Self-assembled biomimetic superhydrophobic hierarchical arrays

Leafhopper-inspired reversibly switchable antireflection coating with sugar apple-like structure arrays

Optical coatings with reversibly tunable antireflective characteristics hold a tremendous potential for next generation optical energy-related applications. Bioinpsired by the camouflage behavior of small yellow leafhoppers, silica hollow sphere/shape memory polymer composites are self-assembled using a non-lithography-based approach. The average visible transmittance of the as-patterned hierarchical structure array-covered substrate is increased by ca. 6.3% at normal incident, and even improved by more than 20% for an incident angle of 75°. Interestingly, the broadband omnidirectional antireflection performance can be reversibly erased and recovered by applying external stimuli under ambient conditions. To gain a better understanding, its reversibility, mechanical robustness, and the structure-shape effect on the antireflective properties are systematically investigated in this research.

Biomimetic superhydrophobic metal/nonmetal surface manufactured by etching methods: A mini review

As an emerging fringe science, bionics integrates the understanding of nature, imitation of nature, and surpassing nature in one aspect, and it organically combines the synergistic complementarity of function and structure-function integrated materials which is of great scientific interest. By imitating the microstructure of a natural biological surface, the bionic superhydrophobic surface prepared by human beings has the properties of self-cleaning, anti-icing, water collection, anti-corrosion and oil-water separation, and the preparation research methods are increasing. The preparation methods of superhydrophobic surface include vapor deposition, etching modification, sol-gel, template, electrostatic spinning, and electrostatic spraying, which can be applied to fields such as medical care, military industry, ship industry, and textile. The etching modification method can directly modify the substrate, so there is no need to worry about the adhesion between the coating and the substrate. The most obvious advantage of this method is that the obtained superhydrophobic surface is integrated with the substrate and has good stability and corrosion resistance. In this article, the different preparation methods of bionic superhydrophobic materials were summarized, especially the etching modification methods, we discussed the detailed classification, advantages, and disadvantages of these methods, and the future development direction of the field was prospected.

Visual Luminescent Probes Constructed by Eu3+ Complex-Functionalized Silica Nanocomposites and Their Langmuir-Blodgett Films at Interfaces

The trivalent europium ion (Eu³⁺) has garnered a great deal of interest for the design of luminescent materials possessing compound-independent emission bands, strong luminescent intensity, and long emission lifetimes. We herein introduce a synthetic methodology capable of constructing visual luminescent probes from Eu³⁺ complex-functionalized silica nanocomposites and their Langmuir-Blodgett (LB) films at interfaces. In order to facilitate the coordinative stabilization of Eu³⁺ over carrier surfaces, silica nanoparticles (nanoSiO₂) were pregrafted with terpyridyl (TPy) to make nanoSiO₂TPy linkers. Then, a well-designed coordination reaction of nanoSiO₂TPy with EuCl₃ and 2,6-pyridinedicarboxylic acid (DPA) was carried out at solid-liquid and air-water interfaces, where our desired material (denoted as nanoSiO₂TPy@EuDPA) and its corresponding LB film are obtained. The presence of TPy and DPA interacting with Eu³⁺ plays a key role in regulating the chemical nature of the particle surface, hence giving rise to closely packed nanocomposite arrays in the film. As a result, the improvement in uniformity and stability is achieved alongside the enhancement in emission intensity and lifetime. With such advantageous optical properties, we find them workable as facile, green, and affordable luminescent sensors, by which a range of common toxic anions (Cr₂O₇^2-, MnO₄^-, and PO₄^3-) can be visually and quantitatively recognized. Notably, the LB film-based material could afford a higher K_sv value (1.53 × 10⁵ M^-1), a lower detection limit (0.157 μM), and better recyclability than its original powder analogue, showcasing its utility as a more promising candidate for practical use.

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.

Nanotextured Si surfaces derived from block-copolymer self-assembly with superhydrophobic, superhydrophilic, or superamphiphobic properties

We demonstrate the use of wafer-scale nanolithography based on block-copolymer (BCP) self-assembly for the fabrication of surfaces with enhanced wetting properties.

TiO2 nanowire-templated hierarchical nanowire network as water-repelling coating

Extraordinary water-repelling properties of superhydrophobic surfaces make them novel candidates for a great variety of potential applications. A general approach to achieve superhydrophobicity requires low-energy coating on the surface and roughness on nano- and micrometre scale. However, typical construction of superhydrophobic surfaces with micro-nano structure through top-down fabrication is restricted by sophisticated fabrication techniques and limited choices of substrate materials. Micro-nanoscale topographies templated by conventional microparticles through surface coating may produce large variations in roughness and uncontrollable defects, resulting in poorly controlled surface morphology and wettability. In this work, micro-nanoscale hierarchical nanowire network was fabricated to construct self-cleaning coating using one-dimensional TiO2 nanowires as microscale templates. Hierarchical structure with homogeneous morphology was achieved by branching ZnO nanowires on the TiO2 nanowire backbones through hydrothermal reaction. The hierarchical nanowire network displayed homogeneous micro/nano-topography, in contrast to hierarchical structure templated by traditional microparticles. This hierarchical nanowire network film exhibited high repellency to both water and cell culture medium after functionalization with fluorinated organic molecules. The hierarchical structure templated by TiO2 nanowire coating significantly increased the surface superhydrophobicity compared to vertical ZnO nanowires with nanotopography alone. Our results demonstrated a promising strategy of using nanowires as microscale templates for the rational design of hierarchical coatings with desired superhydrophobicity that can also be applied to various substrate materials.

Electric-Field Assisted Assembly of Colloidal Particles into Ordered Nonclose-Packed Arrays

Nonclose-packed colloidal arrays have many potential applications ranging from plasmonic sensors, light trapping for photovoltaics, to transparent electrodes. However, scalable fabrication of those structures remains a challenge. In this Article, we investigate the robustness of an electric-field assisted approach systematically. A monolayer of nonclose-packed crystalline array is first created under a low-frequency alternating-current electric field in solution. We then apply a sequence of direct-current pulses to fix the particle array onto the substrate so that it remains intact even after both field removal and solvent evaporation. Key process parameters such as the alternating-current field strength, direct-current magnitude, particle concentration, and solvent-evaporation rate that affect both ordering and fixing of colloidal particles have been studied systematically. We find that direct currents with an intermediate magnitude induce electrophoretic motion of particles toward the substrate and facilitate their permanent adhesion on the substrate due to strong van der Waals attraction. A higher current, however, causes lateral aggregation of particles arising from electroosmotic flow of solvent and destroys the periodic ordering between particles. This approach, in principle, can be conveniently adapted into the continuous convective assembly process, thus making the fabrication of nonclose-packed colloidal arrays scalable.

Superhydrophobic hierarchical arrays fabricated by a scalable colloidal lithography approach

Here we report an unconventional colloidal lithography approach for fabricating a variety of periodic polymer nanostructures with tunable geometries and hydrophobic properties. Wafer-sized, double-layer, non-close-packed silica colloidal crystal embedded in a polymer matrix is first assembled by a scalable spin-coating technology. The unusual non-close-packed crystal structure combined with a thin polymer film separating the top and the bottom colloidal layers render great versatility in templating periodic nanostructures, including arrays of nanovoids, nanorings, and hierarchical nanovoids. These different geometries result in varied fractions of entrapped air in between the templated nanostructures, which in turn lead to different apparent water contact angles. Superhydrophobic surfaces with >150° water contact angles and <5° contact angle hysteresis are achieved on fluorosilane-modified polymer hierarchical nanovoid arrays with large fractions of entrapped air. The experimental contact angle measurements are complemented with theoretical predictions using the Cassie's model to gain insights into the fundamental microstructure-dewetting property relationships. The experimental and theoretical contact angles follow the same trends as determined by the unique hierarchical structures of the templated periodic arrays.

Wettability of natural superhydrophobic surfaces

Since the description of the 'Lotus Effect' by Barthlott and Neinhuis in 1997, the existence of superhydrophobic surfaces in the natural world has become common knowledge. Superhydrophobicity is associated with a number of possible evolutionary benefits that may be bestowed upon an organism, ranging from the ease of dewetting of their surfaces and therefore prevention of encumbrance by water droplets, self-cleaning and removal of particulates and potential pathogens, and even to antimicrobial activity. The superhydrophobic properties of natural surfaces have been attributed to the presence of hierarchical microscale (>1 μm) and nanoscale (typically below 200 nm) structures on the surface, and as a result, the generation of topographical hierarchy is usually considered of high importance in the fabrication of synthetic superhydrophobic surfaces. When one surveys the breadth of data available on naturally existing superhydrophobic surfaces, however, it can be observed that topographical hierarchy is not present on all naturally superhydrophobic surfaces; in fact, the only universal feature of these surfaces is the presence of a sophisticated nanoscale structure. Additionally, several natural surfaces, e.g. those present on rose petals and gecko feet, display high water contact angles and high adhesion of droplets, due to the pinning effect. These surfaces are not truly superhydrophobic, and lack significant degrees of nanoscale roughness. Here, we discuss the phenomena of superhydrophobicity and pseudo-superhydrophobicity in nature, and present an argument that while hierarchical surface roughness may aid in the stability of the superhydrophobic effect, it is nanoscale surface architecture alone that is the true determinant of superhydrophobicity.