Many biological surfaces in both the plant and animal kingdom possess unusual structural features at the micro- and nanometre-scale that control their interaction with water and hence wettability. An intriguing example is provided by desert beetles, which use micrometre-sized patterns of hydrophobic and hydrophilic regions on their backs to capture water from humid air. As anyone who has admired spider webs adorned with dew drops will appreciate, spider silk is also capable of efficiently collecting water from air. Here we show that the water-collecting ability of the capture silk of the cribellate spider Uloborus walckenaerius is the result of a unique fibre structure that forms after wetting, with the 'wet-rebuilt' fibres characterized by periodic spindle-knots made of random nanofibrils and separated by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots. Submillimetre-sized liquid drops have been driven by surface energy gradients or a difference in Laplace pressure, but until now neither force on its own has been used to overcome the larger hysteresis effects that make the movement of micrometre-sized drops more difficult. By tapping into both driving forces, spider silk achieves this task. Inspired by this finding, we designed artificial fibres that mimic the structural features of silk and exhibit its directional water-collecting ability.

Directional water collection on wetted spider silk

Research into extreme water-repellent surfaces began many decades ago, although it was only relatively recently that the term superhydrophobicity appeared in literature Here we review the work on the preparation of superhydrophobic surfaces, with focus on the different techniques used and how they have developed over the years, with particular focus on the last two years We discuss the origins of water-repellent surfaces, examining how size and shape of surface features are used to control surface characteristics, in particular how techniques have progressed to form multi-scaled roughness to mimic the lotus leaf effect There are notable differences in the terminology used to describe the varying properties of water-repellent surfaces, so we suggest some key definitions

Progess in superhydrophobic surface development.

A novel superhydrophilic and underwater superoleophobic hydrogel-coated mesh for oil/water separation.

Design, and Applications Shutao Wang,†,‡ Kesong Liu, Xi Yao, and Lei Jiang*,†,‡,§ †Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and Chemistry, and ‡Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, BeiHang University, Beijing 100191, People’s Republic of China Department of Biomedical Sciences, City University of Hong Kong, Hong Kong P6903, People’s Republic of China

Bioinspired Surfaces with Superwettability: New Insight on Theory, Design, and Applications

The development of efficient and robust earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) is an ongoing challenge. We report metallic cobalt pyrite (cobalt disulfide, CoS2) as one such high-activity candidate material and demonstrate that its specific morphology—film, microwire, or nanowire, made available through controlled synthesis—plays a crucial role in determining its overall catalytic efficacy. The increase in effective electrode surface area that accompanies CoS2 micro- and nanostructuring substantially boosts its HER catalytic performance, with CoS2 nanowire electrodes achieving geometric current densities of −10 mA cm–2 at overpotentials as low as −145 mV vs the reversible hydrogen electrode. Moreover, micro- and nanostructuring of the CoS2 material has the synergistic effect of increasing its operational stability, cyclability, and maximum achievable rate of hydrogen generation by promoting the release of evolved gas bubbles from the electrode surface. The benefits of ca...

High-Performance Electrocatalysis Using Metallic Cobalt Pyrite (CoS2) Micro- and Nanostructures

A smart role of the leaf margin makes the lotus surface dry when a lotus leaf floats on the water’s surface. It is mainly attributed to anisotropic topography composed of flat folds at the margin, which are quite different from the upper surface of the lotus leaf including the micro-papillae. The flat folds around the leaf margin forms a ring-band, which introduces a strong energy barrier against water that tends to return to the surface of leaf. As a result, the water underneath the lotus surface hardly overflows onto the upper surface by getting across the energy barrier of the margin. Thanks to the ultra narrowness of the margin, water on the upper surface can still be shed from the surface through the margin. This finding helps the understanding of the structure effect of the margin on many natural nonwetting surfaces of aqueous biology. It can inspire us to design a smart margin that can be applied to micro-devices on water.

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How does the leaf margin make the lotus surface dry as the lotus leaf floats on water

Surfaces that control fluids are important in self-cleaning, liquid-transport and cell-directing. They are significantly observed on biological surfaces that control wettability and adhesion by means of micro-/nanostructures, and have aroused interest in foundational and biomimetic research. Here, we report a novel taper-ratchet array on ryegrass leaf. It integrates a gradient of retention at solid–liquid interfaces in contrasting directions to reversibly generate the release or the pinning of solid–liquid contact lines, and accordingly, achieves effective directional water shedding-off properties. By mimicking taper-ratchets from ryegrass leaf, the polymer surfaces are fabricated successfully. They display a robust property of directional water shedding-off. When external vibrations are executed on polymer surfaces, the drops achieve a unidirectional self-shedding along the oriented direction of tips of taper-ratchets, because asymmetric retention forces are formed in the contrasting oriented directions. This investigation will be helpful to design a novel fluid-controlling surface that can be extended to applications such as self-cleaning, liquid-transport and cell-directed projects.

Directional shedding-off of water on natural/bio-mimetic taper-ratchet array surfaces

Issues of surfaces, e.g., inspired from beetle's back, spider silk, cactus stem, etc., become the active area of research on designing novel materials in need of human beings to acquire fresh water resource from air. However, the design of materials on surface structure is little achieved on controlling of micro-scale drop transport in a long distance. Here, we report the ability of micro-drop transport in a long distance on a bioinspired Fibers with Gradient Spindle-knots (BFGS), which are fabricated by tilt angle dip-coating method. The micro-drop of ~0.25 μL transports in distance of ~5.00 mm, with velocity of 0.10–0.22 m s−1 on BFGS. It is attributed to the multi-level cooperation of the release energy of drop coalescence along the gradient spindle-knots, in addition to capillary adhesion force and continuous difference of Laplace pressure, accordingly, water drops are driven to move fast directionally in a long distance on BFGS.

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Bioinspired tilt-angle fabricated structure gradient fibers: micro-drops fast transport in a long-distance

The observation of the surface of a red rose petal indicates that there are micropapillae on the surface and many nanofolders exist on each papilla. Here, much tinier nanorods with periodic pattern on the nanofolders can be seen by in situ atomic force microscopy (AFM). Angle-resolved UV−vis spectral measurement and reflectance UV−vis spectra by immersion red rose petal in solvents with different refractive indices demonstrate that such periodic nanostructures can induce structural color. The combination of structural color, driven by the nanostructures, and chemical color, driven by pigments, provide flowers bright color and special functions for human and animals’ visual system. Biomimic polymer films, that fabricated by duplicating the petal’s hierarchical micro/nano structures, exhibit only structural color by UV−vis spectra since there is no pigment introduced.

The structural color of red rose petals and their duplicates.

Controlled directional spreading of a droplet on a smart high-adhesion surface was made possible by simply controlling anodic oxidation. The wettability gradient of the surface was controlled from 0.14 to 3.38° mm(-1) by adjusting the anodic oxidation conditions. When a water droplet made contact with the substrate, the droplet immediately spread in the direction of the wettability gradient but did not move in other directions, such as those perpendicular to the gradient direction, even when the surface was turned upside down. The spreading behavior was mainly controlled by the wettability gradient. Surfaces with a V- or inverse-V-shaped wettability gradient were also formed by the same method, and two droplets on these surfaces spread either toward or away from one another as designed. This method could be used to oxidize many conductive substrates (e.g., copper, aluminum) to form surfaces with variously shaped wettability gradients. It has potential for application in microfluidic devices.

Yongmei Zheng

Papers

How does the leaf margin make the lotus surface dry as the lotus leaf floats on water

Directional shedding-off of water on natural/bio-mimetic taper-ratchet array surfaces

Bioinspired tilt-angle fabricated structure gradient fibers: micro-drops fast transport in a long-distance

The structural color of red rose petals and their duplicates.

Controlled directional water-droplet spreading on a high-adhesion surface.