Strelitzia reginae leaf as a natural template for anisotropic wetting and superhydrophobicity

Anisotropic Superhydrophobic Properties Replicated from Leek Leaves

A bio-inspired approach to fabricate robust superhydrophobic (SHB) surfaces with anisotropic properties replicated from a leek leaf is presented. The polydimethylsiloxane (PDMS) replica surfaces exhibit anisotropic wetting, anti-icing, and light scattering properties due to microgrooves replicated from leek leaves. Superhydrophobicity is achieved by a novel modified candle soot (CS) coating that mimics leek's epicuticular wax. The resulting surfaces show a contact angle (CA) difference of ≈30° in the directions perpendicular and parallel to the grooves, which is similar to the anisotropic properties of the original leek leaf. The coated replica is durable, withstanding cyclic bending tests (up to 10 000 cycles) and mechanical sand abrasion (up to 60 g of sand). The coated replica shows low ice adhesion (10 kPa) after the first cycle; and then, increases to ≈70 kPa after ten icing-shearing cycles; while, anisotropy in ice adhesion becomes more evident with more cycles. In addition, the candle soot-coated positive replica (CS-coated PR) demonstrates a transmittance of ≈73% and a haze of ≈65% at the wavelength of 550 nm. The results show that the properties depend on the replicated surface features of the leek leaf, which means that the leek leaf appears to be a highly useful template for bioinspired surfaces.

Optimizing anisotropic transport on bioinspired sawtooth surfaces

Species ranging from butterflies and other insects, to cactuses and lotus plants have evolved to use geometrically patterned surfaces to influence the transport of water droplets. While this phenomenon is well known, an ideal geometry has yet to be discovered. To determine the impact of surface geometry on droplet transport, we have studied the contact angle and droplet motion across anisotropically wetting patterned surfaces. The surface geometries tested were sawtooth patterns with angles (8.62-26.70°) and lengths (0.56-1.67 μm). The droplet contact angles were measured on 45° angled surfaces to simulate the droplet in motion. Velocities were measured using a high-speed camera shooting at 500 frames per second and the tailing edges of the droplets were hand tracked over 18 frames. It was found that travel along the sawtooth ridges is significantly faster than travel against the ridges for geometries with shallow angles. The optimal geometry was determined to be α = 8.62° and b = 1.67 μm and was replicated using nanoimprint lithography using materials with different surface energies. When replicated with acrylate resins and PDMS, the contact angles remained high, regardless of wettability, but we find that the overall velocity and velocity hysteresis depends on the hydrophobicity. More hydrophobic surfaces have overall higher hysteresis. The ability to tune imprinted surfaces to achieve ideal wetting characteristics using geometry will lead to interesting anisotropic material design.

A wear and heat-resistant hydrophobic fluoride-free coating based on modified nanoparticles and waterborne-modified polyacrylic resin

Hydrophobic coatings have attracted extensive research due to their broad application prospects. However, hydrophobic coatings in practical applications are often limited by their insufficient stability and are difficult to be applied on a large scale. In this regard, wear and heat resistance are key aspects that must be considered. In this paper, a method for preparing a robust hydrophobic coating with modified ZrO₂ particles as the core component and modified acrylic resin is proposed. First, γ-aminopropyltriethoxysilane (APTES) was used to silanize ZrO₂ to obtain Si-ZrO₂ nanoparticles, which were grafted with amino groups. Then, the nanoparticles reacted with isocyanates to be grafted with hydrophobic groups. A simple spray method was developed to deposit a hydrophobic (141.8°) coating using the mixture containing the modified nanoparticles and non-fluorinated water-based silicon-modified acrylic resin (WSAR) that was prepared by free radical polymerization. The obtained coating exhibited a rough surface and the particles and resin were closely combined. Compared with pure resin coating, the composite coating exhibited 150% enhancement in wear resistance and it could wear 45 meters at a pressure of 20 kPa. Moreover, the coating could maintain the hydrophobic property even when it lost 70% quality or after it was heated at 390 °C. The thermogravimetric results showed that the temperature could reach 400 °C before the quality of the fluorine-free coating dropped to 90%. In addition, the coating could easily take away graphite or silicon carbide powder under the impact of water droplets, showing excellent self-cleaning performance.

Influences of Stent Design on In-Stent Restenosis and Major Cardiac Outcomes: A Scoping Review and Meta-Analysis

Thanks to the developments in implantable biomaterial technologies, invasive operating procedures, and widespread applications especially in vascular disease treatment, a milestone for interventional surgery was achieved with the introduction of vascular stents. Despite vascular stents providing a solution for embolisms, this technology includes various challenges, such as mechanical, electro-chemical complications, or in-stent restenosis (ISR) risks with long-term usage. Therefore, further development of biomaterial technologies is vital to overcome such risks and problems. For this purpose, recent research has focused mainly on the applications of surface modification techniques on biomaterials and vascular stents to increase their hemocompatibility. ISR risk has been reduced with the development and prevalent usage of the art technology stent designs of drug-eluting and biodegradable stents. Nevertheless, their problems have not been overcome completely. Furthermore, patients using drug-eluting stents are faced with further clinical challenges. Therefore, the bare metal stent, which is the first form of the vascular stent technology and includes the highest ISR risk, is still in common usage for vascular treatment applications. For this reason, further research is necessary to solve the remaining vital problems. In this scoping review, stent-based major cardiac events including ISR are analyzed depending on different designs and material selection in stent manufacturing. Recent and novel approaches to overcome such challenges are stated in detail.

Tailoring silicon for dew water harvesting panels

Dew water, mostly ignored until now, can provide clean freshwater resources, just by extracting the atmospheric vapor available in surrounding air. Inspired by silicon-based solar panels, the vapor can be harvested by a concept of water condensing panels. Efficient water harvesting requires not only a considerable yield but also a timely water removal from the surface since the very beginning of condensation to avoid the huge evaporation losses. This translates into strict surface properties, which are difficult to simultaneously realize. Herein, we study various functionalized silicon surfaces, including the so-called Black Silicon, which supports two droplet motion modes-out-of-plane jumping and in-plane sweeping, due to its unique surface morphology, synergistically leading to a pioneering combination of above two required characteristics. According to silicon material's scalability, the proposed silicon-based water panels would benefit from existing infrastructures toward dual functions of energy harvesting in daytime and water harvesting in nighttime.

Long-Lived T-Shaped Micropillars with Submicron-Villi on PP/POE Surfaces with Grinding-Enhanced Water Repellency Fabricated via Hot Compression Molding

Superhydrophobic properties are derived from the roughness of the surface of micro/nanostructures and low-surface-energy materials. However, they are both easy to damage on superhydrophobic surfaces after mechanical abrasion in practical applications, resulting in the transition from the Cassie-Baxter state to the Wenzel state and even the loss of water repellency. In this work, the mechanical properties of polypropylene (PP) toughened with poly(ethylene-co-octene) (POE) were improved for the fabrication of long-lived T-shaped micropillars with submicron-villi on top by a combined method of compression molding and grinding. A universal testing machine was modified as equipment for the precise control of the traveling distance of specimens on sandpaper in precise. The PP/POE blend possessed high tensile strength of up to ∼23.84 MPa as well as elongation at break of ∼533.60%. The abrasive grains on sandpaper reshaped their surface morphologies from micropillars to T-shaped microstructures, on which the submicron-villi as secondary structures formed. The abraded microstructured PP/POE surface exhibited the highest contact angle of 154.4° and the most stable wetting state with a bouncing height of 7.68 mm (3.2 times the diameter of the 7-μL droplet) after a traveling distance of 1000 mm on 3000-grit sandpaper among the abraded and unabraded PP/POE surfaces.

Biased Motions of a Droplet on the Inclined Micro-conical Superhydrophobic Surface

Anisotropic superhydrophobic surfaces that have many superior properties, such as directional self-cleaning, droplet transport, heat transfer, and so on, are widely used in various fields. Different from symmetric surfaces, a water droplet often shows directional spreading, moving, and bouncing on asymmetric surfaces. To investigate the mechanisms and achieve controllability of droplet motions on asymmetric surfaces, a series of surfaces with inclined micro-conical arrays are fabricated by integrating the methods of soft lithography, hot-pressing, and crystal growth. We found that the droplet would spread along the reverse direction of micro-cone's orientation but bounce and detach off the surface and move toward the direction of micro-cone's orientation. To understand these interesting performances, a mathematical model is established from the perspective of force balance, and a series of numerical simulations are performed. Additionally, the relationship between the droplet motions and the micro-structural parameters, including the inclined angle, line space, and height, are studied. This work may provide useful insights into droplet controlling, anisotropic surface designing, and its applications.

Fabrication of Impact-Resistant and Wear-Recoverable Superhydrophobic Surfaces

Robustness of superhydrophobic materials has been gradually taken into consideration for practical applications; however, little attention has been paid to the impact resistance of the superhydrophobicity of the materials. The present study demonstrated a new route for improving the mechanical durability, especially the impact resistance, of the superhydrophobic materials. First, poly(styrene-co-butadiene)/poly(ethylene-vinyl acetate) (SBR/EVA) composite monoliths with microscale cellular structures were manufactured by vulcanization-foaming processes. Then the composite monoliths were treated with sandpaper to create nanostructures above the revealed micropores after removing the uppermost skin, forming micro/nanotextured surfaces and giving improvements in superhydrophobicity. Due to the elastomeric nature of SBR and EVA, the superhydrophobicity of the monoliths can be maintained even while the material is mechanically impacted or compressed, and wearing helps improvement or recovery of the superhydrophobicity because of the self-similarity of the cellular structure inside the monoliths. Additionally, the obtained superhydrophobic materials are resistant to acidic, alkali, and salt liquors as well as organic solvents and have easy healing capacity of superhydrophobicity with a simple sanding treatment when destroyed by exposure to oxygen plasma.

3D Arrays of Super-Hydrophobic Microtubes from Polypore Mushrooms as Naturally-Derived Systems for Oil Absorption

Porous materials derived from natural resources, such as Luffa sponges, pomelo peel and jute fibres, have recently emerged as oil adsorbents for water purification, due to their suitability, low environmental impact, biodegradability and low cost. Here we show, for the first time, that the porosity of the fruiting body of polypore mushrooms can be used to absorb oils and organic solvents while repelling water. We engineered the surface properties of Ganoderma applanatum fungi, of which the fruiting body consists of a regular array of long capillaries embedded in a fibrous matrix, with paraffin wax, octadecyltrichlorosilane (OTS) and trichloro(1H,1H,2H,2H-perfluorooctyl)silane. Morphological and wettability analyses of the modified fungus revealed that the OTS treatment was effective in preserving the 3D porosity of the natural material, inducing super-hydrophobicity (water contact angle higher than 150°) and improving oil sorption capacity (1.8⁻3.1 g/g). The treated fungus was also inserted into fluidic networks as a filtration element, and its ability to separate water from chloroform was demonstrated.

Multifunctional WS2&M-AgNPs superhydrophobic conductive sponges for application in various sensors

Superhydrophobic conductive sponge is prepared by an easy method based on WS₂ nanosheets and modified Ag nanoparticles in this work, which is promising to apply in various sensors derived from superior liquid repellence, thermo stability, conductive property, mechanical and chemical durability.

Hydrodynamic Parameters of Strelitzia Gum

The flower of Strelitzia reginae generates abundant and viscous mucilage as exudate, which is purified in periods of heating–cooling, and finally precipitated with ethanol, obtaining strelitzia gum (StrG). By means of intrinsic viscosity measurement, the viscometric molecular weight (MWv) is determined, with a value of 200,000 g/mol, as well as a hydrodynamic radius of 20 ± 1 nm and a hydration value of 445 ± 34 g/g. The size of StrG was compared against dynamic light scattering data with a value of 16 ± 2 nm and a MWDLS of 230,000 g/mol. StrG is a biopolyelectrolyte with an “a” value of 0.85, which corresponds to a flexible behavior with a great effect of volume exclusion. This statement is based on the difficulty of gum dissolution, that should be performed at 80 °C. This macromolecule is very promising and can potentially be used in several industrial applications, such as in film forming, and as a gel, thickener, and coemulsifier.

Size dependence of static polymer droplet behavior from many-body dissipative particle dynamics simulation

We used molecular simulation to study the static behavior of polymer droplets in vacuum and on solid surfaces, namely the size of the droplet and the contact angle, respectively. The effects of the polymer chain length and the total number of particles were calculated by the many-body dissipative particle dynamics method. For the spherical droplet containing the same number of particles, we show that its radius depends on the polymer chain length. The radius of the droplet is also proportional to one-third power of the total number of particles for all given chain lengths. For the hemispherical droplet, the contact angle increases with the number of particles in the droplet, and this effect is relatively strong, especially for longer polymer chains. The effect of wettability of the solid surface was also investigated by using polymerphobic (low-affinity) and polymerphilic (high-affinity) surfaces. As the chain length increases, the contact angle on the low-affinity surface decreases, while that on the hydrophilic surface increases. The simulation reveals that there is a critical affinity for the monomer on the solid surface; above and below which the wettability increases and decreases as the molecular length increases, respectively.

Anisotropic sliding on dual-rail hydrophilic tracks

Biomimetic surfaces with sliding angle (SA) anisotropy have the capacity to directionally control the motion of water droplets and therefore have wide applications in various domains. Parallel and narrowing dual-rail hydrophilic tracks (DRHTs) are fabricated on etched superhydrophobic Al surfaces using a micromilling technique. Orthogonal and linear SA anisotropies are observed and investigated on the parallel and narrowing DRHTs, respectively. Track spacings of the parallel DRHTs are designed to regulate the orthogonal SA anisotropy of the water droplet. Experimental data shows that the along-track droplet-substrate interfacial widths, together with the sliding anisotropy, decrease with the increase of the track spacings. SA contrast (linear SA anisotropy) in two opposite directions along the tracks is observed and discussed on the narrowing DRHTs. Results indicate that droplets slide with more difficulty in the spacing-expanding direction than those in the shrinking direction, and when a droplet is dispensed at the tail end of a DRHT segment, the along-track outward detaching SAs and inward SAs also show sharp linear anisotropy due to the droplet-track interfacial liquid tension. On the basis of the discussed orthogonal and linear SAs, potential lab-on-a-chip applications for intelligent droplet transport, mixing and capture & release are explored.

Biomimetic polymeric superhydrophobic surfaces and nanostructures: from fabrication to applications

Numerous research studies have contributed to the development of mature superhydrophobic systems. The fabrication and applications of polymeric superhydrophobic surfaces have been discussed and these have attracted tremendous attention over the past few years due to their excellent properties. In general, roughness and chemical composition, the two most crucial factors with respect to surface wetting, provide the basic criteria for yielding polymeric superhydrophobic materials. Furthermore, with their unique properties and flexible configurations, polymers have been one of the most efficient materials for fabricating superhydrophobic materials. This review aims to summarize the most recent progress in polymeric superhydrophobic surfaces. Significantly, the fundamental theories for designing these materials will be presented, and the original methods will be introduced, followed by a summary of multifunctional superhydrophobic polymers and their applications. The principles of these methods can be divided into two categories: the first involves adding nanoparticles to a low surface energy polymer, and the other involves combining a low surface energy material with a textured surface, followed by chemical modification. Notably, surface-initiated radical polymerization is a versatile method for a variety of vinyl monomers, resulting in controlled molecular weights and low polydispersities. The surfaces produced by these methods not only possess superhydrophobicity but also have many applications, such as self-cleaning, self-healing, anti-icing, anti-bioadhesion, oil-water separation, and even superamphiphobic surfaces. Interestingly, the combination of responsive materials and roughness enhances the responsiveness, which allows the achievement of intelligent transformation between superhydrophobicity and superhydrophilicity. Nevertheless, surfaces with poor physical and chemical properties are generally unable to withstand the severe conditions of the outside world; thus, it is necessary to optimize the performances of such materials to yield durable superhydrophobic surfaces. To sum up, some challenges and perspectives regarding the future research and development of polymeric superhydrophobic surfaces are presented.

Superoleophobic surfaces

This review systematically summarizes the recent developments of superoleophobic surfaces, focusing on their design, fabrication, characteristics, functions, and important applications.

New insights and perspectives into biological materials for flexible electronics

Materials based on biological materials are becoming increasingly competitive and are likely to be critical components in flexible electronic devices.

Controllable anisotropic wetting characteristics on silicon patterned by slit-based spatial focusing of femtosecond laser

We propose a promising method to fabricate controllable anisotropic morphologies in which the slit-based spatial focusing of femtosecond laser is used to create an elliptical-shaped intensity distribution at focal plane, inducing elliptical-shaped morphology with micro/nano-dual-scale structures. Our study shows that 1) by increasing slit width, minor axis increases while major axis and axial ratio decrease; 2) with fixed slit width and laser fluence above the threshold, axial ratio is independent of irradiation pulse number; and 3) when polarization direction is changed from 0° to 90°, the axial ratio of anisotropic morphology declines. As a case study, large-area periodic anisotropic hierarchical structures are fabricated with the bidirectional anisotropic wetting.

Plant leaves as templates for soft lithography

Complex microvascular venation patterns of natural leaves are replicated into PDMS replicas, which allows for a leakage-tight seal with a flat substrate despite the surface topography.

Femtosecond laser controlled wettability of solid surfaces

Femtosecond laser microfabrication is emerging as a hot tool for controlling the wettability of solid surfaces. This paper introduces four typical aspects of femtosecond laser induced special wettability: superhydrophobicity, underwater superoleophobicity, anisotropic wettability, and smart wettability. The static properties are characterized by the contact angle measurement, while the dynamic features are investigated by the sliding behavior of a liquid droplet. Using different materials and machining methods results in different rough microstructures, patterns, and even chemistry on the solid substrates. So, various beautiful wettabilities can be realized because wettability is mainly dependent on the surface topography and chemical composition. The distinctions of the underlying formation mechanism of these wettabilities are also described in detail.

Effective directional self-gathering of drops on spine of cactus with splayed capillary arrays

We report that the fast droplet transport without additional energy expenditure can be achieved on the spine of cactus (Gymnocalycium baldianum) with the assistance of its special surface structure: the cactus spine exhibits a cone-like structure covered with tilted scales. A single scale and the spine surface under it cooperatively construct a splayed capillary tube. The arrays of capillary tube formed by the overlapping scales build up the out layer of the spine. The serial drops would be driven by the asymmetric structure resulted from tilt-up scales-by-scales on the cone-shaped spine, and move directionally toward the bottom from top of spine, by means of the Laplace pressure in differences. In addition, after the past of the first droplet, thin liquid film of drop is trapped in the splayed capillary micro-tube on the surface of spine, which greatly reduces the friction of subsequential droplet transport in efficiency. This finding provides a new biological model which could be used to transport droplet spontaneously and directionally. Also this work offers a way to reduce the surface adhesion by constructing liquid film on the surface, which has great significance in prompting droplet transport efficiency.

Controlled Smart Anisotropic Unidirectional Spreading of Droplet on a Fibrous Surface

Smart anisotropic-unidirectional spreading is displayed on a wettable-gradient-aligned fibrous surface due to a synergetic directing effect from the aligned structure and the ratio of hydrophilic components.

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.

Durability and restoring of superhydrophobic properties in silica-based coatings

The durability and restorable properties of superhydrophobicity are a most important issue essential for utilization of superhydrophobic materials in industrial and domestic fields. In this work, we have focused on the sol-gel dip coating synthesis, durability, and restoring properties of a superhydrophobic surface composed of spherical silica particles with low energy free surface successfully achieved on the surface of glass substrates. The water contact angle (WCA) of the as-prepared superhydrophobic coatings reached as high as 170±1° (WCA=170±1°) and the sliding angle was 3±1° (WSA=3±1°). The behavior of wettability on superhydrophobic coating surface under various external disturbances including outdoor environments, heat, and mild acidic condition is investigated. It is shown in particular that degradation and repairing of surface methyl groups between two such surfaces leads to a fully restorable coating surface by the surface modification with trimethylchlorosilane (TMCS) at room temperature. The coating also holds good thermal stability; it holds superhydrophobicity up to 550 °C and gets degrade into superhydrophilicity above 600 °C without deforming surface morphology. The results confirm the durability of the silica coating under different external disturbances and sol-gel dip coating method and restoring property provide the best solution to fabricate superhydrophobic silica coating surface with extended durability at low cost.

Superhydrophobic surfaces by electrochemical processes

This review is an exhaustive representation of the electrochemical processes reported in the literature to produce superhydrophobic surfaces. Due to the intensive demand in the elaboration of superhydrophobic materials using low-cost, reproducible and fast methods, the use of strategies based on electrochemical processes have exponentially grown these last five years. These strategies are separated in two parts: the oxidation processes, such as oxidation of metals in solution, the anodization of metals or the electrodeposition of conducting polymers, and the reduction processed such as the electrodeposition of metals or the galvanic deposition. One of the main advantages of the electrochemical processes is the relative easiness to produce various surface morphologies and a precise control of the structures at a micro- or a nanoscale.