Anti-wetting Surfaces Research Articles

The bubble electrostatic spraying a new technology for fabrication of superhydrophobic nanofiber membranes

Researchers are excited about the latest advances in the long needle electrospinning and the bubble electrospinning, which have triggered wide-spread concern. This paper offers a new angle for modifying both methods, the former is developed into a modified one with an auxiliary helix needle, which is used for fabrication of super-hydrophobic polyvinylidene difluoride-copolypolyhexafluoropropylene nanofiber membrane (PH-E membrane for short), and the latter is extended to a bubble electrostatic spaying, which is used for spraying PDMS microspheres on the PH-E membrane surface, and the resultant membrane is called as the PDMS-PH-E membrane. Both membranes hydrophobicity, surface roughness, porosity, and wetting property are measured and compared, and the drop impact dynamical property of the PDMS-PH-E membrane has opened the path for a new way to design of self-clearing and anti-fouling and anti-wetting surfaces. Far-reaching applications of the membranes include energy harvesting devices and sensors. We anticipate this article to be a starting point for more sophisticated study of the bubble electrostatic spaying and PDMS-PH-E membrane advanced applications.

Towards development of sustainable metallic superhydrophobic materials

In the present study, an environment friendly, scalable, and cost-effective approach is proposed, combining thermo-mechanical and microwave-assisted hydrothermal processing techniques, resulting in the augmentation of nanoscale structures on metallic surfaces. The thermo-mechanical processing of the aluminium alloy performed at different strain rates resulted in significant grain refinement (∼ 1 µm for processed and ∼ 30 µm for unprocessed samples). After hydrothermal treatment, highly dense and networked nanostructured morphology was evoked by refined grains. Post silanization, samples exhibited high contact angle (θs > 155°) with a low tilt angle (θt < 10°) and CAH (< 5°). The low adhesion with water (∼ 16 µN) for the processed sample (∼ 50 μN for unprocessed) with utmost refined grains is attributed to the high interfacial energy of Cassie state (EC−B>1.0J) due to effective entrapment of air. The processed samples were highly de-wetting and mechanically resilient owing to the strong negative capillary pressure (PC > 1100 kPa) generated by highly dense networked nanostructures. As a result of thermo-mechanical processing, samples displayed higher resilience to abrasion, simulated rain, and long-term immersion tests with low tilt angles (θt < 10°) and CAH (< 5 °). This paper provides a practical approach to achieve anti-wetting surfaces in various industrial settings.

Micro-Nano Dual-Scale All-Polymer particles for construction of superwetting surface with controllable wettability and Hot-Water-Repellency

Based on the principle of like dissolves like, the reactive oil phases, consisted of monomers, cross-linked monomers, fluorinated monomers and initiators, were absorbed into cavity of hollow polymer microspheres to obtain reactants-loaded particles in the stirring condition at room temperature; in the effect of heating, the internal reactants could polymerize to glut its core cavity and shell pores, hereupon a quantity of nanoscale copolymer surface protrusions were produced; ultimately, the micro-nano dual-scale all-polymer particles (MNDPs) were integrally formed by dispensing with any chemical modification, which physico-chemical properties could be regularly controlled by utilizing alteration of absorbed fluorinated monomers types. In view of the special wetting behaviour for various temperature water droplets to anti-wetting MNDPs-based surfaces, the superhydrophobic fabric, applicable to anti-fouling and self-cleaning, was prepared by coating with short fluoroalkyl chain-containing MNDPs; additionally, near-spherical organic fluorine MNDPs were in a position to build hot-water-repellent fabric, which preventing high-temperature droplets from infiltrating or spreading. Furthermore, mathematical analysis of the hot-water-repellent surface was carried out to establish its theoretical wetting model. We believe that the constructed MNDPs, equipped with controlled microstructure and chemical composition, are conducive to realizing the fine regulation of wettability for superwetting MNDPs-coated surface; by means of discussion for static and dynamic contact status of different temperature droplets onto anti-wetting-adjustable MNDPs-based surface, functional coated fabric, that exhibited superhydrophobicity or hot-water-repellency, would be formed, it has favourable research value on high durable non-wetted flexible textile coating; based on mathematical model of wetting state for hot-water-repellent surface, the mechanism, as regards effective blocking thermal-droplet at its contact area from wetting, is clearly clarified, which is beneficial to develop special industrial textiles with high-temperature water proofing.

Fabrication of zirconium based coordination polymers for fluorine/silane free superhydrophobic coatings

Biomimicking of superhydrophobic surfaces popularly known as ‘Lotus effect’ has been widely researched as a solution for fabrication of anti-wetting, self-cleaning and functional surfaces. With the ban on the use of toxic perfluorinated materials by Environmental Protection Agency, the alternatives for fabrication of such materials with excellent stability in various conditions has been an area of great research interest. In this work, this property was realized by exploring the imine chemistry as a potential route to fabricate superhydrophobic materials through the modification of the amine group present in aromatic dicarboxylic acids with non-toxic long chain aldehydes followed by the formation of highly stable coordination polymers by reaction with biocompatible metal ions in aqueous conditions. A polymeric base coat served as a means for improving the fixation of the superhydrophobic material on the coated surfaces. The water contact angle of the coated materials increased to 155° with water sliding angles less than 5° making the surface superhydrophobic. Also, a procedure for directed growth of superhydrophobic particles on cellulose mesh was developed and its mechanism was evaluated using FTIR and XPS studies. The prepared material was extremely tolerant towards the action of acids and alkali and was very stable up to 350 °C. Additionally, a simple spray coating technique was demonstrated to overcome the loss of intended property due to the abrasion caused by prolonged use. The prepared material also exhibited good anti-wetting and anti-adhesion properties making this a suitable non-toxic alternative for fabrication of self-cleaning materials and superhydrophobic finish.

Ferromagnetic Liquid Droplet on a Superhydrophobic Surface for the Transduction of Mechanical Energy to Electricity Based on Electromagnetic Induction.

Ferromagnetic liquids undergo reversible magnetization changes upon varying external magnetic field levels. The movement of ferromagnetic liquid droplets across a coil under an external magnetic field holds promise as an energy transducer from mechanical force to electricity; however, it suffers from an adhesive issue between the ferromagnetic liquid and the solid pedestal. We introduce a superhydrophobic support that uses antiwetting surfaces to remarkably reduce adhesion during the movement of ferromagnetic liquid droplets. Maxwell numerical simulation was utilized to analyze the working mechanism and improve further electrical outputs. By controlling the droplet size, the strength of the magnetic bottom and the tilting speed of the test condition, we generated a ferromagnetic liquid droplet-based superhydrophobic magnetoelectric energy transducer (FLD-SMET) that can convert vibrational energy to electricity. When a 100 μL ferromagnetic liquid droplet was used for FLD-SMET under a 13 mT magnetic field, an electrical voltage response of 280 μV and electrical current response of ∼7.5 μA were generated using a shaking machine with a tilting speed of 9.5°/s. We thus show that such a device can serve as a self-powered light buoy floating on a water surface. Our study presents an applied concept for the design of droplet-based energy harvesters to convert surrounding vibrational energy to electricity.

Droplet Dynamics and Freezing Delay on Nanoporous Microstructured Surfaces at Condensing Environment

Superhydrophobic surfaces have aroused great interest for being promising candidates in applications such as self-cleaning, anti-icing, and corrosion resistance. However, most of the superhydrophobic surfaces lose their anti-wettability in low surface temperature and high humidity. The loss of superhydrophobicity by condensed liquid is a very common practical incident, yet to be understood properly. Here we report the wettability of the superhydrophobic nanoporous surfaces in condensation and freezing environments. Various structured surfaces fabricated with carbon nanotubes (CNT) and coated by an ultrathin, conformal, and low surface energy layer of poly (1H,1H,2H,2H-perfluorodecylacrylate) (pPFDA) are exploited in humid conditions. Droplet impact dynamics, condensate characteristics, and freezing time delays are investigated on the CNT micropillars with various geometries along with the CNT forest and two commercially available anti-wetting coatings, NeverWet and WX2100. Nanoporous microstructured CNT pillars with the favorable topological configuration demonstrated complete droplet bouncing, significant freezing delays, and considerable durability during several icing/de-icing cycles. This study provides an understanding on the preferable geometry of the highly porous CNT micropillars for retaining hydrophobicity and preventing ice formation, which is of practical importance for the rational development of anti-wetting surfaces and their applications in low temperatures and humid conditions.

Biomimetic Slippery PDMS Film with Papillae-Like Microstructures for Antifogging and Self-Cleaning

Transparent materials with antifogging and self-cleaning ability are of extreme significance for utilization in outdoor solar cell devices to alleviate the performance loss and maintenance costs. Herein, with inspiration from the anti-wetting surfaces in nature, regular papillae-like microstructure arrays (PMAs) inspired by lotus leaves were designed via a common UV lithography combined with a soft replication. Subsequently, the biomimetic slippery polydimethylsiloxane (PDMS) film (BSPF) inspired by the pitcher plant was fabricated successfully by infusing with hydrophobic liquid lubricant. The resultant surface has hydrophobic surface chemistry, a slippery interface, PMAs structure. The wettability, optical characteristic, antifogging property and self-cleaning ability of the PMAs-based BSPF were characterized experimentally. The film displays excellent optical transmittance, antireflection, antifogging, and self-cleaning properties, which is superior to the flat PDMS film (FPF). Remarkably, an average reflection of ∼11.3% in the FPF was reduced to ∼8.9% of the BSPF. In addition, after gradient spray test for 120 s, the antifogging efficiency was close to 100% for the BSPF surface in comparison with the flat PDMS film (FPF), biomimetic PDMS film (BPF) and flat slippery PDMS film (FSPF) (35%, 70% and 85%). Furthermore, we also discovered that the BSPF surface exhibited a better self-cleaning performance toward a variety of liquids than solid dust.

Superhydrophobic surfaces with embedded protective sawtooth ring microstructures

Despite the achievements in superhydrophobic surface obtained via geometrically uniform micro-nano scale surface texture, the mechanical robustness of superhydrophobic surfaces remains a challenge where service conditions involve solid-body contact. We introduce a unique sawtooth-ring protective microstructure design as an integrated shield for underlying superhydrophobic micro-pillar arrays. The sawtooth-ring design minimizes the areal occupancy of the surface and merges directly with the anti-wetting micro-pillar array, giving rise to an integrated micro-structure. The integrated surface micro-structure has been fabricated in an intermediate polymer mold via an innovative two-step thermal nanoimprinting process. The integrated sawtooth ring and micro-pillar array have been replicated on UV polymerized resin, thermally cured polydimethylsiloxane (PDMS) and solvent cast polycarbonate (PC) using the intermediate polymer mold. The post-compression water contact angle results demonstrate the reliance of the sawtooth ring design on the underlying material’s resistance to plastic yield. This reliance can be met either in the form of relatively high yield stress, as is the case with PC or in the form of a large elastic strain tolerance, as is the case with PDMS. In both cases, compressive testing of sawtooth ring protected anti-wetting surfaces resulted in the retention of the Cassie-Baxter 2-component wetting state and negligible degradation in their respective contact angles.

Liquid-Superrepellent Bioinspired Fibrillar Adhesives.

Bioinspired elastomeric fibrillar surfaces have significant potential as reversible dry adhesives, but their adhesion performance is sensitive to the presence of liquids at the contact interface. Like their models in nature, many artificial mimics can effectively repel water, but fail when low-surface-tension liquids are introduced at the contact interface. A bioinspired fibrillar adhesive surface that is liquid-superrepellent even toward ultralow-surface-tension liquids while retaining its adhesive properties is proposed herein. This surface combines the effective adhesion principle of mushroom-shaped fibrillar arrays with liquid repellency based on double re-entrant fibril tip geometry. The adhesion performance of the proposed microfibril structures is retained even when low-surface-tension liquids are added to the contact interface. The extreme liquid repellency enables real-world applications of fibrillar adhesives for surfaces covered with water, oil, and other liquids. Moreover, fully elastomeric liquid-superrepellent surfaces are mechanically not brittle, highly robust against physical contact, and highly deformable and stretchable, which can increase the real-world uses of such antiwetting surfaces.

Review on Blueprint of Designing Anti-Wetting Polymeric Membrane Surfaces for Enhanced Membrane Distillation Performance.

Recently, membrane distillation (MD) has emerged as a versatile technology for treating saline water and industrial wastewater. However, the long-term use of MD wets the polymeric membrane and prevents the membrane from working as a semi-permeable barrier. Currently, the concept of antiwetting interfaces has been utilized for reducing the wetting issue of MD. This review paper discusses the fundamentals and roles of surface energy and hierarchical structures on both the hydrophobic characteristics and wetting tolerance of MD membranes. Designing stable antiwetting interfaces with their basic working principle is illustrated with high scientific discussions. The capability of antiwetting surfaces in terms of their self-cleaning properties has also been demonstrated. This comprehensive review paper can be utilized as the fundamental basis for developing antiwetting surfaces to minimize fouling, as well as the wetting issue in the MD process.

Interaction of Surface Energy Components between Solid and Liquid on Wettability, and Its Application to Textile Anti-Wetting Finish.

With various options of anti-wetting finish methods, this study intends to provide basic information that can be applied in selecting a relevant anti-wetting chemical to grant protection from spreading of liquids with different surface energy profiles. With such an aim, the anti-wetting effectiveness of fluorinated coating and silane coating was investigated for liquids having different surface energy components, water (WA), methylene iodide (MI) and formamide (FA). The wetting thermodynamics was experimentally investigated by analyzing dispersive and polar component surface energies of solids and liquids. The role of surface roughness in wettability was examined for fibrous nonwoven substrates that have varied surface roughness. The presence of roughness enhanced the anti-wetting performance of the anti-wetting treated surfaces. While the effectiveness of different anti-wetting treatments was varied depending on the liquid polarities, the distinction of different treatments was less apparent for the roughened fibrous surfaces than the film surfaces. This study provides experimental validation of wetting thermodynamics and the practical interpretation of anti-wetting finishing.

Superamphiphobic Cu/CuO Micropillar Arrays with High Repellency Towards Liquids of Extremely High Viscosity and Low Surface Tension

For almost all the research of super anti-wetting surfaces, pure liquids like water and n-hexadecane are used as the probes. However, liquids of diverse compositions are used in academic research, industrial production and our daily life. Obviously, the liquid repellency of super anti-wetting coatings is highly dependent on properties of the liquids. Here, we report the first superamphiphobic surface with high repellency towards liquids of extremely high viscosity and low surface tension. The surfaces were prepared by constructing a hierarchical micro-/nanostructure on the Cu micropillar arrays followed by modification with perfluorosilane. The surfaces are superamphiphobic towards the liquids with extremely high viscosity and low surface tension because of (i) the micro-/nanostructured surface composed of micropillars with proper pillar distance and CuO nano-flowers, and (ii) the abundant perfluorodecyl groups on the surface. The contact angles, sliding angles, apparent contact line at the solid-liquid interface and adhesion forces are the end products of micropillar distance, viscosity and surface tension. Smaller micropillar distance, higher viscosity and higher surface tension contribute to reducing the adhesion force. We in situ observed the process of microcapillary bridge rupture for the first time using highly viscous liquids. We also successfully reduced the adhesion forces and enhanced the average rolling velocity of liquids with extremely high viscosity and low surface tension by regulating the micropillar distance.

Effect of superamphiphobic macrotextures on dynamics of viscous liquid droplets

The ability of hydrophobic surfaces to repel impinging liquid droplets is important in applications ranging from self-cleaning of solar panels to avoiding ice formation in freezing rain environments. In quest of maximizing water repellency, modification of droplet dynamics and subsequent reduction of contact time have been achieved by incorporating macrotexture on the superhydrophobic surfaces. However, the dynamics of low temperature water, and other viscous liquid droplets impacting anti-wetting surfaces with macrotextures is not well explored. Here, we investigate the effect of viscosity on the bouncing dynamics of liquid droplets impacting macrotextured superamphiphobic surfaces using various glycerol-water mixtures as model liquids at different impacting conditions. We demonstrate that the changes of reduction in contact times by macrotextures due to the increasing viscosity are in opposite trends at low and at high impact velocities. Since macrotexture executes substantial contact time reduction for the droplets which exhibit splitting after the impact, a preliminary model for predicting the minimum impact velocity to observe droplet splitting by macrotexture is proposed considering the important parameters of an impinging droplet along with the surface characteristics and the macrotexture size. This work aims to provide an insight on several possible outcomes of viscous droplets impacting on the macrotextured surfaces and a model that will help to design the desired superamphiphobic surfaces capable of exhibiting reduced contact time and enhanced repellency of low-temperature water droplets (such as freezing rain) and other viscous liquids (such as oils) under different impacting conditions.

Transparent, fluorine-free, heat-resistant, water repellent coating by infusing slippery silicone oil on polysiloxane nanofilament layers prepared by gas phase reaction of n-propyltrichlorosilane and methyltrichlorosilane

The subject of slippery liquid-infused porous surfaces (SLIPS) have attracted many researchers due to their excellent liquid repellent properties and widespread usage in many fields such as anti-wetting, anti-icing, omniphobic, self-cleaning and anti-biofouling surfaces. Choosing an environmentally friendly, durable lubricant and easy-clean preparation method is very important when designing SLIPS. In this work, fluorine-free, transparent, water and oil repellent slippery liquid-infused porous surfaces (SLIPS) were successfully obtained via infusing silicone oil, which is a fluorine free and durable liquid, onto superhydrophobic polysiloxane nanofilament surfaces. Polysiloxane nanofilaments were prepared on activated glass slides by an easy, clean and environmentally friendly gas phase reactions of n-propyltrichlorosilane (n-PTCS) for the first time and also methyltrichlorosilane (MTCS) and the results were compared. Advancing contact angles of the n-PTCS based polysiloxane nanofilament layers were between 143 and 172° and contact angle hysteresis as low as 7° was obtained on these surfaces by altering the relative humidity and reaction time during filament growth. Then, transparent SLIPS were obtained by infusing silicone oil into polysiloxane filament layers of MTCS and n-PTCS. The obtained SLIPS shows excellent liquid repellency, very low contact angle hysteresis for long durations and maintains its water repellency after heating up to 80 °C for 6 days and also after exposure to continuous water droplet flow up to 2 h.

Biomimicking wetting properties of spider web from Linothele megatheloides with electrospun fibers

In this study, we showed a simple approach to biomimic the wetting properties of spider webs, which can be mainly attributed to the geometry of fibers. We created biomimetic fibers using electrospun polyvinylidene fluoride (PVDF) with a wrinkled surface similar to the morphology of spider silk bundles produced by Linothele megatheloides. Without any chemical modification and copying the silk bundles geometry, we successfully translated the similar hydrophobic properties to an electrospun network of fibers. The novelty of this approach lays in obtaining similar macroscale roughness parameters, responsible here for wetting contact angles, due to the substitution of spider silk bundles with individual wrinkled electrospun fibers. The presented methods open new creative solutions for manufacturing anti-wetting surfaces.

Super Antiwetting Surfaces for Mitigating Drag-Out of Deep Eutectic Solvents.

Deep eutectic solvents (DESs) are, at room temperature, about dozens to hundreds of times more viscous than water, which brings pretty thick residues on solid surfaces, for example, causing drag-out and weight loss in the transfer process. Unfortunately, until now little work had been done for solving this knotty problem. In this study, the super antiwetting surface, i.e., super-DES-phobic surface (defined as DES contact angle > 150°) is proposed and fabricated successfully by a facile coating technique. Hierarchical silver dendrites on copper foam substrate provide effective dual-roughness surfaces showing stable superDESphobicity. The superDESphobic surface can repel the DESs and their derived solutions even under elevated temperature of about 120 °C and the impact attack of drops. It is also found that the superDESphobic surface can significantly delay the DESs freezing and reduce the adhesion strength of the frozen DESs. Interestingly, the superDESphobic surface can be applied as an effective tool for gauging the density of DES using an ∼2 μL droplet in virtue of its super antiwetting property. The super antiwetting surfaces show promise for potential applications in DES self-cleaning and antifreezing.

Bioinspired Interfaces with Superwettability: From Materials to Chemistry.

Superwettability is a special case of the wetting phenomenon among liquids, gases, and solids. The superhydrophobic/superhydrophilic effect discovered initially has undergone a century of development based on materials science and biomimetics. With the rapid development of research on anti-wetting materials, superoleophobic/superoleophilic surfaces have been fabricated to repel organic liquids besides water. Further studies of underwater superoleophobic/superoleophilic/superaerophobic/superaerophilic materials provide an alternative way to fabricate anti-wetting surfaces rather than lowering the surface energy. Owing to a series of efforts on the studying of extreme wettabilities, a mature superwettability system gradually evolved and has since become a vibrant area of active research, covering topics of superhydrophobicity/superhydrophilicity, superoleophobicity/superoleophilicity in gas or under liquid, superaerophobicity/superaerophilicity under liquid, and combinations of these states. The kinetic study of the superwettability system includes statics and dynamics, while the studied material structures range from traditional two-dimensional materials to three-dimensional, one-dimensional, and zero-dimensional materials. Furthermore, the wetting liquids range from water to oil, aqueous solutions, and ionic liquids, as well as liquid crystals and other types of liquids. The wetting conditions extend over a wide range of temperatures, pressures, and other external fields. With the development of this series of research, many new theories and functional interfacial materials have been fabricated, including self-cleaning textiles, oil/water separation systems, and water collection systems, and some of these have already been applied in industry. Moreover, the study of superwettability has also introduced many new phenomena and principles to the field of interfacial chemistry that display its vast potential in both materials and chemistry. The present Perspective aims to summarize the most recent research on these materials and their interfacial chemistry. An overview of novel materials in superwettability systems and interfacial materials is presented. Specifically, the evolution of superwettable materials will be introduced, and the fundamental rules for building these superwetting materials will be discussed, followed by a summary of recent progress in the application of superwettable materials to alter the behaviors of chemical reactants and products. Specific emphasis is placed on recent strategies that exploit superwettable materials to influence the performance of traditional chemical reactions and their unique contributions to chemistry, including the effective collection of reaction products, unique growth models of precipitates, and a simple strategy for the alignment/assembly of nanoscale building blocks. Finally, a short perspective is provided on the potential for future developments in the field.

Single etch fabrication and characterization of robust nanoparticle tipped bi-level superhydrophobic surfaces

Though hierarchical roughness gives best anti-wetting surfaces, they fail even under small mechanical stresses. In contrast dual level surfaces fabricated using a single etch step provides robust superhydrophobicity.

Stable slippery liquid-infused anti-wetting surface at high temperatures

Recently developed slippery liquid-infused surfaces (LIS) offer a new approach to construct anti-wetting surfaces due to their excellent repellence of various liquids.

Water-Repellent Properties of Superhydrophobic and Lubricant-Infused "Slippery" Surfaces: A Brief Study on the Functions and Applications.

Bioinspired water-repellent materials offer a wealth of opportunities to solve scientific and technological issues. Lotus-leaf and pitcher plants represent two types of antiwetting surfaces, i.e., superhydrophobic and lubricant-infused "slippery" surfaces. Here we investigate the functions and applications of those two types of interfacial materials. The superhydrophobic surface was fabricated on the basis of a hydrophobic fumed silica nanoparticle/poly(dimethylsiloxane) composite layer, and the lubricant-infused "slippery" surface was prepared on the basis of silicone oil infusion. The fabrication, characteristics, and functions of both substrates were studied, including the wettability, transparency, adhesive force, dynamic droplet impact, antifogging, self-cleaning ability, etc. The advantages and disadvantages of the surfaces were briefly discussed, indicating the most suitable applications of the antiwetting materials. This contribution is aimed at providing meaningful information on how to select water-repellent substrates to solve the scientific and practical issues, which can also stimulate new thinking for the development of antiwetting interfacial materials.