Mechanically durable superhydrophobic surfaces

A scalable, self-healing and hot liquid repelling superamphiphobic spray coating with remarkable mechanochemical robustness for real-life applications

A simultaneous demonstration of scalability, mechanochemical robustness, self-healing and hot liquid repelling features is still a major challenge in fabricating superamphiphobic coatings. In this work, we developed a facile and effective silica-inorganic adhesive-based spray coating for the preparation of self-healing and hot liquid repelling superamphiphobic coatings that demonstrate good mechanical durability (under repeated adhesive tape-peeling tests, ultrasonic treatment, sandpaper abrasion and sand flow impact tests) and superstrong chemical robustness when exposed to highly corrosive media, such as 98% sulfuric acid and 5% chromic acid, for a long time. In addition, our superamphiphobic paints can be coated on large-sized substrates to create large robust coatings for real-world applications, which are still regarded as the tightest bottlenecks in the development of superamphiphobic materials. The large coatings also showed excellent liquid repellence when placed for a long time in the outdoor environment, and upon repeatable quartz sand abrasion and treading stepping test cycles. Moreover, the anti-smudge ability, semitransparency, repeated self-healing ability, self-cleaning behaviour both in air and oil, and hot liquid repelling behavior of the resultant coatings are also investigated. Taking multifaceted stability and scalability into consideration, our described coatings are promising for more vital applications such as windows, infrastructures, crude oil pipelines, in harsh chemical engineering, etc.

Assembly Mechanism and the Morphological Analysis of the Robust Superhydrophobic Surface

Robust superhydrophobic surfaces are fabricated on different substrates by a scalable spray coating process. The developed superhydrophobic surface consists of thin layers of surface functionalized silica nanoparticle (SiO2) bound to the substrate by acrylate-polyurethane (PU) binder. The influence of the SiO2/PU ratio on the superhydrophobicity, and the robustness of the developed surface, is systematically analyzed. The optimized SiO2/PU ratio for prepared superhydrophobic surfaces is obtained between 0.9 and 1.2. The mechanism which yields superhydrophobicity to the surface is deduced for the first time with the help of scanning electron microscopy and profilometer. The effect of mechanical abrasion on the surface roughness and superhydrophobicity are analyzed by using profilometer and contact angle measurement, respectively. Finally, it is concluded that the binder plays a key role in controlling the surface roughness and superhydrophobicity through the capillary mechanism. Additionally, the reason for the reduction in performance is also discussed with respect to the morphology variation.

Enhancing the Robustness of Superhydrophobic Coatings via the Addition of Sulfide

Micro/nano hierarchical structures with special wettability impart a wide spectrum of unique properties to the superhydrophobic surfaces that are applicable in different potential fields. Therefore, it is necessary to develop advanced superhydrophobic materials with excellent wear-resistance properties. In this study, PDMS-based robust superhydrophobic coatings, which used MoS₂ or WS₂ as a solid lubricant, PDMS as a binder, and SiO₂ as a filler, were prepared on glass substrate by the one-step air spaying method. Lamellar MoS₂ and WS₂ with high crystallinity had intrinsic hydrophobic properties. The MoS₂@SiO₂-PDMS (MSP) and WS₂@SiO₂-PDMS (WSP) coatings with very rough textures showed good water-repellent behavior with water contact angles of 167.8 and 166.2°, respectively. The results demonstrated that the addition of microsized MoS₂ or WS₂ could easily format micro/nano second-level hierarchical structures, thus realizing the superhydrophobic properties. The friction coefficient decreased gradually with the increasing in MoS₂ or WS₂. A 4:1 ratio of SiO₂ to MoS₂/WS₂ could cause the samples to preserve their superhydrophobic properties even after 100 cycles on the abraser. As a result, superhydrophobic coatings with excellent wear resistance will be good candidates for water-repellent surfaces to meet practical emerging needs in industry applications.

Chemically reactive protein nanoparticles for synthesis of a durable and deformable superhydrophobic material

The past few decades have witnessed significant development in the field of artificially biomimicking extremely water repellent interfaces, developed mostly through tedious synthetic processes using synthetic/non-biodegradable polymers and fluorinated derivatives rendering health and environment related hazards. Only a few approaches furnish superhydrophobic materials that can withstand different harsh environments. Here, in this current design, naturally abundant and biodegradable bovine serum albumin (BSA) protein nanoparticles and cotton fibers are rationally selected for environment-friendly green synthesis of a highly sustainable and deformable artificial superhydrophobic material through strategic association of facile and rapid Michael addition reactions between amine and acrylate moieties under ambient conditions without the aid of any catalyst. This protein based nature-inspired interface can endure severe repetitive physical manipulations, abrasions and prolonged (30 days) chemical exposure i.e. extremes of pH, artificial sea water, river water and surfactant contaminated water. This highly durable and compressible superhydrophobic material was successfully exploited for efficient (above 2000 wt%), selective and repetitive removal of contaminating oils from aqueous phases under harsh chemical conditions. Such a durable biomimicking interface derived directly from serum protein following a facile synthetic approach would be useful for developing various other functional materials.

Development of Durable, Fluorine-free, and Transparent Superhydrophobic Surfaces for Oil/Water Separation

Although artificial superhydrophobic materials have extensive and significant applications in antifouling, self-cleaning, anti-icing, fluid transport, oil/water separation, and so forth, the poor robustness of these surfaces has always been a bottleneck for their development in practical industrial applications. Here, we report a facile, economical, efficient, and versatile strategy to prepare environmentally friendly, mechanically robust, and transparent superhydrophobic surfaces by combining adhesive and hydrophobic paint, which is applicable for both hard and soft substrates. The coated substrates exhibit excellent superhydrophobic property and ultralow adhesion with water (contact angle ≈ 160° and sliding angle <2°). Additionally, the coated surface maintained its superhydrophobicity even after 325 sandpaper abrasion cycles, showing remarkable mechanical robustness. Furthermore, the coated surfaces were applied to separate oil/water mixtures because of their unique characteristics of being simultaneously superhydrophobic and superoleophilic. In addition, it is believed that this fabrication method is significant, promising, and feasible for mass production of superhydrophobic surfaces for industrial applications.

Durability and Wear Resistance of Laser-Textured Hardened Stainless Steel Surfaces with Hydrophobic Properties

Hydrophobic surfaces are of high interest to industry. While surface functionalization has attracted significant interest, from both industry and research, the durability of engineered surfaces remains a challenge, as wear and scratches deteriorate their functional response. In this work, a cost-effective combination of surface engineering processes on stainless steel was investigated. Low-temperature plasma surface alloying was applied to increase surface hardness from 172 to 305 HV. Then, near-infrared nanosecond laser patterning was deployed to fabricate channel-like patterns that enabled superhydrophobicity. Abrasion tests were carried out to examine the durability of such engineered surfaces during daily use. In particular, the evolution of surface topographies, chemical composition, and water contact angle with increasing abrasion cycles were studied. Hydrophobicity deteriorated progressively on both hardened and raw stainless steel samples, suggesting that the major contributing factor to hydrophobicity was the surface chemical composition. At the same time, samples with increased surface hardness exhibited a slower deterioration of their topographies when compared with nontreated surfaces. A conclusion is made about the durability of laser-textured hardened stainless steel surfaces produced by applying the proposed combined surface engineering approach.

Super-Hydrophobic Co–Ni Coating with High Abrasion Resistance Prepared by Electrodeposition

Although super-hydrophobic surfaces have great application prospects in industry, their preparation cost and mechanical durability have limited their practical utilization. In this work, we presented a new low-cost process preparation for super-hydrophobic Co–Ni coating on carbon steel substrate via an electrodeposition route. The deposited Co–Ni coating with cauliflower-shaped micro-nano structures exhibited high super-hydrophobic properties with water contact angles over 161° after modification with 1H,1H,2H,2H-Perfluorooctyltrichlorosilane (PFTEOS). Evaluated by the linear abrasion methods, the super-hydrophobic coating can maintain super-hydrophobicity after abrasion distance of 12 m under the applied pressure of 5 kPa, which was attributed to the high cobalt content of the Co–Ni coating. Moreover, electrochemical tests showed that the super-hydrophobic Co–Ni coatings exhibited a good anti-corrosion performance thus providing an adequate protection to the carbon steel substrates.

Amphiphobic Nanostructured Coatings for Industrial Applications

The search for surfaces with non-wetting behavior towards water and low-surface tension liquids affects a wide range of industries. Surface wetting is regulated by morphological and chemical features interacting with liquid phases under different ambient conditions. Most of the approaches to the fabrication of liquid-repellent surfaces are inspired by living organisms and require the fabrication of hierarchically organized structures, coupled with low surface energy chemical composition. This paper deals with the design of amphiphobic metals (AM) and alloys by deposition of nano-oxides suspensions in alcoholic or aqueous media, coupled with perfluorinated compounds and optional infused lubricant liquids resulting in, respectively, solid⁻liquid⁻air and solid⁻liquid⁻liquid working interfaces. Nanostructured organic/inorganic hybrid coatings with contact angles against water above 170°, contact angle with n-hexadecane (surface tension γ = 27 mN/m at 20 °C) in the 140⁻150° range and contact angle hysteresis lower than 5° have been produced. A full characterization of surface chemistry has been undertaken by X-ray photoelectron spectroscopy (XPS) analyses, while field-emission scanning electron microscope (FE-SEM) observations allowed the estimation of coatings thicknesses (300⁻400 nm) and their morphological features. The durability of fabricated amphiphobic surfaces was also assessed with a wide range of tests that showed their remarkable resistance to chemically aggressive environments, mechanical stresses and ultraviolet (UV) radiation. Moreover, this work analyzes the behavior of amphiphobic surfaces in terms of anti-soiling, snow-repellent and friction-reduction properties-all originated from their non-wetting behavior. The achieved results make AM materials viable solutions to be applied in different sectors answering several and pressing technical needs.

TDI/TiO2 Hybrid Networks for Superhydrophobic Coatings with Superior UV Durability and Cation Adsorption Functionality

Durability under UV illumination remains a big challenge of TiO₂-based superhydrophobic coatings, with the photocatalytic effect causing degradation of low-surface-energy material over time, resulting in the surfaces losing their hydrophobicity. We report surfaces made from tolylene-2,4-diisocyanate (TDI)/TiO₂ hybrid networks that demonstrate superhydrophobicity and superior UV durability. Structural and morphological studies reveal that the TDI/TiO₂ hybrid networks are composed of TiO₂ nanoparticles interconnected with TDI bridges and then encapsulated by a TDI layer. Through controlling the fraction of TDI in the synthesis process, the thickness of the TDI encapsulation layer around the TDI/TiO₂ hybrid networks can be varied. When the weight ratio of TDI/TiO₂ is 5:1, the superhydrophobicity of the hybrid network surface remains almost unchanged after a month of continuous UV illumination. This hybrid network surface can also clean methylene blue solution through the synergistic effects of cation adsorption and photocatalysis, holding promising potential for applications toward reducing cation pollutions in both liquid and air environments.

Flexible and Stable Omniphobic Surfaces Based on Biomimetic Repulsive Air-Spring Structures

In artificial biological circulation systems such as extracorporeal membrane oxygenation, surface wettability is a critical factor in blood clotting problems. Therefore, to prevent blood from clotting, omniphobic surfaces are required to repel both hydrophilic and oleophilic liquids and reduce surface friction. However, most omniphobic surfaces have been fabricated by combining chemical reagent coating and physical structures and/or using rigid materials such as silicon and metal. It is almost impossible for chemicals to be used in the omniphobic surface for biomedical devices due to durability and toxicity. Moreover, a flexible and stable omniphobic surface is difficult to be fabricated by using conventional rigid materials. This study demonstrates a flexible and stable omniphobic surface by mimicking the re-entrant structure of springtail's skin. Our surface consists of a thin nanohole membrane on supporting microstructures. This structure traps air under the membrane, which can repel the liquid on the surface like a spring and increase the contact angle regardless of liquid type. By theoretical wetting model and simulation, we confirm that the omniphobic property is derived from air trapped in the structure. Also, our surface well maintains the omniphobicity under a highly pressurized condition. As a proof of our concept and one of the real-life applications, blood experiments are performed with our flat and curved surfaces and the results including contact angle, advancing/receding angles, and residuals show significant omniphobicity. We hope that our omniphobic surface has a significant impact on blood-contacting biomedical applications.

Nonfluorinated Superomniphobic Surfaces through Shape-Tunable Mushroom-like Polymeric Micropillar Arrays

Superomniphobic surfaces showing extremely liquid-repellent properties have received a great amount of attention as they can be used in various industrial and biomedical applications. However, so far, the fabrication processes of these materials mostly have involved the coating of perfluorocarbons onto micro- and nanohierarchical structures of these surfaces, which inevitably causes environmental pollution, leading to health concerns. Herein, we developed a facile method to obtain flexible superomniphobic surfaces without perfluorocarbon coatings that have shape-tunable mushroom-like micropillars (MPs). Inspired by the unique structures on the skin of springtails, we fabricated mushroom-like structures with downward facing edges (i.e., a doubly re-entrant structure) on a surface. The flexible MP structures were fabricated using a conventional micromolding technique, and the shapes of the mushroom caps were made highly tunable via the deposition of a thin aluminum (Al) layer. Due to the compressive residual stress of the Al, the mushroom caps were observed to bend toward the polymer upon forming doubly re-entrant-MP structures. The obtained surface was found to repel most low-surface-tension liquids such as oils, alcohols, and even fluorinated solvents. The developed flexible superomniphobic surface showed liquid repellency even upon mechanical stretching and after surface energy modification. We envision that the developed superomniphobic surface with high flexibility and wetting resistance after surface energy modification will be used in a wide range of applications such as self-cleaning clothes and gloves.

Instant Tuning of Superhydrophilic to Robust Superhydrophobic and Self-Cleaning Metallic Coating: Simple, Direct, One-Step, and Scalable Technique

We present a simple, direct, one-step, scalable technique for instant tuning of all the different states of wetting characteristics using atmospheric plasma spray (APS) technique. We observed that, just by changing the process parameters in the APS technique, the wetting characteristics of an intrinsically hydrophilic aluminum metallic surface can be tuned to superhydrophilic (contact angle (CA): 0°), hydrophilic (CA: 19.6°), hydrophobic (CA: 97.6°), and superhydrophobic (CA: 156.5°) surfaces. Also, tuned superhydrophobic surface showed an excellent self-cleaning property. Further, we demonstrated that these surfaces retain their superhydrophobic nature even after exposure at elevated temperatures (up to 773 K) and on application of mechanical abrasion. Manipulation in different wetting behavior was possible mainly due to the presence of varying degrees of smooth surface as well as micropillars, which incorporated the multiscale roughness to the surface. "Re-entrant"-like microstructures such as mushroom, cauliflower, and cornet microstructures were observed in the case of tuned superhydrophobic surface, which is well-known for achieving the excellent water repellency over the hydrophilic surface.

A novel dual-layer approach towards omniphobic polyurethane coatings

Omniphobic surfaces have a plethora of applications ranging from household paints to sensors. The predominant practice of fabricating those materials/surfaces is to use fluorinated materials which are environmentally harmful, and thus have limited practical applications. In this study, we report a novel dual-layer approach of fabrication towards omniphobic surfaces using polyurethane (PU) as a matrix and polydimethylsiloxane (PDMS) as a self-cleaning ingredient. This approach was also used to produce omniphobic PU nanocomposites, where nanofillers (e.g., nanoclay, cellulose nanocrystals (CNCs) and graphene oxide (GO)) were incorporated. The resultant coatings were investigated for their performance, such as optical clarity, durability, and self-cleaning properties. In addition, scanning electron microscopy (SEM) was used for microstructural analysis of the obtained coatings. The facile nature of fabrication and the use of PDMS, an environmentally benign material relative to fluorinated chemicals, thus offer an eco-friendly sustainable scheme for practical applications aimed at omniphobic purposes.

Omniphobic surfaces have a plethora of applications ranging from household paints to sensors.

How to Coat the Inside of Narrow and Long Tubes with a Super-Liquid-Repellent Layer-A Promising Candidate for Antibacterial Catheters

Fouling of thin tubes is a major problem, leading to various infections and associated morbidities, while cleaning is difficult or even impossible. Here, a generic method is introduced to activate and coat the inside of meter-long and at the same time thin (down to 1 mm) tubes with a super-liquid-repellent layer of nanofilaments, exhibiting even antibacterial properties. Activation is facilitated by pumping an oxidative Fenton solution through the tubes. Subsequent pumping of a silane solution renders the surface of the tubes super-liquid-repellent. The wide applicability of the method is demonstrated by coating stiff and flexible tubes made of polymers, inorganic/organic hybrids, metals, and ceramics. Coated medical catheters show excellent antibacterial properties. Notably, the nanofilaments retain their antibacterial properties even in the superhydrophilic state. These findings open new avenues toward the design of biocide-free, antibacterial tubings and catheters.

Preparation and characterization of POSS-containing poly(perfluoropolyether)methacrylate hybrid copolymer and its superhydrophobic coating performance

To design a mechanically stable and superhydrophobic coating, a polyhedral oligomeric silsesquioxane (POSS)-containing poly(perfluoropolyether)methacrylate (PFPEM) hybrid copolymer (PFPEM–POSS) was synthesized via a free-radical solution polymerization with PFPEM, 1H,1H,2H,2H-perfluorooctyl acrylate, methyl (meth)acrylate, n-butyl acrylate, hydroxypropyl acrylate, methacryloxy propyl trimethoxy silane, and methacrylisobutyl POSS; and azobisisobutyronitrile as an initiator. Hydrophobic coatings were formed on substrates by a facile one-step dip-coating method in a solution mixture of diethylene glycol dimethyl ether with the PFPEM–POSS hybrid copolymer. The chemical structure of the PFPEM–POSS copolymer and the surface morphology and performance of the PFPEM–POSS coatings were investigated. The results indicate that, under POSS aggregation via the fluorophilic/oleophilic co-monomer phase separation and owing to the low-surface-energy poly(perfluoropolyether)methacrylate incorporated into the copolymer, PFPEM–POSS exhibited a hierarchical micro-nano roughness in atomic force microscopy observations and provided the treated substrates with excellent hydrophobicity and abrasion resistance. As a result, the water contact angle reached 152.3° on the treated glass.

A coating with excellent superhydrophobicity and durability was built via incorporating an environmentally-friendly poly(perfluoropolyether)methacrylate copolymer into polyhedral oligomeric silsesquioxane (POSS).

3D-Printed Surface Architecture Enhancing Superhydrophobicity and Viscous Droplet Repellency

Macrotextured superhydrophobic surfaces can reduce droplet-substrate contact times of impacting water droplets; however, surface designs with similar performance for significantly more viscous liquids are missing, despite their importance in nature and technology such as for chemical shielding, food-staining repellency, and supercooled (viscous) water droplet removal in anti-icing applications. Here, we introduce a deterministic, controllable, and upscalable method to fabricate superhydrophobic surfaces with a 3D-printed architecture, combining arrays of alternating surface protrusions and indentations. We show a more than threefold contact time reduction of impacting viscous droplets up to a fluid viscosity of 3.7 mPa·s, which equals 3.7 times the viscosity of water at room temperature, covering the viscosity of many chemicals and supercooled water. On the basis of the combined consideration of the fluid flow within and the simultaneous droplet dynamics above the texture, we recommend future pathways to rationally architecture such surfaces, all realizable with the methodology presented here.

Drop Cargo Transfer via Unidirectional Lubricant Spreading on Peristome-Mimetic Surface

To promote drop mobility, lubricating the gap between liquid drop and solid surface is a facile method which has been widely exploited by nature. Examples include lotus and rice leaves using entrapped air to "lubricate" water and Nepenthes pitcher plant using a slippery water layer to trap insects. Inspired by these, here, we report a strategy for transporting drop cargoes via the unidirectional spreading of immiscible lubricants on the peristome-mimetic surface. Oleophilic/hydrophobic peristome-mimetic surfaces were fabricated through replicating three-dimensional printed samples. The peristome-mimetic surface, via unidirectional immiscible hexadecane spreading, can transport a wide diversity of drop cargoes over a long distance with no loss with controllable drop volumes and velocities, hence mixing multiphase liquids and even reacting liquids. We anticipate this unidirectional drop cargo transport technique will find use in microfluidics, microreactors, water harvesting systems, etc.

Cascade Freezing of Supercooled Water Droplet Collectives

Surface icing affects the safety and performance of numerous processes in technology. Previous studies mostly investigated freezing of individual droplets. The interaction among multiple droplets during freezing is investigated less, especially on nanotextured icephobic surfaces, despite its practical importance as water droplets never appear in isolation, but in groups. Here we show that freezing of a supercooled droplet leads to spontaneous self-heating and induces strong vaporization. The resulting, rapidly propagating vapor front causes immediate cascading freezing of neighboring supercooled droplets upon reaching them. We put forth the explanation that, as the vapor approaches cold neighboring droplets, it can lead to local supersaturation and formation of airborne microscopic ice crystals, which act as freezing nucleation sites. The sequential triggering and propagation of this mechanism results in the rapid freezing of an entire droplet ensemble, resulting in ice coverage of the nanotextured surface. Although cascade freezing is observed in a low-pressure environment, it introduces an unexpected pathway of freezing propagation that can be crucial for the performance of rationally designed icephobic surfaces.

Recent Hydrophobic Metal-Organic Frameworks and Their Applications

The focus of discussion of this review is the application of the most recent synthesized hydrophobic metal-organic frameworks (MOFs). The most promising hydrophobic MOFs are mentioned with their applications and discussed. The various MOFs considered are sub-sectioned into the main application areas, namely alcohol adsorption and oil/water-alcohol/water separation, gas separation and storage, and other applications such as self-cleaning and liquid marbles. Again, the methods of synthesis are briefly described, showing how the features of the end product aid in their applications. The efficiency of the MOF materials and synthesis methods are highlighted and briefly discussed. Lastly, the summary and outlook section concludes the write-up giving suggestions that would be useful to present-day researchers.

Highly Reliable Superhydrophobic Surface with Carbon Nanotubes Immobilized on a PDMS/Adhesive Multilayer

We propose a new superhydrophobic surface that contains a carbon nanotube (CNT)-implanted poly(dimethylsiloxane) (PDMS)/adhesive multilayer. The adhesive provides very strong adhesion between the CNT-implanted PDMS layer and the substrate, and the CNTs on the surface exhibit superhydrophobicity. Therefore, the CNT-implanted PDMS/adhesive (CIPA) layer provides a highly reliable surface for superhydrophobicity. The fabricated CIPA surface performs far better than previously reported surfaces in terms of stability tests, such as contamination and solvent tests, and physical contact, including thermal pressure, bending, adhesion, and water jet tests. If a portion of the CIPA surface is destroyed, the surface is immediately restored because the material can regenerate the surface to its initial state. The surface can therefore maintain its superhydrophobicity even when damaged in rough environments, without self-healing or additional repair. Furthermore, because the adhesive is sprayed and coated on the surface of the substrate, a CIPA surface can be formed on three-dimensional shapes, including curved surfaces, and on various substrates.

A study on the wetting properties of broccoli leaf surfaces and their time dependent self-healing after mechanical damage

Plants are protected from the elements by a complex hierarchical epicuticular wax layer which has inspired the creation of super-hydrophobic and self-cleaning surfaces. Although many studies have been conducted on different plant wax systems to determine the mechanisms of water repulsion hardly any have studied the recovery of the epicuticular wax layer. In the current study the wetting properties and crystallographic nature of the wax surface of Brassica oleracea var. italica (broccoli) has been studied, as well as the time-dependent recovery of the surface after mechanical damage. It was found that the surface of the broccoli leaves is not only super-repulsive and self-cleaning in regards to water but also in regards to glycerol and formamide, both of which have considerably lower surface tension values. Furthermore, it was shown that the surface properties do indeed recover after damage and that this recovery is multi-stepped and strongly dependent on the recovery of the roughness of the surface.

Collective Shape Actuation of Polymer Double Emulsions by Solvent Evaporation

We demonstrate that the shape actuation of water-in-oil-in-water double emulsion droplets can be achieved by controlling solvent evaporation in a model system, where the oil phase consists of hydrophobic homopolymer/amphiphilic block copolymer/solvent. A gradient of interfacial tension is created in the polymer shell, which drives significant deformation of the droplets in constant volume. The deformed droplets recover to their initial shape spontaneously, and shape actuation of droplets can be further tuned by osmotic pressure. Our model system provides a new prototype for developing shape-responsive droplets in a solvent environment.

Omniphobic Metal Surfaces with Low Contact Angle Hysteresis and Tilt Angles

Various metal (Al, Ti, Fe, Ni, and Cu) surfaces with native oxide layers were rendered "omniphobic" by a simple thermal treatment of neat liquid trimethylsiloxy-terminated polymethylhydrosiloxanes (PMHSs) with a range of different molecular weights (MWs). Because of this treatment, the PMHS chains were covalently attached to the oxidized metal surfaces, giving 2-10 nm thick PMHS layers. The resulting surfaces were fairly smooth, liquid-like, and showed excellent dynamic omniphobicity with both low contact angle hysteresis (≲5°) and substrate tilt angles (≲8°) toward small-volume liquid drops (5 μL) with surface tensions ranging from 20.5 to 72.8 mN/m. Droplet mobility was improved overall as a result of heating the substrates to 70 °C. The reaction kinetics and final dynamic dewetting properties were found to be not dependent of the types of metals employed or MWs of PMHS, but mainly dominated by both reaction temperatures and reaction times.

Biomimetic coating-free surfaces for long-term entrapment of air under wetting liquids

Trapping air at the solid-liquid interface is a promising strategy for reducing frictional drag and desalting water, although it has thus far remained unachievable without perfluorinated coatings. Here, we report on biomimetic microtextures composed of doubly reentrant cavities (DRCs) and reentrant cavities (RCs) that can enable even intrinsically wetting materials to entrap air for long periods upon immersion in liquids. Using SiO2/Si wafers as the model system, we demonstrate that while the air entrapped in simple cylindrical cavities immersed in hexadecane is lost after 0.2 s, the air entrapped in the DRCs remained intact even after 27 days (~106 s). To understand the factors and mechanisms underlying this ten-million-fold enhancement, we compared the behaviors of DRCs, RCs and simple cavities of circular and non-circular shapes on immersion in liquids of low and high vapor pressures through high-speed imaging, confocal microscopy, and pressure cells. Those results might advance the development of coating-free liquid repellent surfaces.

Desublimation Frosting on Nanoengineered Surfaces

Ice nucleation from vapor presents a variety of challenges across a wide range of industries and applications including refrigeration, transportation, and energy generation. However, a rational comprehensive approach to fabricating intrinsically icephobic surfaces for frost formation-both from water condensation (followed by freezing) and in particular from desublimation (direct growth of ice crystals from vapor)-remains elusive. Here, guided by nucleation physics, we investigate the effect of material composition and surface texturing (atomically smooth to nanorough) on the nucleation and growth mechanism of frost for a range of conditions within the sublimation domain (0 °C to -55 °C; partial water vapor pressures 6 to 0.02 mbar). Surprisingly, we observe that on silicon at very cold temperatures-below the homogeneous ice solidification nucleation limit (<-46 °C)-desublimation does not become the favorable pathway to frosting. Furthermore, we show that surface nanoroughness makes frost formation on silicon more probable. We experimentally demonstrate at temperatures between -48 °C and -55 °C that nanotexture with radii of curvature within 1 order of magnitude of the critical radius of nucleation favors frost growth, facilitated by capillary condensation, consistent with Kelvin's equation. Our findings show that such nanoscale surface morphology imposed by design to impart desired functionalities-such as superhydrophobicity-or from defects can be highly detrimental for frost icephobicity at low temperatures and water vapor partial pressures (<0.05 mbar). Our work contributes to the fundamental understanding of phase transitions well within the equilibrium sublimation domain and has implications for applications such as travel, power generation, and refrigeration.

Effect of Varying Chain Length and Content of Poly(dimethylsiloxane) on Dynamic Dewetting Performance of NP-GLIDE Polyurethane Coatings

Polyurethane coatings containing nanopools of a grafted lubricating liquid ingredient for dewetting enablement (NP-GLIDE) are prepared by curing a commercial polyol P0, a hexamethylene diisocyanate trimer, and P1- g-PDMS, which is a graft copolymer consisting of a polyol backbone P1 bearing poly(dimethylsiloxane) (PDMS) side chains. These materials are known as NP-GLIDE because most test liquids have no problem to cleanly glide off them and because segregated nanopools of the grafted lubricating ingredient (PDMS) for dewetting enablement are dispersed throughout the coating matrix. To optimize the dewetting performance of the NP-GLIDE coatings, the molecular weights of the PDMS side chains in the P1- g-PDMS samples were increased from 1.0 kDa (1k) to 5.0 kDa (5k) and 10.0 kDa (10k). A comparative study of the coatings containing three different P1- g-PDMS samples at a constant PDMS mass fraction of either 6.0 or 2.00% (m/m) showed that P1- g-PDMS_5k-based coatings exhibited the best dewetting properties. These properties included the lowest sliding angles for test liquids that were incompatible with PDMS and the fastest and most effective contraction of marker ink traces and a paint. Coatings containing 0.50 and 1.00% (m/m) of PDMS_5k were also prepared from P1- g-PDMS_5k and compared with those containing 2.00 and 6.0% (m/m) of PDMS_5k. The coatings were shown to retain their dewetting properties with the PDMS contents as low as 1.00% (m/m). Although the results of this study provided valuable insight into the design of future practical NP-GLIDE coatings, a model has also been proposed for the surface structure of the coatings to justify our observations.

Extending the Lotus Effect: Repairing Superhydrophobic Surfaces after Contamination or Damage by CHic Chemistry

Superhydrophobic surfaces have gained a reputation to show a self-cleaning behavior ("Lotus effect") as drops rolling off the surface take along loosely adhering dust particles. However, this self-cleaning process reaches its limits when such surfaces are brought in contact with sticky contaminants such as oils and smaller particles. Once intimate contact is established between the surface and a small particle, it will be almost impossible to remove it because of strong surface interactions. Such contaminations, however, lead to contact line pinning and destroy the superhydrophobic effect. Because the fragility of the micro- and nanostructures prohibits any mechanical cleaning, the sample is usually doomed. Here, we report a universal method for restoring superhydrophobicity: by simple dip-coating, a conformal ultrathin layer (≈10 nm) of a highly hydrophobic and photoreactive fluoropolymer is deposited. Through short UV irradiation (5 min), this thin layer is cross-linked and chemically attached to the underlying surface by C,H-insertion cross-linking, thus covering the contaminant like a thin veil. We use this "cover-up" strategy of masking the contaminants to restore superhydrophobicity. We demonstrate this principle by deliberately soiling the surface with various model contaminants, such as oily substances and particles, and study the repair process.

Green and Rapid Synthesis of Durable and Super-Oil (under Water) and Water (in Air) Repellent Interfaces

In this letter, a single polymer is rapidly and covalently transformed into a chemically reactive and functional bulk polymeric coatings through a catalyst-free mutual chemical reaction between acrylates and amine groups at ambient condition-in the absence of any external reaction solvent, which is unprecedented in the literature. This facile and green chemical approach provided a common basis for achieving two distinct biomimicked wettabilities-that are superhydrophobicity (lotus-leaf mimicked) in air and superoleophobicity (fish-scale inspired) under water. The essential chemistry that conferred bioinspired wettability was optimized in the hierarchically featured polymeric material by postcovalent modulation of chemically reactive polymeric material with primary-amine-containing small moleculess, glucamine and octadecylamine. The inherently sticky and "chemically reactive" polymeric material having appropriate hierarchical topography is highly capable of providing substrate-independent (irrespective of chemical compositions and mechanical strength of the substrates) stable coatings with robust bioinspired (i.e., lotus leaf and fish scale) wettability.

Fabrication of a scratch & heat resistant superhydrophobic SiO2 surface with self-cleaning and semi-transparent performance

Herein, we report the fabrication of a superhydrophobic surface with a new and effective silica nanocomposite. A facile synthesis was developed by spraying the as-prepared silica suspension on a glass substrate, where the SiO2 nanoparticles were composed of methylated aerogel particles wrapped by hydroxyl-terminated polydimethylsiloxane (PDMS). Three types of methylated silica aerogel nanoparticles with different surface roughness and porosities were prepared using specific precursors and methylation agents. The coating of the silica aerogels (sodium silicate and trimethylchlorosilane) wrapped in PDMS was exceptionally superhydrophobic with a superior water contact angle of 169.80 ± 3° and a sliding angle of less than 4°. The semi-transparent coating maintained its excellent water repellency at 350 °C for least 4 h and exhibited durable superhydrophobic properties for 6 months at ambient conditions. Additionally, the coating also showed good mechanical stability and remarkable self-cleaning behaviour.

Superhydrophobic Blood-Repellent Surfaces

Superhydrophobic surfaces repel water and, in some cases, other liquids as well. The repellency is caused by topographical features at the nano-/microscale and low surface energy. Blood is a challenging liquid to repel due to its high propensity for activation of intrinsic hemostatic mechanisms, induction of coagulation, and platelet activation upon contact with foreign surfaces. Imbalanced activation of coagulation drives thrombogenesis or formation of blood clots that can occlude the blood flow either on-site or further downstream as emboli, exposing tissues to ischemia and infarction. Blood-repellent superhydrophobic surfaces aim toward reducing the thrombogenicity of surfaces of blood-contacting devices and implants. Several mechanisms that lead to blood repellency are proposed, focusing mainly on platelet antiadhesion. Structured surfaces can: (i) reduce the effective area exposed to platelets, (ii) reduce the adhesion area available to individual platelets, (iii) cause hydrodynamic effects that reduce platelet adhesion, and (iv) reduce or alter protein adsorption in a way that is not conducive to thrombus formation. These mechanisms benefit from the superhydrophobic Cassie state, in which a thin layer of air is trapped between the solid surface and the liquid. The connections between water- and blood repellency are discussed and several recent examples of blood-repellent superhydrophobic surfaces are highlighted.

Superhydrophobic Natural and Artificial Surfaces-A Structural Approach

Since ancient times humans observed animal and plants features and tried to adapt them according to their own needs. Biomimetics represents the foundation of many inventions from various fields: From transportation devices (helicopter, airplane, submarine) and flying techniques, to sports' wear industry (swimming suits, scuba diving gear, Velcro closure system), bullet proof vests made from Kevlar etc. It is true that nature provides numerous noteworthy models (shark skin, spider web, lotus leaves), referring both to the plant and animal kingdom. This review paper summarizes a few of "nature's interventions" in human evolution, regarding understanding of surface wettability and development of innovative special surfaces. Empirical models are described in order to reveal the science behind special wettable surfaces (superhydrophobic /superhydrophilic). Materials and methods used in order to artificially obtain special wettable surfaces are described in correlation with plants' and animals' unique features. Emphasis is placed on joining superhydrophobic and superhydrophilic surfaces, with important applications in cell culturing, microorganism isolation/separation and molecule screening techniques. Bio-inspired wettability is presented as a constitutive part of traditional devices/systems, intended to improve their characteristics and extend performances.

Rational design of materials interface at nanoscale towards intelligent oil-water separation

Oil-water separation is critical for the water treatment of oily wastewater or oil-spill accidents. The oil contamination in water not only induces severe water pollution but also threatens human beings' health and all living species in the ecological system. To address this challenge, different nanoscale fabrication methods have been applied for endowing biomimetic porous materials, which provide a promising solution for oily-water remediation. In this review, we present the state-of-the-art developments in the rational design of materials interface with special wettability for the intelligent separation of immiscible/emulsified oil-water mixtures. A mechanistic understanding of oil-water separation is firstly described, followed by a summary of separation solutions for traditional oil-water mixtures and special oil-water emulsions enabled by self-amplified wettability due to nanostructures. Guided by the basic theory, the rational design of interfaces of various porous materials at nanoscale with special wettability towards superhydrophobicity-superoleophilicity, superhydrophilicity-superoleophobicity, and superhydrophilicity-underwater superoleophobicity is discussed in detail. Although the above nanoscale fabrication strategies are able to address most of the current challenges, intelligent superwetting materials developed to meet special oil-water separation demands and to further promote the separation efficiency are also reviewed for various special application demands. Finally, challenges and future perspectives in the development of more efficient oil-water separation materials and devices by nanoscale control are provided. It is expected that the biomimetic porous materials with nanoscale interface engineering will overcome the current challenges of oil-water emulsion separation, realizing their practical applications in the near future with continuous efforts in this field.

Slippery Liquid-Infused Porous Surfaces (SLIPS) Using Layer-by-Layer Polyelectrolyte Assembly in Organic Solvent

Slippery liquid-infused porous surfaces (SLIPS) have potential impact on a wide range of industries, including healthcare, food packaging, and automobile. A tremendouseffort has been focused on developing novel fabrication methods for making SLIPS. However, current fabrication methods usually involve harsh conditions and complicated postfabrication modifications or are limited to specific substrates. Presented here is a novel method for the fast and facile fabrication of SLIPS. Layer-by-layer (LBL) assembly of branched polyethylenimine and Nafion, a perfluorinated polyelectrolyte, is performed with methanol as the solvent. Hierarchically rough and superhydrophobic surface is obtained directly without further modification on various substrates. The surface properties are shown to highly depend on the LBL assembly parameters, including deposition cycles, dipping time, rinsing time, and drying time between baths. The polyelectrolyte multilayers obtained with this method are infused with Krytox 100 to form SLIPS surfaces, which show excellent omniphobic, antifouling, self-cleaning, flexible, and optical properties. The result of this study not only simplifies the fabrication of SLIPS surfaces, but also provides great insight for making LBL films with specific morphologies.