Transition metals and their oxide materials have been widely employed to fabricate superhydrophobic surfaces, not only because of their surface topography with controllable microstructures leading to water-repellence, diverse adhesion even tunable wettability, but also due to a variety of special properties like optical performance, magnetism, anti-bacterial, transparency and so on. At the meantime, biomimetic superhydrophobic surfaces have attracted great interest from fabricating hierarchical micro-/nano-structures inspired by nature to imitate creature’s properties and many potential applications, including self-cleaning, antifogging, antireflection, low drag and great stability and durability. In this review, natural surfaces and biomimetic materials with special wettability are introduced by classification according to the similar microstructure of morphology, like array structure, sheet overlapped structure, high density hairs and seta shaped structure. Not only do we exhibit their special performances, but also try to find out the true reasons behind the phenomenon. Then, the recent progress of a series of superhydrophobic transition mental and their oxide materials, including TiO2, ZnO, Fe3O4, CuO, Ag, Au and so on, is presented with a focus on fabricating methods, microstructures, wettability, and other properties. As followed, these superhydrophobic surfaces can be applied in many fields, such as oil/water separation, self-cleaning, photo-controlled reversible wettability, surface-enhanced Raman scattering, antibacterial, anticorrosion, and synthesis of various applications. However, few of them have been applied in practical life. Hence, we discuss the remaining challenges at present and the development tendency in future at the end of this article. This review aims to present recent development of transition metals and their oxides applied in biomimetic superhydrophobic surfaces about fabrication, microstructure, water repellence, various properties, and potential applications.

The ability to control cell patterning on artificial substrates with various physicochemical properties is of essence for important implications in cytology and biomedical fields. Despite extensive progress, the ability to control the cell-surface interaction is complicated by the complexity in the physiochemical features of bioactive surfaces. In particular, the manifestation of special wettability rendered by the combination of surface roughness and surface chemistry further enriches the cell-surface interaction. Herein we investigated the cell adhesion behaviors of Circulating Tumor Cells (CTCs) on topographically patterned but chemically homogeneous surfaces. Harnessing the distinctive cell adhesion on surfaces with different topography, we further explored the feasibility of controlled cell patterning using periodic lattices of alternative topographies. We envision that our method provides a designer’s toolbox to manage the extracellular environment.

 

The response of human osteoblast-like osteosarcoma cells (MG63) to surface modification of Ti-6Al-4V implant alloy was investigated by Laser Interference Lithography (LIL). In this work, laser interference lithography was employed to fabricate the microstructures of grooves, dots and dimples onto the surfaces of Ti-6Al-4V samples. Two and three beam LIL systems were developed to carry out the experiments. The laser treatment resulted in the increases of the roughness and the contact angle of water on the implant alloy surfaces. The proliferation of osteoblasts was analyzed by MTT (3-(4,5-dimethyl- 2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide) assay for the time periods of 4 hours, 2 days, 3 days, and
6 days. The MTT test results demonstrated that the laser treatment surfaces had a positive impact on the proliferation of osteoblast cells after 24 hours. The alloy surface morphology and the morphological changes of MG63 cells cultured on the laser textured Ti-6Al-4V surface were observed by Scanning Electron Microscope (SEM). The SEM results indicated that the osteoblast cells were aligned on grooved surfaces and they were prolonged with the structures. Enzymatic detachment results showed that the 20 µm grooved structures provided the better cell adhesion to the textured Ti-6Al-4V surfaces.

The aim of the present paper is to characterize bioinspired chitosan (CS) + hydroxyapatite (HA) coatings with various components ratio on a zirconium alloy with titanium. The coatings were characterized by FT-IR, SEM, hydrophilic/hydrophobic balance, adherence, roughness, electrochemical stability and in vitro cell response. Electrochemical tests, including potentiodynamic polarization curves and electrochemical impedance spectroscopy, were performed in normal saline physiological solution. Cell viability of MC3T3-E1 osteoblasts, lactate dehydrogenase, nitric oxide, and Reactive Oxygen Species (ROS) levels, as well as actin cytoskeleton morphology, were evaluated as biological in vitro tests. The results on in vitro cell response indicated good cell membrane integrity and viability for all samples, but an increased cell number, a decreased ROS level and a better cytoskeleton organization were noticed for the sample with a higher CS content. The coating with highest CS concentration indicated the best performance based on the experimental data. The highest hydrophilic character, highest resistance to corrosion and best biocompatibility as well recommend this coating for bioapplications in tissue engineering.

Numerous exceptional properties can be observed in nature. Among these properties, parahydrophobic feature is of interest. This property describes material with high adhesion with water such as rose petals or gecko foot. Such kind of surface presents a real potential for applications in the field of water harvesting systems. In this work, we report a new synthetic strategy to mimic this property. Here, we combine three strategies in one. First, a monomer is electropolymerized in order to form the starting structured surface. Then, nanoparticles are grafted on the surface to increase the structuration and consequently to create the reactive surface. Finally, the grafted surface is post-functionalized (Huisgen reaction) with various aryl alkynes to control the surface chemistry and energy. This strategy allows to reach surfaces with both very high hydrophobic properties (? = 140?) and high water adhesion. This work also includes the surface wettability, roughness and morphology investigation in order to study the impact of the starting monomer structure and post-functionalization on the surface properties.

Superhydrophobic surfaces are often found in nature, such as plant leaves and insect wings. Inspired by superhydrophobic phenomenon of the rose petals and the lotus leaves, biomimetic hydrophobic surfaces with high or low adhesion were prepared with a facile drop-coating approach in this paper. Poly(vinyl alcohol) (PVA) was used as adhesive and SiO2 nanoparticles were used to fabricate surface micro-structure. Stearic acid or dodecafluoroheptyl-propyl-trimethoxysilane (DFTMS) were used as low surface energy materials to modify the prepared PVA/SiO2 coating surfaces. The effects of size of SiO2 nanoparticles, concentration of SiO2 nanoparticle suspensions and the modifications on the wettability of the surface were investigated. The morphology of the PVA/SiO2 coating surfaces was observed by using scanning electron microscope. Water contact angle of the obtained superhydrophilic surface could reach to 3?. Stearic acid modified PVA/SiO2 coating surfaces showed hydrophobicity with high adhesion. By mixing the SiO2 nanoparticles with sizes of 40 nm and 200 nm and modifying with DFTMS, water contact angle of the obtained coating surface could be up to 155? and slide angle was only 5?. This work provides a facile and useful method to control surface wettability through changing the roughness and chemical composition of a surface.

The hydrophobicity of natural surfaces has drawn much attention of scientific communities in recent years. By mimicking natural surfaces, the manufactured biomimetic hydrophobic surfaces have been widely applied to green technologies such as self-cleaning surfaces. Although the theories for wetting and hydrophobicity have been developed, the mechanism of wetting transitions between heterogeneous wetting state and homogeneous wetting state is still not fully clarified. As understanding of wetting transitions is crucial for manufacturing a biomimetic superhydrophobic surface, more fundamental discussions in this area should be carried out. In the present work, the wetting transitions are numerically studied using a phase field lattice Boltzmann approach with large density ratio, which should be helpful in understanding the mechanism of wetting transitions. The dynamic wetting transition processes between Cassie-Baxter state and Wenzel state are presented, and the energy barrier and the gravity effect on transition are discussed. It is found that the two wetting transition processes are irreversible for specific inherent contact angles and have different transition routes, the energy barrier exists on an ideally patterned surface and the gravity can be crucial to overcome the energy barrier and trigger the transition.

Oily water treatment has attracted the attention of many researchers. The development of effective and cheap oil/water separation materials is urgent for treating this problem. Herein, inspired by superhydrophobic typical plant leaves such as lotus, red rose and marigold, superhydrophobic and superoleophilic copper mesh was fabricated by etching and then surface modification with 1-dodecanethiol (HS(CH2)11CH3). A rough silver layer is formed on the mesh surface after immersion. The obtained mesh surface exhibits superhydrophobicity and superoleophilicity and the static water contact angle was 153? ± 3?. In addition, the as-prepared copper mesh shows self-cleaning character with water and chemical stability. The as-prepared copper foam can easily remove the organic solvents either on water or underwater. We demonstrate that by using the as-prepared mesh, oils can be absorbed and separated, and that high separation efficiencies of larger than 92% are retained for various oils. Thus, such superhydrophobic and superoleophilic copper mesh is a very promising material for the application of oil spill cleanup and industrial oily wastewater treatment.

This paper addresses the potential to use Lotus leaf bioinspired surfaces in applications involving heat transfer with phase change, namely pool boiling and spray impingement. Besides describing the role of bioinspired topographical features, using an innovative technique combining high-speed visualization and time-resolved infrared thermography, surface durability is also addressed. Water is used for pool boiling and for spray impingement systems (simplified as single droplet impact), while HFE7000 is used in a pool boiling cooler for electronic components. Results show that surface durability is quickly compromised for water pool boiling applications, as the chemical treatment does not withstand high temperatures (T > 100 ?C) during long time intervals (3 h – 4 h). For HFE7000 pool boiling (depicting lower saturation temperature − 34 ?C), heat transfer enhancement is governed by the topography. The regular hierarchical pattern of the bioinspired surfaces promotes the heat transfer coefficient to increase up to 22.2%, when compared to smooth surfaces, while allowing good control of the interaction mechanisms until a distance between micro-structures of 300 μm – 400 μm. Droplet impingement was studied for surface temperatures ranging between 60 ?C – 100 ?C. The results do not support the use of superhydrophobic surfaces for cooling applications, but reveal great potential for other applications involving droplet impact on heated surfaces (e.g. metallurgy industry).

Bionic surface structures, inspired by the flora, were developed for Sheet-Bulk Metal Forming (SBMF) in order to locally control the friction condition by adjusting the wetting behavior. Five bionic structures were micromilled on ASP®2023 in annealed as well as hardened and tempered conditions. Subsequently, the structured surfaces were plasma-nitrided and coated with a CrAlN thin film. The influence of the treatment method on the structural geometry was investigated with the aid of a scanning electron microscope and 3D-profilometer. The wetting behaviors of water and deep drawing oil (Berufluid ST6007) on bionic surfaces were evaluated using contact angle measurements. The resulting micro-milled structures exhibit an almost identical shape as their bionic models. However, the roughness of the structured surfaces is influenced by the microstructure. The combination of plasma-nitriding and Physical Vapor Deposition (PVD) leads to an increase in roughness. All bionic structures possess higher contact angles than that of the unstructured surfaces when wetted by water. This can be explained by the fact that the structural elevations block the spreading. When the bionic surfaces are wetted by deep drawing oil, the lubricant spreads in the structural cavities, leading to smaller contact angles. Furthermore, the anisotropy of the structure has an influence on the wetting behavior.

TiO2–SiO2 composite films were produced on commercially pure titanium (CP-Ti) substrate by a sol–gel method to investigate the behavior of sol aging time and its potential effects on the structural and electrochemical properties of composite coatings. Anatase-TiO2 and quartz-SiO2 peaks were observed on all composite coated samples according to XRD results. It was observed that the average grain size increased with sol aging time. Also, the average smallest grain size was seen at composite coatings prepared from unaged sol according to the width of the peaks. Electrochemical behavior of coated samples was mainly investigated by potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS) in Simulated Body Fluid (SBF) solution. In corrosion tests, the composite coatings showed better anti-corrosion behavior than that of uncoated samples. In addition, the corrosion properties of the composite films were considerably affected by sol aging time. Corrosion resistance of coatings decreased with increasing aging time and the best result was obtained from composite coatings prepared from unaged sol.

The anti-adhesive surfaces have always aroused great interest of worldwide scientists and engineers. But in practical applications, it often faces the threat and impact of temperature and humidity. In this work, the excellent anti-adhesive performance of maize leaf under high temperature and humidity were investigated in detail. Firstly, the adhesion forces of the maize leaf surface under different temperature and humidity were measured by using Atomic Force Microscopy (AFM). The temperature of the substrate was varied between 23 ?C to 100 ?C, and the ambient relative humidity is from 18% to 100%. It was found that the adhesion force of maize leaf decreased with the increase of temperature and humidity. The mechanism of its excellent anti-adhesive performance of maize leaf under high temperature and relative humidity was revealed. The transverse and longitudinal ridges on maize leaf surface interlace with each other, forming small air pockets, which reduces the actual contact area between the object and the maize leaf. With the increase of humidity, the liquid film will be formed in the air pockets gradually and so much water vapor is produced with increase of temperature. Then the air flow rate increases though the wavy top of transverse ridges, inducing the dramatic decrease of adhesion force. Inspired by this mechanism, four samples with this bionic structure were made. This functional “biomimetic structure” would have potential value in the wide medical equipments such as high frequency electric knife with anti-adhesion surface under high temperature and high humidity.

Honeybees have received public attention for their remarkable performance in low-altitude flying and their outstanding airborne hovering capability. However, minimal attention has been given to their capability to fly through the harshest climatic conditions. In this study, we used a high-speed camera and recorded an interesting phenomenon in which honeybees (Apis mellifera ligustica) flew effortlessly through mists or drizzling rain. To identify the mechanism behind honeybees flying through mists, the microstructure of their wings was examined via atomic force microscopy and scanning electron microscopy. Experimental results showed that the surface of a honeybee wing was rough, with bristles distributed on both the dorsal and ventral sides. The measurement results of the contact angle proved that the surface of honeybee wings was hydrophobic. Furthermore, hydrophobic proteins, which contained at least one hydrophobic tetra-peptide (i.e., AAPA/V), were obtained. The rugged surface and hydrophobic proteins caused the hydrophobicity of honeybee wings. These results identify the hydrophobic mechanism of honeybee wings, which will be useful in designing hydrophobic structures.

The microstructure of the main longitudinal veins of the dragonfly wing and the aerodynamic behaviors of the wing were investigated in this paper. The microstructure of longitudinal vein presents two circumferential chitin layers and a protein-fiber soft layer. The dragonfly wing is corrugated due to the spatial arrangement of longitudinal veins. It was found that the corrugation angle could significantly influence the lift/drag ratio across a range of attack angles by the wind tunnel experiments. The results of the finite element analysis indicate that the protein soft layer of vein facilitates the change of the corrugation angle by allowing substantial relative twisting deformation between two neighboring veins, which is not possible in veins without a soft sandwich layer.

An advanced electro-active dry adhesive, which was composed of a mushroom-shaped fibrillar dry adhesive array actuated by an Ionic Polymer Metal Composite (IPMC) artificial muscle reinforced with nitrogen-doped carbon nanocages (NCNCs), was developed to imitate the actuation of a gecko’s toe. The properties of the NCNC-reinforced Nafion membrane, the electro- mechanical properties of the NCNC-reinforced IPMC, and the related electro-active adhesion ability were investigated. The NCNCs were uniformly dispersed in the 0.1 wt% NCNC/Nafion membrane, and there was a seamless connection with no clear interface between the dry adhesive and the IPMC. Our 0.1 wt% NCNC/Nafion-IPMC actuator shows a displacement and force that are 1.6 – 2 times higher than those of the recast Nafion-IPMC. This is due to the increased water uptake (25.39%) and tensile strength (24.5 MPa) of the specific 3D hollow NCNC-reinforced Nafion membrane, as well as interactions between the NCNCs and the sulfonated groups of the Nafion. The NCNC/Nafion-IPMC was used to effectively actuate the mushroom-shaped dry adhesive. The normal adhesion forces were 7.85 mN, 12.1 mN, and 51.7 mN at sinusoidal voltages of 1.5 V, 2.5 V, and 3.5 V, respectively, at 0.1 Hz. Under the bionic leg trail, the normal and shear forces were approximately 713.5 mN (159 mN?cm−2) and 1256.6 mN (279 mN?cm−2), respectively, which satisfy the required adhesion. This new electro-active dry adhesive can be applied for active, distributed actuation and flexible grip in robots.

Windbreak fences in open and urban areas can be used to effectively reduce the wind velocity. In this paper we examine how the geometrical shape of the windbreak fence can optimally mitigate wind velocity. We propose an approach for windbreak fence design based on a bionic parametric model of the shark skin denticle geometry, which improves the reduction of the wind velocity around and behind the windbreak fences. The generative model was used to estimate improvements by variations in the parameters of the fence panel’s geometrical shape, inspired by shark skin denticles. The results of the Computational Fluid Dynamics (CFD) analysis indicates that the fence surface inspired by shark skin performs much better than both flat and corrugated surfaces. Taking into account the complex geometry of the surface inspired by shark skin denticles, we propose a fabrication process using an expanded polystyrene foam (EPS) material, created using an industrial robot arm with a hot-wire tool. Creating EPS moulds for the shark skin denticle panels allows for a richer variety material to be used in the final design, leading both to higher efficiency and a more attractive design.