Biomimetic fabrication and characterization of an artificial rice leaf surface with anisotropic wetting

Anticorrosion Behaviour of SS304 Microgroove Surfaces in Saline Water

The 304 Stainless Steel (SS304) is severely affected by salt water corrosion due to its high surface wettability. By reducing its surface wettability, its corrosion can be reduced. To achieve this, topographical modification of the steel surface is an effective route. In this work, SS304 flat surfaces were topographically modified into microgrooves (ridge width 250 μm to 500 μm, groove width 200 μm, width ratio = ridge width/groove width >1). Wire cut electrical discharge machining was used to fabricate the microgrooves. Long-term wetting characteristics and long-term corrosion behaviour of flat surface and microgrooves were studied. The influence of the nature of wetting of the tested surfaces on their corrosion behaviour was examined. The sessile drop method and potentiodynamic polarization tests in sodium chloride (3.5 wt. % NaCl) solution (intermittent and continuous exposures for 168 h) were studied to characterize their wetting and corrosion behaviours, respectively. Topographical modification imparted long-term hydrophobicity and, as a consequence, long-term anticorrosion ability of the steel surface. Micropatterning reduced the corrosion rate by two orders of magnitude due to reduction in interfacial contact area with the corrosive fluid via composite wetting, i.e., solid–liquid–air interface. Microgrooves showed corrosion inhibition efficiency ≥88%, upon long-term exposure to NaCl solution. By comparing the wetting and corrosion behaviours of the microgrooves with those of the previously studied microgrooves (ridge width/groove width <1), it was found that the surface roughness of their ridges strongly influences their wetting and corrosion properties.

The challenges, achievements and applications of submersible superhydrophobic materials

Superhydrophobic materials have been widely reported throughout the scientific literature. Their properties originate from a highly rough morphology and inherently water repellent surface chemistry. Despite promising an array of functionalities, these materials have seen limited commercial development. This could be attributed to many factors, like material compatibility, low physical resilience, scaling-up complications, etc. In applications where persistent water contact is required, another limitation arises as a major concern, which is the stability of the air layer trapped at the surface when submerged or impacted by water. This review is aimed at examining the diverse array of research focused on monitoring/improving air layer stability, and highlighting the most successful approaches. The reported complexity of monitoring and enhancing air layer stability, in conjunction with the variety of approaches adopted, results in an assortment of suggested routes to achieving success. The review is addressing the challenge of finding a balance between maximising water repulsion and incorporating structures that protect air pockets from removal, along with challenges related to the variant approaches to testing air-layer stability across the research field, and the gap between the achieved progress and the required performance in real-life applications.

Bouncing Dynamics of Impact Droplets on the Biomimetic Plane and Convex Superhydrophobic Surfaces with Dual-Level and Three-Level Structures

Reducing the contact time of a water droplet on non-wetting surfaces has great potential in the areas of self-cleaning and anti-icing, and gradually develops into a hot issue in the field of wettability surfaces. However, the existing literature on dynamic behavior of water drops impacting on superhydrophobic surfaces with various structural shapes is insufficient. Inspired by the microstructure of lotus leaf and rice leaf, dual-level and three-level structures on plane and convex surfaces were successfully fabricated by wire electrical discharge machining on aluminum alloy. After spraying hydrophobic nanoparticles on the surfaces, the plane and convex surfaces with dual-level and three-level structures showed good superhydrophobic property. Bouncing dynamics of impact droplets on the superhydrophobic surfaces wereinvestigated, and the results indicated that the contact time of plane superhydrophobic surface with a three-level structure was minimal, which is 60.4% less than the plane superhydrophobic surface with dual-level structure. The effect of the interval S, width D, and height H of the structure on the plane superhydrophobic surface with three-level structure on contact time was evaluated to obtain the best structural parameters for reducing contact time. This research is believed to guide the direction of the structural design of the droplet impinging on solid surfaces.

Phragmites Communis Leaves with Anisotropy, Superhydrophobicity and Self-Cleaning Effect and Biomimetic Polydimethylsiloxane (PDMS) Replicas

Phragmites communis leaf (PCL) is anisotropic, superhydrophobic and shows a self-cleaning effect. The water contact angle (WCA) values along the vertical and parallel vein directions on PCL are 153° ± 2° and 148° ± 2°, respectively. In contrast, the water sliding angle (WSA) values along the vertical and parallel vein directions for PCL are 12° ± 2° and 7° ± 2°, respectively. The epidermal wax makes the leaves intrinsically hydrophobic. The microstructure of the PCL surface shows sub-millimetre-, micron- and nanometre-scale structures. The sub-millimetre ridge structure is the main reason for the anisotropy of the leaves. The micron-scale papillae structure has a strong hydrophobic enhancement effect, and the nanoscale sheet structure is the key factor in achieving a stable Cassie state, as well as superhydrophobicity and self-cleaning activities. PCL-like polydimethylsiloxane (PDMS) samples fabricated by template transfer technology exhibited the sub-millimetre ridge structure and micron-scale papillae from the natural PCL; they also show obvious anisotropy and strong hydrophobicity and have a certain self-cleaning effect. The WCA and WSA values along the vertical and parallel vein directions on PCL are 146° ± 2°, 23° ± 2°, 142° ± 2° and 19° ± 2°, respectively. The preparation of a biomimetic PCL surface has broad application prospects in micro-fluidic control and the non-destructive transmission of liquids.

Using Nanoimprint Lithography to Create Robust, Buoyant, Superhydrophobic PVB/SiO2 Coatings on wood Surfaces Inspired by Red roses petal

Robust, buoyant, superhydrophobic PVB/SiO₂ coatings were successfully formed on wood surface through a one-step solvothermal method and a nanoimprint lithography method. The as-prepared PVB/SiO₂/wood specimens were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermogravimetric/differential thermogravimetric (TG-DTG) analyses. The superhydrophobic property and abrasion resistance of rose-petal-like wood were measured and assessed by water contact angle (WCA) and sand abrasion tests. The results show that PVB/SiO₂/wood not only exhibited a robust superhydrophobic performance with a WCA of 160° but also had excellent durability and thermostability during the sand abrasion tests and against corrosive liquids. Additionally, the as-prepared PVB/SiO₂/wood specimens show high buoyancy.

Double-grooved nanofibre surfaces with enhanced anisotropic hydrophobicity

This study reports a facile method for fabricating double-grooved fibrous surfaces. The primary grooves of the surface are formed by aligned fibres, while the secondary grooves are achieved by oriented nanogrooves on the fibre surface. Investigation into the formation mechanism reveals that the nanogrooves can be readily tailored through adjusting the solvent ratio and relative humidity. With this understanding, a variety of polymers have been successfully electrospun into fibres having the same nanogrooved feature. These fibres show high resemblance to natural hierarchical structures, and thereby endowing the corresponding double-grooved surface with enhanced anisotropic hydrophobicity. A water droplet at a parallel direction to the grooves exhibits a much higher contact angle and a lower roll-off angle than the droplet at a perpendicular direction. The application potential of such anisotropic hydrophobicity has been demonstrated via a fog collection experiment, in which the double-grooved surface can harvest the largest amount of water. Moreover, the fabrication method requires neither post-treatment nor sophisticated equipment, making us anticipate that the double-grooved surface would be competitive in areas where a highly ordered surface, a large surface area and an anisotropic hydrophobicity are preferred.

A Rapid One-Step Process for Fabrication of Biomimetic Superhydrophobic Surfaces by Pulse Electrodeposition

Inspired by some typical plants such as lotus leaves, superhydrophobic surfaces are commonly prepared by a combination of low surface energy materials and hierarchical micro/nano structures. In this work, superhydrophobic surfaces on copper substrates were prepared by a rapid, facile one-step pulse electrodepositing process, with different duty ratios in an electrolyte containing lanthanum chloride (LaCl₃·6H₂O), myristic acid (CH₃(CH₂)₁₂COOH), and ethanol. The equivalent electrolytic time was only 10 min. The surface morphology, chemical composition and superhydrophobic property of the pulse electrodeposited surfaces were fully investigated with SEM, EDX, XRD, contact angle meter and time-lapse photographs of water droplets bouncing method. The results show that the as-prepared surfaces have micro/nano dual scale structures mainly consisting of La[CH₃(CH₂)₁₂COO]₃ crystals. The maximum water contact angle (WCA) is about 160.9°, and the corresponding sliding angle is about 5°. This method is time-saving and can be easily extended to other conductive materials, having a great potential for future applications.

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.

Wetting transition energy curves for a droplet on a square-post patterned surface

Due to the property of water repellence, biomimetic superhydrophobic surfaces have been widely applied to green technologies, in turn inducing wider and deeper investigations on superhydrophobic surfaces. Theoretical, experimental and numerical studies on wetting transitions have been carried out by researchers, but the mechanism of wetting transitions between Cassie-Baxter state and Wenzel state, which is crucial to develop a stable superhydrophobic surface, is still not fully understood. In this paper, the free energy curves based on the transition processes are presented and discussed in detail. The existence of energy barriers with or without consideration of the gravity effect, and the irreversibility of wetting transition are discussed based on the presented energy curves. The energy curves show that different routes of the Cassie-to-Wenzel transition and the reverse transition are the main reason for the irreversibility. Numerical simulations are implemented via a phase field lattice Boltzmann method of large density ratio, and the simulation results show good consistency with the theoretical analysis.

Biomimetic surfaces with anisotropic sliding wetting by energy-modulation femtosecond laser irradiation for enhanced water collection

Inspired by natural rice leaf surfaces, we report a simple method to prepare three-level macrogrooves and micro/nanostructures on PDMS films by using energy-modulation femtosecond laser scanning for potential applications in water collection.

From macro to micro: structural biomimetic materials by electrospinning

Bionics provides a model for preparation of structural materials.