Superhydrophobic Self-cleaning Surfaces Research Articles

Development of self-cleaning superhydrophobic cotton fabric through silica/PDMS composite coating

The lotus effect informs that self-cleaning superhydrophobic surfaces can be obtained by creating rough surface structures and modifying them with chemicals that have low surface energy. Herein, the composite of hydrophobic silica nanoparticles (SNPs) and polydimethylsiloxane (PDMS) was deposited on cotton fabric by multiple dip cycles. At optimal condition, the agglomerated SNPs in PDMS produces a hierarchical rough surface, as a result the coated cotton fabric has revealed a water contact angle (WCA) of 158.41 ± 1.58° and 4° of sliding angle. Due to negligible water adhesion to a superhydrophobic surface, coated cotton fabric reveals excellent self-cleaning behavior, which was tested by dust particles, muddy water and tea droplets. Furthermore, coated cotton fabric sustains superhydrophobicity over the mechanical robustness tests including adhesive tape peeling test, sandpaper abrasion test, and ultrasonication. Therefore, such an approach may be applicable in textile industries for self-cleaning purposes.

Layer-by-layer fabrication of self-cleaning superhydrophobic surface made from Carboxymethylcellulose and ZnO quantum dots: A combined experimental and computational study

Non-health hazardous self-clean coating materials have fascinated and assumed immense significance from both academia and industries. Herein, we put forward a facile and efficient technique to design a self-clean coating material by employing selective organic (carboxymethylcellulose (CMC)) and inorganic (ZnO quantum dots (QDs)) fillers. Computed water repellence nature through interfacial solid-liquid-vapor interactions well compared with experimental outcomes. Interestingly, experimental water contact angle (WCA) (159.2 ± 2°) and simulated binding energy with aluminum (Al) substrate (−3044.12 kcal/mol) revealed the subtle balance of the excellent water repellence on one side while interfacial interactions confirmed sufficient adhesion with the substrate on the other side of the proposed coating. Fluoro functionalized CMC and ZnO QDs weaken the matrix interactions with water molecules by providing a much desirable combination of micro and nano-scale grooves to the exposed coating surface. The adopted technique for synthesis of such a superior coating is simple for application with commercially available materials and across surfaces in automobiles, biomedical instruments, and structural glasses.

Image analysis of PDMS/ZnO nanocomposite surfaces for optimized superhydrophobic and self-cleaning surface design

Optimized superhydrophobic and self-cleaning nanocomposite surfaces were obtained by spraying surface modified ZnO nanoparticles (NPs) onto PDMS, using octadecylphosphonic acid and octadecanethiol as hydrophobic modifiers. In this study, it is the first time to our knowledge that surface parameters such as topography, morphology, superhydrophobicity, and self-cleaning are correlated to particle surface distribution and agglomeration parameters obtained by image analysis. The topography, morphology, and wettability of the surfaces were analyzed using atomic force microscopy, scanning electron microscopy, static contact angle (SCA), and contact angle hysteresis measurements. Image analysis was performed using the new enhanced graphical user interface of a previously self-developed Matlab® algorithm. Both hydrophobization methodologies increased the NPs’ surface coverage and the hierarchical rough structure formation on the substrates, resulting in more homogenous superhydrophobic self-cleaning surfaces. A higher coated fraction and lower degree of interconnected uncoated PDMS paths are correlated to an increase in SCA. The combination of a higher agglomerates fraction, lower agglomerate radius, and lower distance between agglomerates obtained for the surfaces with hydrophobized ZnO-NPs rendered self-cleaning surfaces. The observed correlations increase the understanding of the design and modelling of superhydrophobic self-cleaning PDMS/ZnO nanocomposite surfaces for use in high voltage outdoor insulators.

Robust and UV-resistant multifunctional surface for self-cleaning, navigated oil absorption and oil/water separation

Traditional self-cleaning surfaces are always limited to defects of poor robustness, complex preparation process and single function. In order to address these problems, we have constructed a versatile self-cleaning superhydrophobic surface with reliable robustness, photodegradation, ease of manufacturing and magnetic navigation. This function integrated surface is conveniently fabricated by spraying a fluorosilane modified TiO2/Fe3O4 nanoparticles suspension onto an adhesive coated substrate, which was made by directly spraying the adhesive onto both soft and hard substrates to improve the robustness. During the volatilization of the ethanol-based suspension, due to the superior compatibility between the solvent and the adhesive, the functional nanoparticles are embedded in the adhesive layer and are firmly bound to it. This superhydrophobic surface shows significant robustness to damage tests and remain self-cleaning property even immersed in oil. By using this surface in oil-water separation, it indicates excellent separation efficiency for various oi-water mixtures. Meanwhile, the superhydrophobic surface shows excellent potential applications as a function integrated oil-water separation device contributed by the photodegradation property of TiO2 nanoparticles and magnetic navigation property from Fe3O4 nanoparticles.

Development of modern nanotecnologies and combined biotoxicity problems

Fast progress in modern nanotechnologies based on use of nanoparticles, nanofibers and nanotubes with different composition, shape and size allows elaboration of materials with superhigh strength, thermal and electric conductivity, acoustical and optical properties. Those materials are already widely used in industry, transportation, aerospace, marine and civil engineering, food processing and medicine. Some examples of nanoreinforces composites, superhydrophobic self-cleaning surfaces, nanodyes and suspensions of nanoparticles are described. The problem of uncontrolled accumulation of some types of nanoparticles in our cells and tissues is discussed within the concept of nanotoxicity. Since the history of permanent observation of human health in connection with nanodust accumulation in the atmosphere, waters and soils is not enough long, the detailed evidences must be documented, systematized and discussed.
 In this study a brief systematic review of literature on the biotoxicity problems caused by modern nanotechnologies is given. Production of the nanoparticles, nanofibers and nanotubes for industry, transportation, food processing, as well as utilization of the used materials which properties were modified by the nanotechnologies leads to permanent rise of the nanodust in the atmosphere, soils, river waters, lakes and the sea bottom. Their uncontrolled interaction with flora and fauna could be catastrophic for human health and life on the Earth. Promising ways for the problem solution and perspectives are discussed. Some own results on the protective action of nanodiamonds, silver and some other nanoparticles are presented. A vital necessity of an open access database on known types of nanoparticles, their use in the materials and documented influence of health of animals and humans is discussed

One-step fabrication of self-cleaning superhydrophobic surfaces: A combined experimental and molecular dynamics study

A simple one-step approach was developed to fabricate self-cleaning superhydrophobic surfaces by soaking the copper substrates into a mixed solution containing zinc oxide, stearic acid and polyphenylene sulfide molecules. X-ray photoelectron spectra confirmed that the hybridization between organic polymers and inorganic molecules happened. Further scanning electron microscopy analyses revealed that hierarchical structure was formed. A synergy between modified surface chemistry and surface roughness provided copper substrates with superhydrophobic properties and good self-cleaning performance. The fabricated superhydrophobic surfaces exhibited a water contact angle as high as ∼164.7 ± 1.5° and a sliding angle as low as ∼4.0 ± 0.8°. Molecular dynamics simulations were applied to study the adsorption behavior of stearic acid molecules on the zinc oxide crystal surfaces and the wettability of water molecules on as-prepared surfaces at molecular level. Molecular dynamics simulation results illustrated that the hydrophilic interactions between water droplets and zinc oxide surfaces involved hydrogen-bond formation, whereas the hydrophobic interaction between water molecules and superhydrophobic surfaces involved Van der Waals's interaction between the alkyl chains.

Fluorine-Free Durable Superhydrophobic Surfaces Fabricated via a Simple Spraying Method

Superhydrophobic self-cleaning surfaces are based on the surface micro/nanomorphologies and special chemical regents with low surface free energy. Such surface structures are highly susceptible to mechanical wear. Moreover, the use of fluorinated compounds is harmful to both human health and environment. Herein, a simple method based on spin coating and spraying is proposed to fabricate durable fluorine-free superhydrophobic surfaces. A glass substrate was coated with a layer of water-based paint via a simple spinning process. Due to the adhesive layer, modified SiO2 nanoparticles were strongly bonded to the glass substrate. After the coating was completely cured, the surfaces realized a water contact angle of 165.1° and maintained their water repellency after 50 abrasion cycles with sandpaper and 50 times of stripping test. No chemical reagents harmful for the environment were used.

Development of natural wax based durable superhydrophobic coatings

In the present work, environment friendly non-fluorinated superhydrophobic composite coatings were developed using naturally available carnauba wax from palm tree, Copernicia prunifera. The hierarchically structured coating morphologies were composed of micro-scale protrusions along with submicroscopic pores (non-composite) and protrusion (composite). The hierarchically structured carnauba wax coatings showed extreme de-wetting with superhydrophobicity (θ > 150°) and low hysteresis (<5°). In contrast, the other mineral and animal waxes showed mild hydrophobic behavior with high hysteresis (>10°) owing to flat morphology. These peculiarities in morphology and hence de-wetting properties are related with the viscosity differences. Further, the composite wax coating also showed high resilience to simulated rain and weathering due to high roughness and mechanical strength. The composite wax showed high ultimate strength (14 MPa) and elastic modulus (427 MPa) than the non-reinforced wax (8.8 MPa and 97 MPa, respectively). The results showed that composite carnauba wax coatings provide an effective and environment friendly solution for developing superhydrophobic self-cleaning surfaces.

A General Strategy towards Superhydrophobic Self-Cleaning and Anti-Corrosion Metallic Surfaces: An Example with Aluminum Alloy

Corrosion and contamination of metallic structures can cause loss of their functionality as well as aesthetic values. In this study, we describe a general strategy to prepare superhydrophobic self-cleaning and anti-corrosion surfaces for metallic structures. As a specific example, a superhydrophobic coating (SHC) on aluminum alloy was prepared by a simple etching combined with the decoration of a low-surface-energy material. The optimal SHC has a water contact angle (CA) at ~157.4° and a sliding angle (SA) of ~8.3° due to the synergy of binary hierarchical structures and chemical modification. The SHC showed low adhesion to dry contaminants and a series of liquids, displaying a good self-cleaning effect. The SHC maintained superhydrophobicity after exposure to air and humid condition at 60 °C for 7 days. In addition, the electrochemical measurements reveal that the anti-corrosion performance was enhanced by reducing the corrosion current density (Jcorr) by 1 order of magnitude and increasing the corrosion potential (Ecorr) by 0.527 V as compared to the bare Al alloy substrate after immersion for 168 h.

Potential of combating transmission of COVID-19 using novel self-cleaning superhydrophobic surfaces: part I-protection strategies against fomites.

According to the World Health Organisation, one of the main concerns of COVID-19 virus is its tenacity to spread from droplets that either land directly on a surface or are transmitted to a surface by an infected person. In this study, we report the potential of using superhydrophobic surfaces to combat the transmission and spread of fomites infected by COVID-19 virus strand. Fomites include clothes, utensils, furniture, regularly touched objects and personal protective equipment used by Health Care Workers to act as barriers against fluid transmission and/or fluid penetration. In this effort, we propose three strategies to combat the transmission and the spread of the virus: encapsulation, contamination suppression, and elimination. We believe that this can be achieved by the use of our recently developed superhydrophobic coating and regenerative monolith to encapsulate and suppress the virus. The newly developed superhydrophobic coating and monolith are scalable, economical, and facile with the monolith capable of regeneration. The elimination of the virus will be through the use of antiviral and antibacterial copper nanoparticles or dedicated copper surfaces.

Potential of combating transmission of COVID-19 using novel self-cleaning superhydrophobic surfaces: part II-thermal, chemical, and mechanical durability.

In part I, we identified encapsulation, contamination suppression, and virus elimination as our three governing strategies for developing surfaces to combat the transmission and spread of COVID-19. We showed that our recent superhydrophobic nanocomposites has the potential of encapsulating and suppressing the virus so as to limit its transmission and spread. In this study, we examine the durability of the newly developed surfaces when subjected to elevated temperature, chemical attack and mechanical damage in the form of abrasion and compressive load. Extensive tests were conducted to reveal the effect of these parameters on the surface performance. Three aspects of the work were accordingly examined. The first was concerned with controlled thermal stability tests in which the surfaces were subjected to elevated temperatures approaching 350 °C for silicone-based nanocomposites and 150 °C for epoxy-based nanocomposites. The second was concerned with subjecting the surfaces to alkaline and acidic solutions with pH concentrations ranging between 1 and 13. Finally, the third involved surface damage by abrasion tests. Our results show clearly that the newly developed superhydrophobic surfaces are capable of resisting the adverse effects of thermal and chemical attacks as well as mechanical abrasion owing to the excellent structural stability and mechanical properties of the constituents of the nanocomposite. Moreover, our superhydrophobic monolith demonstrated exceptional regenerative capabilities even after being subjected to damaging compressive stresses of up to 10 MPa.

A Durable and Self-Cleaning Superhydrophobic Surface Prepared by Precipitating Flower-Like Crystals on a Glass-Ceramic Surface.

Herein, a superhydrophobic surface with superior durability was fabricated on a glass-ceramic surface by crystallization, hydrofluoric acid (HF) etching, and surface grafting. The as-prepared glass-ceramic surface was composed of three-dimensional flower-like micro-clusters, which were self-assembled from numerous nanosheets. Such a dual-scale rough surface exhibited superhydrophobicity, with a water contact angle (WCA) of 170.3° ± 0.1° and a sliding angle (SA) of ~2° after grafting with 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (FAS-17). This can be attributed to the synergistic effect between the dual-scale structure and surface chemistry. Furthermore, this surface exhibited excellent self-cleaning properties, stability against strong acid and strong alkali corrosion, and anti-stripping properties.

Fabrication of Self-Cleaning Superhydrophobic Surfaces with Improved Corrosion Resistance on 6061 Aluminum Alloys.

Aluminum alloys are widely used, but they are prone to contamination or damage under harsh working environments. In this paper, a self-cleaning superhydrophobic aluminum alloy surface with good corrosion resistance was successfully fabricated via the combination of sand peening and electrochemical oxidation, and it was subsequently covered with a fluoroalkylsilane (FAS) film. The surface morphology, surface wettability, and corrosion resistance were investigated using a scanning electron microscope (SEM), an optical contact angle measurement, and an electrochemical workstation. The results show that binary rough structures and an FAS film with a low surface energy on the Al alloy surfaces confer good superhydrophobicity with a water contact angle of 167.5 ± 1.1° and a sliding angle of 2.5 ± 0.7°. Meanwhile, the potentiodynamic polarization curve shows that the corrosion potential has a positively shifted trend, and the corrosion current density decreases by three orders of magnitude compared with that of the original aluminum alloy sample. In addition, the chemical stability of the as-prepared superhydrophobic surface was evaluated by dripping test using solutions with different pH values for different immersion time. It indicates that the superhydrophobic surface could provide long-term corrosion protection for aluminum alloys. Consequently, the as-prepared superhydrophobic surface has excellent contamination resistance and self-cleaning efficacy, which are important for practical applications.

Superhydrophobic Self-cleaning Surfaces in Nature

A global interest was awarded to study the natural superhydrophobic surfaces since the description of the Lotus Effect by Barthlott and Neinhuis in 1997. Natural biomimetic surface merits of micro/nano-roughness, water contact ˃ 150°, sliding angles ˂10°, and minimized free-energy characteristics would motivate the dynamic fabrication of superhydrophobic surfaces. This critical review introduces an architectural panorama of numerous structural designs of natural superhydrophobic surfaces. Also, it discussed the fundamentals of self-cleaning and wetting theories to develop superhydrophobic structures. This progress review concentrates on superhydrophobic materials' applications for self-cleaning marine antifouling surfaces. It introduced an in-depth understanding of the structural design-superhydrophobic property relationship of the natural nano-wettable surfaces. It is technically first to shed light on the inner basics and platform for surface non-wettability and facilitates the way for design biomimetic self-cleaning antifouling surfaces.

Wood-based composite phase change materials with self-cleaning superhydrophobic surface for thermal energy storage

Form-stable composite phase change materials, as thermal energy storage technology, show great promise for reducing energy consumption and relieving current energy shortage problems. However, porous supporting materials and most phase change materials are hydrophilic and hygroscopic, which cause crack-formation at the interfaces between supporting materials and phase change materials and decrease in thermal energy storage capacity of composite phase change material in wet or humid environment. There are almost no reports concerning this topic. Herein, form-stable and superhydrophobic composite phase change materials are fabricated by spraying superhydrophobic coating on the surface of composite phase change materials, in which delignified wood acts as a supporting material to protect against liquid leakage of 1-tetradecanol. The superhydrophobic composite phase change materials possess large water contact angle of 155° and superhydrophobic stability at 20–100 °C and pH 3–12, which prevents supporting materials and phase change materials from contacting with moisture in wet environment. In addition, the superhydrophobic composite phase change materials exhibit large latent heat of fusion (125.40 J/g), 29.58 J/g higher than that of composite phase change materials without superhydrophobic coating in wet environment. Moreover, the superhydrophobic composite phase change materials possess excellent thermal reliability and stability, efficient solar-to-thermal energy conversion and self-cleaning property, which are potential in the application of advanced energy-related devices and systems for thermal energy storage in wet or humid environment.

Study of self-cleaning superhydrophobic surface based on titanium dioxide nanomaterial

Biological and chemical pollutants produced by man-made activities has been a serious worldwide concern. These pollutants will be embedded onto the exterior of a building where it will affect the appearance of the glass as façade of the building. The pollutants pose not only environmental issue but also significant health problems upon frequent exposure. Therefore, the research and development of an effective solution is essential to reduce the impact of these pollutants. Since nanotechnology has become one of the most significant discussion among researchers, self-cleaning based on nanomaterial could be the possible solution to these problems. Generally, self-cleaning properties refers to a surface that possess the ability to repel contaminants and can be divided into two types which are superhydrophobic and superhydrophilic. This study was focused to the development of superhydrophobic self-cleaning coating surface which offers more effective way of cleaning method at minimal consumption of energy and cost. In this study, a simple dip coating method for the fabrication of superhydrophobic surface onto glass substrate has been developed by using titanium dioxide nanomaterial. This research work was divided into three phases: synthesis, characterization and testing. In the first phase, synthesis of titanium dioxide coating solution was done by mixing methanol with water at the ratio of 0.3:0.7 before the addition of titanium dioxide nanoparticles into the mixture. The coating film was characterized by its physical appearance, scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FT-IR). Lastly the testing of the prepared coating surface was performed to investigate its self-cleaning ability against contaminants. It was concluded that the prepared coated glass substrate showed superhydrophobic properties which has great potential in executing self-cleaning action against pollutants. The developed dip coating method was simple, fast, able to control coating thickness and reproducible.

Eco-friendly preparation of superhydrophobic copper surfaces for oil/water separation

Copper surfaces decorated with micro- and nanostructures can be applied to self-cleaning superhydrophobic surfaces, energy devices and pollution remediation. Wet chemical etching is the classical strategy to grow micro- and nanostructures on copper surfaces. However, such processes usually produce hazardous chemical waste. Here, we present a one-step, eco-friendly and solvent-less alternative to conventional wet chemical methods. Our method involves the use of volatile compounds released from yolk and egg white of boiled chicken eggs as green oxidants for the growth of micro- and nanostructures on the copper surface. After treatment with egg components, copper substrates were modified with stearic acid to make them superhydrophobic. Stearic acid-coated substrates had a contact angle of about 150°, thus confirming their superhydrophobicity. Similar to copper sheets, superhydrophobic copper meshes were also prepared and used for the selective removal of oil from oil/water mixture. Results show that our method produces a wide range of shapes including hollow and solid spheres, nanoflakes, nanoneedles and dome shapes. We found that H2S, the major volatile compound emitted by eggs, is mainly responsible for the growth of microstructures. Therefore, our method can be used both to capture H2S and to design specific surfaces. The mesh exhibited excellent separation efficiency, more than 95% for the organic solvents. A key advantage of our method is that it can be performed also using waste and expired eggs.

Cicada-inspired self-cleaning superhydrophobic surfaces

Abstract Water-based heat transfer surfaces in HVAC&amp;R (Heating, Ventilation, Air Conditioning and Refrigeration) systems have biofouling or microbial growth which might not only impose adverse effects to public health but also deteriorate the heat transfer performance of surfaces. In nature, a living creature is surviving while being exposed to formidable condition such as bacteria and fungi. Cicada wing has known to have multi-functional wings exhibiting nonwetting, and anti-microbial due to nanoscale pillars and biochemicals to achieve non-wetting characteristics. We conducted experiments on cicada wings and cicada-inspired engineered superhydrophobic samples to show that droplets jumping from superhydrophobic wings could carry away individual particles (~100 µm) or cluster of particles. This study was investigated on self-cleaning mechanism using jumping droplet condensation and elucidate further antifouling characteristics of non-wetting heat transfer surfaces.

Highly Sensitive Dissolved Oxygen Sensor with a Sustainable Antifouling, Antiabrasion, and Self-Cleaning Superhydrophobic Surface.

Long-term sensing of dissolved oxygen in aqueous solution always suffers from adherence of algae, barnacles, and clams and formation of biofilms on the sensor surface, which strongly influences the diffusion of oxygen into the sensor film. Metabolism of these adhered species consumes oxygen and causes bias on sensor readout. Therefore, commercial sensors are equipped with mechanical brushes to constantly clean the sensor surface, which significantly complicates the sensor design and causes damage to the sensor surface. In addition, extra energy storage and mechanical structures are required, which make an optical sensor bulky and limit its service life. We have developed a robust and highly sensitive dissolved oxygen sensor with good mechanical stability and self-cleaning capability. The sensor was fabricated by doping oxygen-sensitive probe PtTFPP with superhydrophobic coating. The 3 to 5 nm micro/nanostructures formed from silica sol were solidified with silicone resin, which endowed the sensor film with excellent mechanical stability. The sensor film exhibits antifouling, antiabrasion, and self-cleaning properties. There is no need of mechanical brushes to clean sensor surfaces, which greatly simplifies the sensor design. Owing to the porous structure, the sensor shows high quenchability, with I0/I100 of 77. All these features guarantee that the sensor could be used in harsh and dirty conditions for long-term monitoring of dissolved oxygen concentration.

Ultra-black superhydrophobic multilayer broadband optical absorber

ABSTRACTPerfect absorbers have many application in different fields including photovoltaic, antireflective coatings, defence, energy harvesting and emissivity control. Here, we have designed and fabricated a near perfect optical broadband absorber, having absorption capacity of more than 99% from 300 to 2000 nm wavelength range. Textured Silicon wafer is used as base layer to fabricate multilayered optically graded assembly, while carbon nano tubes layer is used as a top layer for impedance matching. Subsequent layers are selected as per increasing refractive index values and varying micro/nano features for efficient light trapping. Moreover, multilayered assembly shows contact angle of more than 160° with roll-off angle of less than 5° which shows that it is not only a near perfect absorber but also a self-cleaning superhydrophobic surface. Conclusively, the properties of multilayered optically graded assembly imparts attractive applications of this superabsorber in the field of photovoltaic.