Superhydrophobic surfaces by electrochemical processes

Thermal Processing of Silicones for Green, Scalable, and Healable Superhydrophobic Coatings

The thermal degradation of silicones is exploited and engineered to make super-hydrophobic coatings that are scalable, healable, and ecofriendly for various outdoor applications. The coatings can be generated and regenerated at the rate of 1 m(2) min(-1) using premixed flames, adhere to a variety of substrates, and tolerate foot traffic (>1000 steps) after moderate wear and healing.

Recent Advances in TiO2 -Based Nanostructured Surfaces with Controllable Wettability and Adhesion

Bioinspired surfaces with special wettability and adhesion have attracted great interest in both fundamental research and industry applications. Various kinds of special wetting surfaces have been constructed by adjusting the topographical structure and chemical composition. Here, recent progress of the artificial superhydrophobic surfaces with high contrast in solid/liquid adhesion has been reviewed, with a focus on the bioinspired construction and applications of one-dimensional (1D) TiO2-based surfaces. In addition, the significant applications related to artificial super-wetting/antiwetting TiO2-based structure surfaces with controllable adhesion are summarized, e.g., self-cleaning, friction reduction, anti-fogging/icing, microfluidic manipulation, fog/water collection, oil/water separation, anti-bioadhesion, and micro-templates for patterning. Finally, the current challenges and future prospects of this renascent and rapidly developing field, especially with regard to 1D TiO2-based surfaces with special wettability and adhesion, are proposed and discussed.

Recent Advances in Superhydrophobic Electrodeposits

In this review, we present an extensive summary of research on superhydrophobic electrodeposits reported in the literature over the past decade. As a synthesis technique, electrodeposition is a simple and scalable process to produce non-wetting metal surfaces. There are three main categories of superhydrophobic surfaces made by electrodeposition: (i) electrodeposits that are inherently non-wetting due to hierarchical roughness generated from the process; (ii) electrodeposits with plated surface roughness that are further modified with low surface energy material; (iii) composite electrodeposits with co-deposited inert and hydrophobic particles. A recently developed strategy to improve the durability during the application of superhydrophobic electrodeposits by controlling the microstructure of the metal matrix and the co-deposition of hydrophobic ceramic particles will also be addressed.

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.

Spontaneous, Phase-Separation Induced Surface Roughness: A New Method to Design Parahydrophobic Polymer Coatings with Rose Petal-like Morphology

While the development of polymer coatings with controlled surface topography is a growing research topic, a fabrication method that does not rely on lengthy processing times, bulk solvent solution, or secondary functionalization has yet to be identified. This study presents a facile, rapid, in situ method to develop parahydrophobic coatings based on phase separation during photopolymerization. A comonomer resin of ethylene glycol diacrylate (EGDA) and 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) is modified with a thermoplastic additive (PVDF) to induce phase separation during polymerization. If applied to a glass substrate and photopolymerized, the EGDA/PFDA copolymer forms a homogeneous network with a single glass transition temperature (T(g)) and slight hydrophobicity (θ(w) ∼ 114°). When the resin is modified with PVDF, phase separation occurs during photopolymerization producing a heterogeneous network with two T(g) values. The phase separation causes differences in composition and cross-link density within the network, which leads to local variations in polymerization shrinkage across the nonconstrained material interface. Domains with higher cross-link densities shrink and contract toward the bulk material more dramatically, permitting the formation of rough surfaces with submicron sized spheres enriched in PVDF dispersed in a continuous matrix of EGDA/PFDA copolymer. Both the surface roughness and hydrophobic components in the resin render these surfaces parahydrophobic with θ(w) ∼ 150°, high water adhesion, and a similar morphology to rose petals observed in nature.

A Superhydrophobic Surface Templated by Protein Self-Assembly and Emerging Application toward Protein Crystallization

A proteinaceous superhydrophobic material for facile protein crystallization is reported. The lysozyme phase transition is rationally manipulated to form a reliable superhydrophobic coating on virtually arbitrary material surfaces with good thermostability and mechanical robustness. Such a surface exhibits a fascinating capability to drive protein crystallization, and the protein crystal array can be facilitated in a large area at an ultralow protein concentration.

Light-Emitting Diodes with Hierarchical and Multifunctional Surface Structures for High Light Extraction and an Antifouling Effect

Bioinspired hierarchical structures on the surface of vertical light-emitting diodes (VLEDs) are demonstrated by combining a self-assembled dip-coating process and nanopatterning transfer method using thermal release tape. This versatile surface structure can efficiently reduce the total internal reflection and add functions, such as superhydrophobicity and high oleophobicity, to achieve an antifouling effect for VLEDs.

Solution chemistry-based nano-structuring of copper dendrites for efficient use in catalysis and superhydrophobic surfaces

Solution chemistry-based nano-structuring of Cu dendrites is exploited to enhance their efficiency in applications of catalysis and superhydrophobicity.

Wettability and drag reduction of a superhydrophobic aluminum surface

The friction drag versus the velocity of the water flowing over surfaces with different adhesion properties.

Inspired smart materials with external stimuli responsive wettability: a review

Recent progress in smart surfaces with responsive wettability upon external stimuli is reviewed and some of the barriers and potentially promising breakthroughs in this field are also briefly discussed.

Fabrication of a micro-nanostructured superhydrophobic aluminum surface with excellent corrosion resistance and anti-icing performance

Synergy is the key to realizing superhydrophobicity. The as-prepared superhydrophobic Al surface possesses both excellent corrosion resistance and anti-icing performance.

The springtail cuticle as a blueprint for omniphobic surfaces

Omniphobic surfaces found in nature have great potential for enabling novel and emerging products and technologies to facilitate the daily life of human societies. One example is the water and even oil-repellent cuticle of springtails (Collembola). The wingless arthropods evolved a highly textured, hierarchically arranged surface pattern that affords mechanical robustness and wetting resistance even at elevated hydrostatic pressures. Springtail cuticle-derived surfaces therefore promise to overcome limitations of lotus-inspired surfaces (low durability, insufficient repellence of low surface tension liquids). In this review, we report on the liquid-repellent natural surfaces of arthropods living in aqueous or temporarily flooded habitats including water-walking insects or water spiders. In particular, we focus on springtails presenting an overview on the cuticular morphology and chemistry and their biological relevance. Based on the obtained liquid repellence of a variety of liquids with remarkable efficiency, the review provides general design criteria for robust omniphobic surfaces. In particular, the resistance against complete wetting and the mechanical stability strongly both depend on the topographical features of the nano- and micropatterned surface. The current understanding of the underlying principles and approaches to their technological implementation are summarized and discussed.

Superhydrophobic and anti-icing properties at overcooled temperature of a fluorinated hybrid surface prepared via a sol-gel process

A superhydrophobic surface with anti-icing performance has been the focus of research, but few studies have reported the effective and low cost strategy that met the requirements under overcooled conditions. In this article, the fluorinated sol-gel colloid coatings were simply prepared via hydrolytic condensation of nanosilica sol, methyltriethoxysilane (MTES) and 3-[(perfluorohexylsulfonyl)amino]propyltriethoxysilane (HFTES). The multi scale morphology and chemical composition of the artificial surfaces were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The influence of the surface roughness structure and fluorinated groups on the wettability and freezing delay time of the colloid surface under overcooled conditions were explored. As the HFTES content was higher than 6 wt%, the prepared colloid surface showed excellent superhydropobicity with a contact angle (CA) of about 166° at room temperature. The CA gradually reduced with the decrease of the temperature. Only the samples with high HFTES contents (above 30 wt%) exhibited special superhydrophobic and anti-icing properties under freeze temperature. Besides the surface roughness structure, the high fluoride enrichment on the surface plays a major role in the superhydrophobic and anti-icing properties under overcooled conditions.

Electrodeposition of nanoflake Pd structures: structure-dependent wettability and SERS activity

The characteristic properties of metal surfaces, i.e., wettability and surface-enhanced Raman scattering (SERS) activity, have been the subject of intensive research because of their useful applications. In the present work, we report a simple electrodeposition of nanoflake Pd structures onto clean Au surfaces without the use of additives. The fine structure of the nanoflake Pd surfaces was regulated by controlling the deposition charge, and the effect of the structural variations on the wettability and SERS activity was examined. The wettability of nanoflake Pd structures in terms of water contact angle was closely related to the fine structures of Pd deposits and their surface roughness. The SERS activity of the nanoflake Pd surfaces was highly dependent on the presence of sharp edge sites on the Pd structures. Well-defined nanoflake Pd structures prepared using a deposition charge of 0.04 C exhibited superhydrophobic natures and reproducible SERS activity. The effect of the metal surface structures on the wettability and the SERS activity demonstrated in this work provides insight into the fabrication of functional metal nanostructures.

Robust biomimetic-structural superhydrophobic surface on aluminum alloy

The following facile approach has been developed to prepare a biomimetic-structural superhydrophobic surface with high stabilities and strong resistances on 2024 Al alloy that are robust to harsh environments. First, a simple hydrothermal treatment in a La(NO3)3 aqueous solution was used to fabricate ginkgo-leaf like nanostructures, resulting in a superhydrophilic surface on 2024 Al. Then a low-surface-energy compound, dodecafluoroheptyl-propyl-trimethoxylsilane (Actyflon-G502), was used to modify the superhydrophilic 2024 Al, changing the surface character from superhydrophilicity to superhydrophobicity. The water contact angle (WCA) of such a superhydrophobic surface reaches up to 160°, demonstrating excellent superhydrophobicity. Moreover, the as-prepared superhydrophobic surface shows high stabilities in air-storage, chemical and thermal environments, and has strong resistances to UV irradiation, corrosion, and abrasion. The WCAs of such a surface almost remain unchanged (160°) after storage in air for 80 days, exposure in 250 °C atmosphere for 24 h, and being exposed under UV irradiation for 24 h, are more than 144° whether in acidic or alkali medium, and are more than 150° after 48 h corrosion and after abrasion under 0.98 kPa for 1000 mm length. The remarkable durability of the as-prepared superhydrophobic surface can be attributed to its stable structure and composition, which are due to the existence of lanthanum (hydr)oxides in surface layer. The robustness of the as-prepared superhydrophobic surface to harsh environments will open their much wider applications. The fabricating approach for such robust superhydrophobic surface can be easily extended to other metals and alloys.

Ni–P synergetic deposition: electrochemically deposited highly active Ni as a catalyst for chemical deposition

Electrochemically deposited Ni nanoparticles act as a catalyst for chemical deposition, producing a ball-like particle morphology.

Controllable growth of durable superhydrophobic coatings on a copper substrate via electrodeposition

Durable superhydrophobic coatings with excellent properties were created via facile and time-saving electrodeposition combined with annealing without organic modification.

Controllable wettability of micro- and nano-dendritic structures formed on aluminum substrates

A stable superhydrophobic surface with excellent anti-corrosion, anti-icing and deicing properties has been fabricated via annealing treatment from a superhydrophilic surface.

Electrodeposition of Pt nanostructures with reproducible SERS activity and superhydrophobicity

A simple one-step electrodeposition of nanostructured Pt structures exhibiting both reproducible SERS activity and superhydrophobicity was demonstrated.

A short-range ordered-disordered transition of a NiOOH/Ni(OH)2 pair induces switchable wettability

By virtue of its amorphous structure with a short-range order feature, the inorganic nanoporous nickel oxyhydroxide (NiOOH) can reversibly and rapidly switch wettability by alternate treatments of environmental chamber (superhydrophobic) and UV/ozone (superhydrophilic). The switchable mechanism of the NiOOH/Ni(OH)2 pair arising from its exceptional intrinsic short-range order-disorder transition together with chemical composition change is highlighted for the first time, which significantly differs from the current stimuli-responsive materials. This distinct multifunctional thin film not only possesses reversible wettability but also is optically patternable/repairable and electrically conductive, which could be applicable in the manufacturing of various micro- and nanostructures. We demonstrate this potential in the rewritable two-dimensional (2D) microfluidic channels and wetting-contrast enhanced selective electroplating.

Multifunctional superamphiphobic TiO2 nanostructure surfaces with facile wettability and adhesion engineering

Compared to conventional top-down photo-cleavage method, a facile bottom-up ink-combination method to in situ and rapidly achieve water wettability and adhesion transition, with a great contrast on the superamphiphobic TiO2 nanostructured film, is described. Moreover, such combination method is suitable for various kinds of superamphiphobic substrate. Oil-based ink covering or removing changes not only the topographical morphology but also surface chemical composition, and these resultant topographical morphology and composition engineering realize the site-selectively switchable wettability varying from superamphiphobicity to amphiphilicity, and water adhesion between sliding superamphiphobicity and sticky superamphiphobicity in micro-scale. Additionally, positive and negative micro-pattern can be achieved by taking advantage of the inherent photocatalytic property of TiO2 with the assistance of anti-UV light ink mask. Finally, the potential applications of the site-selectively sticky superamphiphobic surface were demonstrated. In a proof-of-concept study, the microdroplet manipulation (storage, moving, mixing, and transfer), specific gas sensing, wettability template for positive and negative ZnO patterning, and site-selective cell immobilization have been demonstrated. This study will give an important input to the field of advanced functional material surfaces with special wettability.

Self-driven one-step oil removal from oil spill on water via selective-wettability steel mesh

Marine oil spills seriously endanger sea ecosystems and coastal environments, resulting in a loss of energy resources. Environmental and economic demands emphasize the need for new methods of effectively separating oil-water mixtures, while collecting oil content at the same time. A new surface-tension-driven, gravity-assisted, one-step, oil-water separation method is presented for sustained filtration and collection of oil from a floating spill. A benchtop prototype oil collection device uses selective-wettability (superhydrophobic and superoleophilic) stainless steel mesh that attracts the floating oil, simultaneously separating it from water and collecting it in a container, requiring no preseparation pumping or pouring. The collection efficiencies for oils with wide ranging kinematic viscosities (0.32-70.4 cSt at 40 °C) are above 94%, including motor oil and heavy mineral oil. The prototype device showed high stability and functionality over repeated use, and can be easily scaled for efficient cleanup of large oil spills on seawater. In addition, a brief consolidation of separation requirements for oil-water mixtures of various oil densities is presented to demonstrate the versatility of the material system developed herein.

Reversibly controlling preferential protein adsorption on bone implants by using an applied weak potential as a switch

A facile method is needed to control the protein adsorption onto biomaterials, such as, bone implants. Herein we doped taurocholic acid (TCA), an amphiphilic biomolecule, into an array of 1D nano-architectured polypyrrole (NAPPy) on the implants. Doping TCA enabled the implant surface to show reversible wettability between 152° (superhydrophobic, switch-on state) and 55° (hydrophilic, switch-off state) in response to periodically switching two weak electrical potentials (+0.50 and -0.80 V as a switch-on and switch-off potential, respectively). The potential-switchable reversible wettability, arising from the potential-tunable orientation of the hydrophobic and hydrophilic face of TCA, led to potential-switchable preferential adsorption of proteins as well as cell adhesion and spreading. This potential-switchable strategy may open up a new avenue to control the biological activities on the implant surface.

Electro-responsively reversible transition of polythiophene films from superhydrophobicity to superhydrophilicity

An electro-responsively reversible switching of wettability between superhydrophobicity and superhydrophilicity has been obtained from a highly porous structured polythiophene film. The polythiophene film was prepared by two-step electrochemical deposition on an indium tin oxide (ITO) substrate. The underlying poly(3,4-ethylenedioxythiophene) (PEDOT) provides a highly porous structured conductive support, and poly(3-methylthiophene) (P(3-MTH)) deposited thereon plays the role of a low-surface-energy conductive coating. The wettability switching of this double-layer film between superhydrophobicity and superhydrophilicity has been investigated by doping and dedoping in an electrolyte solution containing ClO4(-). Electrochromism of the film was also seen to accompany the electrochemical process of conversion between the two superwetting states. On the basis of this porous electro-active film, an in situ electro-wetting device was also demonstrated.

Elaboration of voltage and ion exchange stimuli-responsive conducting polymers with selective switchable liquid-repellency

In this work, we report the possibility to selectively switch by voltage and anion exchange the water-repellent properties, in comparison with the oil-repellent properties, of copolymers containing both fluorinated chain (EDOT-F8) and pyridinium (EDOT-Py(+)) moieties. Here, the fluorinated chains are necessary to reach superhydrophobic properties while the pyridinium moieties allow the switching in the wettability by counterion exchange. Because, conducting polymers can exist in their oxidized and reduced state, here, we report also the switching of their wettability by voltage. The best properties (superhydrophobic properties with low hysteresis and sliding and good oleophobic properties) are obtained for a % of EDOT-Py(+) of 25 %. Surprisingly, by reducing the polymer by changing the voltage, a selective decrease in the contact angle of water is observed, whereas that of oils (diiodomethane and hexadecane) remain unchanged, making it possible to have higher contact angles with diiodomethane than with water. Here, the switching in the wettability is due to the change of the water droplet from the Cassie-Baxter state to the Wenzel state by changing the voltage. Because of the presence of highly polar pyridinium groups and their perchlorate counterions, the wettability of oil droplets (diiodomethane and hexadecane) is not significantly affected. This effect is confirmed by changing the counterions with highly hydrophobic ones (C8F17SO3(-), Tf2N(-), or BF4(-)). Such materials are excellent candidates for oil/water separation membranes.

Electrically conductive PEDOT coating with self-healing superhydrophobicity

A self-healing electrically conductive superhydrophobic poly(3,4-ethylenedioxythiophene) (PEDOT) coating has been prepared by chemical vapor deposition of a fluoroalkylsilane (POTS) onto a PEDOT film, which was obtained by electrochemical deposition. The coating not only maintained high conductivity with a low resistivity of 3.2 × 10(-4) Ω·m, but also displayed a water contact angle larger than 156° and a sliding angle smaller than 10°. After being etched with O2 plasma, the coating showed an excellent self-healing ability, spontaneously regaining its superhydrophobicity when left under ambient conditions for 20 h. This superhydrophobicity recovery process was found to be humidity-dependent, and could be accelerated and completed within 2 h under a high humidity of 84%. The coating also exhibited good superhydrophobicity recovering ability after being corroded by strong acid solution at pH 1 or strong base solution at pH 14 for 3 h.

Fabricating superhydrophobic surfaces via a two-step electrodeposition technique

This work presents a template-free electrochemical route to producing superhydrophobic copper coatings with the water contact angle of 160 ± 6° and contact angle hysteresis of 5 ± 2°. In this technique, copper deposit with multiscale surface features is formed through a two-step electrodeposition process in a concentrated copper sulfate bath. In the first step, applying a high overpotential results in the formation of structures with dense-branching morphology, which are loosely attached to the surface. In the second step, an additional thin layer of the deposit is formed by applying a low overpotential for a short time, which is used to reinforce the loosely attached branches on the surface. The work also presents a theoretical analysis of the effects of the fabrication parameters on the surface textures that cause the superhydrophobic characteristic of the deposit.

Dual-scaled porous nitrocellulose membranes with underwater superoleophobicity for highly efficient oil/water separation

Large-area dual-scaled porous nitrocellulose (p-NC) membranes are fabricated by a facile, inexpensive and scalable perforating approach. These p-NC membranes show stable superhydrophilicity in air and underwater superoleophobicity. The p-NC membranes with intrinsic nanopores and array of microscale perforated pores could selectively and efficiently separate water from various oil/water mixtures with high efficiency (>99%) rapidly.

Polymer/graphene hybrid aerogel with high compressibility, conductivity, and "sticky" superhydrophobicity

The idea of extending functions of graphene aerogels and achieving specific applications has aroused wide attention recently. A solution to this challenge is the formation of a hybrid structure where the graphene aerogels are decorated with other functional nanostructures. An infiltration-evaporation-curing strategy has been proposed by the formation of hybrid structure containing poly(dimethylsiloxane) (PDMS) and compressible graphene aerogel (CGA), where the cellular walls of the CGA are coated uniformly with an integrated polymer layer. The resulting composite shows enhanced compressive strength and a stable Young's modulus that are superior to those of pure CGAs. This unique structure combines the advantages of both components, giving rise to an excellent electromechanical performance, where the bulk resistance repeatedly shows a synchronous and linear response to variation of the volume during compression at a wide range of compressed rates. Furthermore, the foamlike structure delivers a water droplet with "sticky" superhydrophobicity and a size as large as 32 μL that remains tightly pinned to the composite, even when it is turned upside-down. This is the first demonstration of superhydrophobicity with strong adhesion on a foamlike structure. These outstanding properties qualify the PDMS/CGA composites developed here as promising candidates for a wide range of applications such as in sensors, actuators, and materials used for biochemical separation and tissue engineering.

A simple route to morphology-controlled polydimethylsiloxane films based on particle-embedded elastomeric masters for enhanced superhydrophobicity

We present a simple route for controlling the surface morphology of polydimethylsiloxane (PDMS) films based on a standard replica molding technique incorporating a microparticle-embedded elastomeric master for enhancing surface wetting properties. The elastomeric masters are simply prepared by embedding microparticles (MPs) firmly into a surface of PDMS substrates using an abrasive air-jetting (AAJ) that can be potentially scaled up to large-area fabrication. The surface geometries of the PDMS masters can be easily controlled by using MPs with different shape and size in the AAJ process, resulting in easy control of the surface morphologies and resultant wetting and optical properties of the PDMS films after replicating. The PDMS masters are found to be highly durable, enabling repeated use to produce superhydrophobic PDMS films with similar characteristics. In addition, the fabricated PDMS films retain almost constant properties even under repetitive compressing and stretching deformations thanks to the mechanical robustness enabled by their all-elastomeric architectures. We show that the fabricated PDMS surfaces can be potentially employed as self-cleaning films in glass-based applications, even with complex surfaces, owing to their enhanced wetting properties, fairly good optical transparency, and superior mechanical stability.