, is the key to their icephobic performance. The design and engineering of the surface’s rough textures, as well as understanding super-cooled water droplet surface kinetics at the microscopic level, are crucial in developing SHP-based icephobic surfaces.

Testing the performance of superhydrophobic aluminum surfaces

The analysis of wetting properties of superhydrophobic surfaces may be a difficult task due to the restless behavior of drops on this type of surfaces and the limitations of goniometry for high contact angles. A method to validate the performance of superhydrophobic surfaces, rather than standard goniometry, is required. In this work, we used bouncing drop dynamics as a useful tool to predict the water repellency of different superhydrophobic surfaces. From bouncing drop experiments conducted over a wide range of superhydrophobic surfaces, we found that those surfaces with a proper roughness degree and homogeneous chemical composition showed higher water-repellency. We also conducted a drop condensation study at saturating conditions aimed to determine whether there is direct correlation between water repellency and condensation delay. We found that the drop condensation process is strongly related to the surface topography, as well as the intrinsic wettability. The condensation is promoted on rough surfaces but it is delayed on intrinsically hydrophobic surfaces. However, the differences found in condensation delay between the superhydrophobic surfaces explored in this study cannot be justified by their chemical homogeneity nor their efficiency as water repellent surfaces, separately.

Role of Water Solidification Concepts in Designing Nano-Textured Anti-Icing Surfaces

Creating anti-icing surfaces has proven to be a challenging task. With such a wide range of impacting parameters it is important to quantify ones with a large effect. Water solidification mechanisms play a fundamental role in designing anti-icing surfaces. In this Review Article, we will consider the effects of surface roughening on the mechanisms of nucleation and ice growth to show how surface roughening can be an alternative to overcome the limitations of icing of superhydrophobic coatings and surfaces. The results from various studies of anti-icing properties of superhydrophobic surfaces are reviewed and expanded to incorporate water solidification mechanisms to provide a more comprehensive approach to the design of anti-icing surfaces. The literature within this review shows that by applying the necessary roughness to either hydrophilic or hydrophobic surfaces and adjusting the surface topography, we can significantly suppress ice nucleation on various surfaces.

Anti-wetting behavior of negatively charged superhydrophobic PVDF membranes in direct contact membrane distillation of emulsified wastewaters

Membrane wetting is a key issue in causing deleterious performance in membrane distillation. In this work, superhydrophobic membranes were challenged in a direct contact membrane distillation (DCMD) process using cationic/anionic surfactant emulsified oil-in-water emulsions in order to assess the antiwetting properties of superhydrophobic surfaces in a highly demanding environment. Contact angle, Zeta potential and DCMD performance of the membrane were determined. It was found that superhydrophobic PVDF membranes exhibited strongly negative charges on the surface, as well as stable MD performance in concentrating the anionic surfactant emulsified waste water, but suffered from severe wetting when cationic surfactant was used. In contrast, the virgin hydrophobic PVDF membrane experienced severe wetting for either surfactant stabilized emulsions. Extended DLVO theory was utilized to explain the membrane-foulant interaction based on the EDL, acid-base and hydrophobic-hydrophobic interaction. Cleaning of superhydrophobic membranes was performed and restore of membrane performance was achieved for membranes that were used for treatment of negatively charged emulsions. This research outlined the experimental evidence and the importance of negative charges of a superhydrophobic surface for a stable membrane distillation performance, which may be helpful for future development of distillation membranes.

Fabrication and Properties of Superhydrophobic Surfaces Based on Low-Density Polyethylene/Ethylene–Propylene–Diene Terpolymer Thermoplastic Vulcanizate

ABSTRACTA simple, effective, and inexpensive method has been developed to fabricate superhydrophobic surfaces based on low-density-polyethylene/ethylene–propylene–diene terpolymer thermoplastic vulcanizate. Field emission scanning electron microscope research showed that the rough microstructure could be fabricated successfully in the thermoplastic vulcanizate surface where metallographic sandpaper was used as molding template; moreover, the micrometer scale plastic deformation of thermoplastic vulcanizate matrix could be observed obviously, leading to the increasing surface roughness. Wettability test showed that the series rough thermoplastic vulcanizate surfaces that were molded with sandpapers had strong hydrophobic property; furthermore, the sliding angle value was less than 10°. Metallographic sandpaper template exhibited well re-used property.

Biomimetic taro leaf-like films decorated on wood surfaces using soft lithography for superparamagnetic and superhydrophobic performance

The surfaces of plants represent multifunctional interfaces between the organisms and the environment. In this paper, biomimetic taro leaf-like structures with superparamagnetic and superhydrophobic performances were exactly copied on the wood surface through the soft lithography to improve the wood properties. Fe3O4 nanoparticles were mixed into poly(dimethylsiloxane) PDMS suspensions to obtain Fe3O4/PDMS suspensions that commonly endow coats magnetic and microwave absorption properties, which were then cast onto the wood surface and packaged by PDMS stamps replicated from fresh taro leaves. Fe3O4/PDMS films, which coexisted superhydrophobic surface and superparamagnetic property, were created on the wood surface after the being dried and stamps were peeled off. The as-prepared wood surface exhibited unique taro leaf-like micro- and nanostructures, microwave absorption, superparamagnetism performances with maximum saturation magnetization (M s) of 22.9 emu g−1 and superior static superhydrophobicity with a water contact angle of 152° ± 2°. This research may provide a feasible pathway for constructing naturally biomorphic structures on the wood surface with tailored functions.

Robust anti-icing coatings via enhanced superhydrophobicity on fiberglass cloth

Robust superhydrophobic coatings with anti-icing properties were obtained by a simple approach, i.e., immersing and spraying of adhesive/hydrophobic silica dispersion on fiberglass cloth. Scanning electron microscopy, water contact angles, abrasion tests and dynamic icing tests were performed to characterize the morphology, hydrophobicity, mechanical durability and icephobicity of the coatings. The experimental results demonstrated that the coatings have good abrasion resistance. The water contact angle and the water sliding angle remain greater than 150° and lower than 10°, respectively, after 60cycles of abrasion. Furthermore, the anti-icing properties of superhydrophobic surfaces were investigated under dynamic flow conditions. It was found that the coatings had the ability to delay ice nucleation without electrical heating, and avoid ice adhesion partly or even completely, under a given mass of heat, which was much less than that of uncoated surfaces. The fiberglass cloth based coatings could reduce by up to 51% of the heating power required to avoid ice accumulation, compared to uncoated surfaces.

Facile Adhesion-Tuning of Superhydrophobic Surfaces between "Lotus" and "Petal" Effect and Their Influence on Icing and Deicing Properties.

Adhesion behavior of superhydrophobic (SH) surfaces is an active research field related to various engineering applications in controlled microdroplet transportation, self-cleaning, deicing, biochemical separation, tissue engineering, and water harvesting. Herein, we report a facile approach to control droplet adhesion, bouncing and rolling on properties of SH surfaces by tuning their air-gap and roughness-height by altering the concentrations of poly dimethyl-siloxane (PDMS). The optimal use of PDMS (4-16 wt %) in a dual-scale (nano- and microparticles) composite enables control of the specific surface area (SSA), pore volume, and roughness of matrices that result in a well-controlled adhesion between water droplets and SH surfaces. The sliding angles of these surfaces were tuned to be varied between 2 ± 1 and 87 ± 2°, which are attributed to the transformation of the contact type between droplet and surface from "point contact" to "area contact". We further explored the effectiveness of these low and high adhesive SH surfaces in icing and deicing actions, which provides a new insight into design highly efficient and low-cost ice-release surface for cold temperature applications. Low adhesion (lotus effect) surface with higher pore-volume exhibited relatively excellent ice-release properties with significant icing delay ability principally attributed to the large air gap in the coating matrix than SH matrix with high adhesion (petal effect).

Water-repellent and corrosion-resistance properties of superhydrophobic and lubricant-infused super slippery surfaces

Inspired by lotus leaves and pitcher plants, superhydrophobic surfaces and super slippery surfaces have been fabricated to improve the characteristics of AZ31 magnesium alloy surfaces.

Dual-action smart coatings with a self-healing superhydrophobic surface and anti-corrosion properties

This work introduces a new self-healing superhydrophobic coating based on dual actions by the corrosion inhibitor benzotriazole (BTA) and an epoxy-based shape memory polymer (SMP).

Robust Ni/WC superhydrophobic surfaces by electrodeposition

The mechanical properties of superhydrophobic surfaces based on nickel composite coatings was enhanced by co-deposition of tungsten carbide. After modified by stearic acid, the surfaces showed excellent water-repellence and abrasive resistance.

Anti-icing property of bio-inspired micro-structure superhydrophobic surfaces and heat transfer model

Ice accumulation is a thorny problem which may inflict serious damage even disasters in many areas, such as aircraft, power line maintenance, offshore oil platform and locators of ships. Recent researches have shed light on some promising bio-inspired anti-icing strategies to solve this problem. Inspired by typical plant surfaces with super-hydrophobic character such as lotus leaves and rose petals, structured superhydrophobic surface are prepared to discuss the anti-icing property. 7075 Al alloy, an extensively used materials in aircrafts and marine vessels, is employed as the substrates. As-prepared surfaces are acquired by laser processing after being modified by stearic acid for 1h at room temperature. The surface morphology, chemical composition and wettability are characterized by means of SEM, XPS, Fourier transform infrared (FTIR) spectroscopy and contact angle measurements. The morphologies of structured as-prepared samples include round hump, square protuberance and mountain-range-like structure, and that the as-prepared structured surfaces shows an excellent superhydrophobic property with a WCA as high as 166±2°. Furthermore, the anti-icing property of as-prepared surfaces was tested by a self-established apparatus, and the crystallization process of a cooling water on the sample was recorded. More importantly, we introduced a model to analyze heat transfer process between the droplet and the structured surfaces. This study offers an insight into understanding the heat transfer process of the superhydrophobic surface, so as to further research about its unique property against ice accumulation.

One-step fabrication of biomimetic superhydrophobic surface by electrodeposition on magnesium alloy and its corrosion inhibition

A facile, rapid and one-step electrodeposition process has been employed to construct a superhydrophobic surface with micro/nano scale structure on a Mg-Sn-Zn (TZ51) alloy, which is expected to be applied as a biodegradable biomedical implant materials. By changing the electrodeposition time, the maximum contact angle of the droplet was observed as high as 160.4°±0.7°. The characteristics of the as-prepared surface were conducted by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR). Besides, the anti-corrosion performance of the coatings in stimulated body fluid (SBF) solution were investigated by electrochemical measurement. The results demonstrated that the anti-corrosion property of superhydrophobic surface was greatly improved. This method show beneficial effects on the wettability and corrosion behavior, and therefore provides a efficient route to mitigate the undesirable rapid corrosion of magnesium alloy in favor of application for clinical field.

Combating Frosting with Joule-Heated Liquid-Infused Superhydrophobic Coatings.

Frost formation is omnipresent when suitable environmental conditions are met. A good portion of research on combating frost formation has revolved around the passive properties of superhydrophobic (SHPO) and slippery lubricant-impregnated porous (SLIP) surfaces. Despite much progress, the need for surfaces that can effectively combat frost formation over prolonged periods still remains. In this work, we report, for the first time, the use of electrically conductive SHPO/SLIP surfaces for active mitigation of frost formation. First, we demonstrate the failure of these surfaces to passively avert prolonged (several hours) frosting. Next, we make use of their electroconductive property for active Joule heating, which results in the removal of any formed frost. We study the role of the impregnating lubricant in the heat transfer across the interface, the surface, and the ambient. We show that, even though the thermal properties of the impregnating lubricant may vary drastically, the lubricant type does not noticeably affect the defrosting behavior of the surface. We attribute this outcome to the dominant thermal resistance of the thick frost layer formed on the cooled surface. We support this claim by drawing parallels between the present system and heat transfer through a one-dimensional (1D) composite medium, and solving the appropriate transient transport equations. Lastly, we propose periodic thermal defrosting for averting frost formation altogether. This methodology utilizes the coating's passive repellent capabilities, while eliminating the dominant effect of thick deposited frost layers. The periodic heating approach takes advantage of lubricants with higher thermal conductivities, which effectively enhance heat transfer through the porous multiphase surface that forms the first line of defense against frosting.

Facile fabrication of iron-based superhydrophobic surfaces via electric corrosion without bath

Superhydrophobic surface is of wide application in the field of catalysis, lubrication, waterproof, biomedical materials, etc. The superhydrophobic surface based on hard metal is worth further study due to its advantages of high strength and wear resistance. This paper investigates the fabrication techniques towards superhydrophobic surface on carbon steel substrate via electric corrosion and studies the properties of as-prepared superhydrophobic surface. The hydrophobic properties were characterized by a water sliding angle (SA) and a water contact angle (CA) measured by the Surface tension instrument. A Scanning electron microscope was used to analyze the structure of the corrosion surface. The surface compositions were characterized by an Energy Dispersive Spectrum. The Electrochemical workstation was used to measure its anti-corrosion property. The anti-icing performance was characterized by a steam-freezing test in Environmental testing chamber. The SiC sandpaper and 500g weight were used to test the friction property. The research result shows that the superhydrophobic surface can be successfully fabricated by electrocorrosion on carbon steel substrate under appropriate process; the contact angle of the as-prepared superhydrophobic surface can be up to 152±0.5°, and the sliding angle is 1–2°; its anti-corrosion property, anti-icing performance and the friction property all show an excellent level. This method provides the possibility of industrialization of superhydrophobic surface based on iron substrate as it can prepare massive superhydrophobic surface quickly.

One-step fabrication of nickel nanocones by electrodeposition using CaCl2·2H2O as capping reagent

In this research, a method for the fabrication of nickel nanocones through the addition of CaCl2·2H2O to an electrodeposition solution was proposed. When electrodeposition was performed after CaCl2·2H2O addition, precipitation of the Ni ions onto the (200) crystal face was suppressed and anisotropic growth of the nickel electrodeposited structures was promoted. Sharper nanocones were produced with increasing concentration of CaCl2·2H2O added to the solution. Moreover, when temperature of the electrodeposition solutions approached 60°C, the apex angle of the nanostructures decreased. In addition, the nanocones produced were applied to superhydrophobic surface modification using a plasma-polymerized fluorocarbon (PPFC) coating. When the solution temperature was maintained at 60°C and the concentration of the added CaCl2·2H2O was 1.2M or higher, the fabricated samples showed superhydrophobic surface properties. The proposed nickel nanocone formation method can be applied to various industrial fields that require metal nanocones, including superhydrophobic surface modification.

Reduced Blood Coagulation on Roll-to-Roll, Shrink-Induced Superhydrophobic Plastics.

The unique antiwetting properties of superhydrophobic (SH) surfaces prevent the adhesion of water and bodily fluids, including blood, urine, and saliva. While typical manufacturable approaches to create SH surfaces rely on chemical and structural modifications, such approaches are expensive, require postprocessing, and are often not biocompatible. By contrast, it is demonstrated that purely structural SH features are easily formed using high throughput roll-to-roll (R2R) manufacturing by shrinking a prestressed thermoplastic with a thin, stiff layer of silver and calcium. These features are subsequently embossed into any commercially available and Food and Drug Administration (FDA)-approved plastic. The R2R SH surfaces have contact angles >150° and contact angle hysteresis <10°. Importantly, the surfaces minimize blood adhesion, leading to reduced blood coagulation without the need for anticoagulants. SH surfaces have >4200× reduction of blood residue area compared to the nonstructured controls of the same material. In addition, blood clotting is reduced >5× using whole blood directly from the patient. Furthermore, these surfaces can be easily configured into 3D shapes, as demonstrated with SH tubes. With the simple scale-up production and the eliminated need for anticoagulants to prevent clotting, the proposed conformable SH surfaces can be impactful for a wide range of medical tools, including catheters and microfluidic channels.

Geometrical Patterning of Super-Hydrophobic Biosensing Transistors Enables Space and Time Resolved Analysis of Biological Mixtures.

PEDOT:PSS is a conductive polymer that can be integrated into last generation Organic Electrochemical Transistor (OECT) devices for biological inspection, identification and analysis. While a variety of reports in literature demonstrated the chemical and biological sensitivity of these devices, still their ability in resolving complex mixtures remains controversial. Similar OECT devices display good time dynamics behavior but lack spatial resolution. In this work, we integrated PEDOT:PSS with patterns of super-hydrophobic pillars in which a finite number of those pillars is independently controlled for site-selective measurement of a solution. We obtained a multifunctional, hierarchical OECT device that bridges the micro- to the nano-scales for specific, combined time and space resolved analysis of the sample. Due to super-hydrophobic surface properties, the biological species in the drop are driven by convection, diffusion, and the externally applied electric field: the balance/unbalance between these forces will cause the molecules to be transported differently within its volume depending on particle size thus realizing a size-selective separation. Within this framework, the separation and identification of two different molecules, namely Cetyl Trimethyl Ammonium Bromid (CTAB) and adrenaline, in a biological mixture have been demonstrated, showing that geometrical control at the micro-nano scale impart unprecedented selectivity to the devices.

A facial approach combining photosensitive sol⿿gel with self-assembly method to fabricate superhydrophobic TiO2 films with patterned surface structure

Superhydrophobic TiO2 films with micro-patterned surface structure was prepared through a facial approach combining photosensitive sol⿿gel method with following surface modification by 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFOTCS). The patterned surface possessed quasi micro-lens array structure resembling processus mastoideus of lotus, leading to excellent hydrophobicity. The relationship between hydrophobic performance and the period of the micro-patterned TiO2 surface was investigated. The water contact angles (CAs) of micro-patterned TiO2 surface increased with the decrease of the periods, and the patterned surface with the lowest period of 0.83μm showed the highest CA of 163°. It suggests that this approach would offer an advantage to control the wettability properties of superhydrophobic surfaces by adjusting the fine pattern structure. Furthermore, the superhydrophobic state could be converted to the state of superhydrophilicity under ultraviolet (UV) illumination as a result of the photocatalytic decomposition of the PFOTCS monolayer on the micro-patterned TiO2 Surface.

Superhydrophobic surfaces based on self-organized TiO2-nanotubes

Abstract In this study, the interaction between a self-organized TiO 2 -nanotube layer formed by an anodizing procedure and a post-treatment with different water repellent coatings is investigated. Therefore, a conceptional model of these interactions is experimentally explored. Ti6Al4V samples are anodized using a fluoride-containing electrolyte. Using an anodizing voltage of 30 V, a homogeneous layer of TiO 2 -nanotubes with a pore diameter of 68.1 ± 3.8 nm and oxide layer thickness of 589.2 ± 23.7 nm could be generated on the surface. The anodization increases the surface area in comparison to the original state. To achieve water repellent surface properties the TiO 2 -nanotubes are functionalized by a selection of coating systems. Two different coating systems (coating 1: fluoroalkylsilane based; coating 2: perfluoropolyether based) are investigated within this study. By means of physical and chemical analyses both morphology and composition of the TiO 2 -nanotube surface are characterized. The scanning electron microscope (SEM) analyses illustrate that the tube morphology is not covered by the different coating systems. The chemical composition of the two different coatings is investigated by X-ray photoelectron spectroscopy (XPS). The results indicate that only the boundaries of the TiO 2 -nanotubes are covered by the coatings. However, the XPS results reveal that the different coatings do not fully infiltrate the tubular structures. Only the first nanometers (15–20 nm) are covered. To characterize and evaluate the superhydrophobic properties of the modified surfaces, the dynamic contact angles are measured. Each individual coating system shows a contact angle in excess of 150°. Surfaces that are simply degreased in an alkaline solution, without post-treatment with a water repellent coating show a contact angle of 21.9 ± 5.4°. Hence, it becomes evident that only the combination of a surface nanostructuring and a water repellent coating leads to superhydrophobic surface properties.