Water Impact Resistant and Antireflective Superhydrophobic Surfaces Fabricated by Spray Coating of Nanoparticles: Interface Engineering via End-Grafted Polymers

Mechanochemical Activation of Silicone for Large-Scale Fabrication of Anti-Biofouling Liquid-like Surfaces

Large-scale preparation of liquid-like coatings with perfect transparency via solventless and room-temperature processes using low-cost and biocompatible materials is of tremendous interest for a broad range of applications. Here, we present a mechanochemical activation strategy for solventless grafting of poly(dimethylsiloxane) (PDMS) onto glass, silicon wafers, and ceramics. Activation is achieved via ball milling PDMS without using any solvents or additives prior to application. Ball milling results in chain scission and generation of free radicals, allowing room-temperature grafting at durations ≤1 h. The deposition of ball-milled PDMS can be facilitated by brushing or drop-casting, enabling large-scale applications. The resulting surfaces facilitate the sliding of droplets at angles <20° for liquids with surface tension ranging from 22 to 73 mN/m. An important application for public health is generating anti-biofouling coatings on sanitary ware. For example, PDMS-grafted surfaces prepared on a regular-size toilet bowl exhibit a 105-fold decrease in the attachment of bacteria from urine. These findings highlight the significant potential of mechanochemical processes for the practical preparation of liquid-like surfaces.

Waterproof and Flame-Retardant Fabric Coating with Nail-Tie Structure was Constructed by Janus Particles with Strong Mechanical, Physical, and Chemical Durability

Oil spills are one of the most dangerous sources that cause serious environmental pollution and fire and explosion. In this work, multifunctional separator silica@polydivinylbenzene/poly 2,6-dimethyl-1, 4-phenyl ether (silica@PDVB/PPE) Janus particles were fabricated via seed emulsion polymerization, causing phase segregation as well as selective modification. The epoxy modified silica is partially covalently bonded to the fabric substrate surface by simple spraying to achieve a strong composite coating. The low surface energy PDVB/PPE forms a micronano rough layered surface, which can achieve a super hydrophobic and lipophile surface (WCA = 155°) and obtain a high flux separation of water and oil at 32,700 L·m-2·h-1. At the same time, the Janus composite fabric coating has the advantages of high heat resistance and flame retardant, which is realized by halogen-free flame-retardant unsaturated polyphosphate (PPE), making Janus fabric have potential value in separating oil-water mixtures and fire protection applications. In addition, the coating shows excellent chemical durability. After soaking in various aqueous solvents and organic solvents for 30 h, it can still maintain superhydrophobicity and flame retardant. The coating still has water repellency and flame retardant after 50 washings and mechanical wear and has good mechanical durability.

Recent Advances in Fabrication of Durable, Transparent, and Superhydrophobic Surfaces

Transparent superhydrophobic coatings have been extensively investigated due to their ability to provide self-cleaning properties for outdoor applications. However, the widespread implementation of these coatings on a large scale is impeded by the challenges of poor durability and complex fabrication procedures. In this review, the fundamentals and theories governing the mutually exclusive properties of superhydrophobicity, optical transparency, and susceptibility to wear are introduced, followed by a discussion of representative examples of advanced surface design and processing optimizations. Also, robust evaluation protocols for assessing mechanical and chemical stabilities are briefed and potential research directions are presented. This review can offer the research community a better understanding of durable and transparent superhydrophobic surfaces, thereby facilitating their development for real-world applications.

Synergistically designed antireflective cover for improving wide-angle photovoltaic efficiencies

We demonstrated that a well-designed nanopatterned cover improves photovoltaic efficiency across a wide range of incident angles (θ). A nanopatterned cover was created using an integrated ray-wave optics simulation to maximize the light absorption of the surface-textured Si photovoltaic device. A hexagonally arranged nanocone array with a 300 nm pitch was formed into a polymer using nanoimprinting, and the nanostructured polymer was then attached to a glass cover with an index-matching adhesive. Angle-resolved current density-voltage measurements on Si photovoltaic devices showed that the nanopatterned glass cover yielded a 2-13% enhancement in power conversion efficiency at θ = 0-60°, which accounted for its broadband antireflective feature. We performed all-season-perspective simulations based on the results of the integrated ray-wave optics simulations and solar altitude database of South Korea, which validated the sustainability of the developed nanopatterned cover during significant seasonal fluctuations.

A Scalable Haze-Free Antireflective Hierarchical Surface with Self-Cleaning Capability

The lotus effect indicates that a superhydrophobic, self-cleaning surface can be obtained by roughening the topography of a hydrophobic surface. However, attaining high transmittance and clarity through a roughened surface remains challenging because of its strong scattering characteristics. Here, a haze-free, antireflective superhydrophobic surface that consists of hierarchically designed nanoparticles is demonstrated. Close-packed, deep-subwavelength-scale colloidal silica nanoparticles and their upper, chain-like fumed silica nanoparticles individually fulfill haze-free broadband antireflection and self-cleaning functions. These double-layered hierarchical surfaces are obtained via a scalable spraying process that permits precise control over the coating morphology to attain the desired optical and wetting properties. They provide a "specular" visible transmittance of >97% when double-side coated and a record-high self-cleaning capability with a near-zero sliding angle. Self-cleaning experiments on photovoltaic devices verify that the developed surfaces can significantly enhance power conversion efficiencies and aid in retaining pristine device performance in a dusty environment.

Superhydrophobic Coating Derived from the Spontaneous Orientation of Janus Particles

A superhydrophobic surface was achieved using a monolayer of the perpendicularly oriented epoxy-silica@polydivinylbenzene (PDVB) Janus particles (JPs) on an epoxy resin substrate. The epoxy-silica@PDVB JPs were synthesized from the silica@PDVB/polystyrene (PS) JPs through selective etching of the PDVB/PS belly and the surface modification of the silica part. The modified silica parts can be covalently bonded with the epoxy resin to make the perpendicular orientation spontaneous as well as the coating more robust. The outward PDVB bellies can constitute the micro-/nanoscale hierarchical structures for the superhydrophobic property. The superhydrophobic coating exhibits water repellence and self-cleaning properties. Moreover, the coating exhibits good chemical durability that it can keep the superhydrophobic property after long-time immersion in various aqueous solutions and organic solvents. The coating is still superhydrophobic after water flushing and mechanical wearing, showing the perfect mechanical durability.

Blood repellent superhydrophobic surfaces constructed from nanoparticle-free and biocompatible materials

Durable and environment friendly superhydrophobic surfaces are needed for a set of important applications. Biomedical applications, in particular, impose stringent requirements on the biocompatibility of the materials used in the fabrication of superhydrophobic surfaces. In this study, we demonstrate the fabrication of mechanically durable superhydrophobic surfaces via an in-situ structuring strategy starting from natural carnauba wax and biocompatible polydimethylsiloxane (PDMS) materials. The transfer of the structure of the paper to a free-standing PDMS film provided the microscale structure. On top of this structured surface, the wax was spray-coated, initially resulting in a relatively homogeneous film with limited liquid repellence. The key in achieving superhydrophobicity was rubbing the surface for in-situ generation of a finely textured wax coating with a water contact angle of 169° and a sliding angle of 3°. The hierarchically structured surface exhibits mechanical robustness as demonstrated with water impact and linear abrasion tests. We finally demonstrate repellence of the surfaces against a range of blood products including platelet suspension, erythrocyte suspension, fresh plasma, and whole blood.

Transparent Super-Repellent Surfaces with Low Haze and High Jet Impact Resistance

Transparent superhydrophobic surfaces are of vital significance for rising applications in optoelectronics, outdoor displays, building windows, and so on. However, facile fabrication of surfaces combining stable superhydrophobicity and high transparency with particularly low haze remains a challenge. Here, we demonstrate a nonfluorinated hierarchical surface, simply prepared by sequential spraying of a primer of poly(ethylene-co-acrylic acid) (EAA) and silica nanoparticles (SiO₂). The resultant surface shows remarkable liquid repellency (e.g., an apparent contact angle of >160° and a sliding angle of <2° for honey) and high transparency (a transmittance of ∼91% and a haze of ∼6%). Especially, flexible EAA adhesive enables the surface to resist water impinging (up to ∼15.0 m s^-1, higher than the terminal velocities of raindrops) and mechanical damaging. This super-repellent surface also presents excellent UV and chemical stability, sustaining a superhydrophobic state upon UVA exposure for 60 days and acidic corrosion or oil contamination for 7 days. With multirobustness and scalability, our coatings show great potential in related fields.

Physically Unclonable Surfaces via Dewetting of Polymer Thin Films

From anti-counterfeiting to biotechnology applications, there is a strong demand for encoded surfaces with multiple security layers that are prepared by stochastic processes and are adaptable to deterministic fabrication approaches. Here, we present dewetting instabilities in nanoscopic (thickness <100 nm) polymer films as a form of physically unclonable function (PUF). The inherent randomness involved in the dewetting process presents a highly suitable platform for fabricating unclonable surfaces. The thermal annealing-induced dewetting of poly(2-vinyl pyridine) (P2VP) on polystyrene-grafted substrates enables fabrication of randomly positioned functional features that are separated at a microscopic length scale, a requirement set by optical authentication systems. At a first level, PUFs can be simply and readily verified via reflection of visible light. Area-specific electrostatic interactions between P2VP and citrate-stabilized gold nanoparticles allow for fabrication of plasmonic PUFs. The strong surface-enhanced Raman scattering by plasmonic nanoparticles together with incorporation of taggants facilitates a molecular vibration-based security layer. The patterning of P2VP films presents opportunities for fabricating hybrid security labels, which can be resolved through both stochastic and deterministic pathways. The adaptability to a broad range of nanoscale materials, simplicity, versatility, compatibility with conventional fabrication approaches, and high levels of stability offer key opportunities in encoding applications.

Robust superhydrophobicity: mechanisms and strategies

Superhydrophobic surfaces hold great prospects for extremely diverse applications owing to their water repellence property. The essential feature of superhydrophobicity is micro-/nano-scopic roughness to reserve a large portion of air under a liquid drop. However, the vulnerability of the delicate surface textures significantly impedes the practical applications of superhydrophobic surfaces. Robust superhydrophobicity is a must to meet the rigorous industrial requirements and standards for commercial products. In recent years, major advancements have been made in elucidating the mechanisms of wetting transitions, design strategies and fabrication techniques of superhydrophobicity. This review will first introduce the mechanisms of wetting transitions, including the thermodynamic stability of the Cassie state and its breakdown conditions. Then we highlight the development, current status and future prospects of robust superhydrophobicity, including characterization, design strategies and fabrication techniques. In particular, design strategies, which are classified into passive resistance and active regeneration for the first time, are proposed and discussed extensively.

Writing chemical patterns using electrospun fibers as nanoscale inkpots for directed assembly of colloidal nanocrystals

Applications that range from electronics to biotechnology will greatly benefit from low-cost, scalable and multiplex fabrication of spatially defined arrays of colloidal inorganic nanocrystals. In this work, we present a novel additive patterning approach based on the use of electrospun nanofibers (NFs) as inkpots for end-functional polymers. The localized grafting of end-functional polymers from spatially defined nanofibers results in covalently bound chemical patterns. The main factors that determine the width of the nanopatterns are the diameter of the NF and the extent of spreading during the thermal annealing process. Lowering the surface energy of the substrates via silanization and a proper choice of the grafting conditions enable the fabrication of nanoscale patterns over centimeter length scales. The fabricated patterns of end-grafted polymers serve as the templates for spatially defined assembly of colloidal metal and metal oxide nanocrystals of varying sizes (15 to 100 nm), shapes (spherical, cube, rod), and compositions (Au, Ag, Pt, TiO2), as well as semiconductor quantum dots, including the assembly of semiconductor nanoplatelets.

The fabrication of mechanically durable and stretchable superhydrophobic PDMS/SiO2 composite film

Stretchable superhydrophobic film was fabricated by casting silicone rubber polydimethylsiloxane (PDMS) on a SiO₂ nanoparticle-decorated template and subsequent stripping. PDMS endowed the resulting surface with excellent flexibility and stretchability. The use of nanoparticles contributed to the sustained roughening of the surface, even under large strain, offering mechanically durable superhydrophobicity. The resulting composite film could maintain its superhydrophobicity (water contact angle ≈ 161° and sliding angle close to 0°) under a large stretching strain of up to 100% and could withstand 500 stretching–releasing cycles without losing its superhydrophobic properties. Furthermore, the obtained film was resistant to long term exposure to different pH solutions and ultraviolet light irradiation, as well as to manual destruction, sandpaper abrasion, and weight pressing.

Stretchable superhydrophobic film was fabricated by casting silicone rubber polydimethylsiloxane (PDMS) on a SiO₂ nanoparticle-decorated template and subsequent stripping.