Interfacial Effects of Superhydrophobic Plant Surfaces： A Review

Nature is a huge gallery of art involving nearly perfect structures and properties over the millions of years of development. Many plants and animals show water-repellent properties with fine micro-structures, such as lotus leaf, water skipper and wings of butterfly. Inspired by these special surfaces, the artificial superhydrophobic surfaces have attracted wide attention in both basic research and industrial applications. The wetting properties of superhydrophobic surfaces in nature are affected by the chemical compositions and the surface topographies. So it is possible to realize the biomimetic superhydrophobic surfaces by tuning their surface roughness and surface free energy correspondingly. This review briefly introduces the physical-chemical basis of superhydrophobic plant surfaces in nature to explain how the superhydrophobicity of plant surfaces can be applied to different biomimetic functional materials with relevance to technological applications. Then, three classical effects of natural surfaces are classified： lotus effect, salvinia effect, and petal effect, and the promising strategies to fabricate biomimetic su- perhydrophobic materials are highlighted. Finally, the prospects and challenges of this area in the future are proposed.     

Synthesis of Black TiOx Nanoparticles by Mg Reduction of TiO2 Nanocrystals and their Application for Solar Water Evaporation

TiO_x (x < 2) nanoparticles with tunable colors from white to gray to blue–gray to black are synthesized by magnesium (Mg) reduction of white P25 TiO₂ nanocrystals followed by removal of excess Mg with aqueous HCl and distilled water. Increasing amounts of Mg smoothly decrease the oxygen content in TiO_x which is responsible for the gradual increase in light absorption and concomitant darkening of its color from white to black with decreasing values of x. The as‐synthesized TiO_x nanoparticles are spin‐coated onto the surface of a stainless steel mesh followed by surface superhydrophobization in order to test their performance as a solar water evaporator. Results from the tests show that the black TiO_x efficiently generates water vapor with a solar thermal conversion efficiency as high as 50% under solar‐simulated light irradiance at an intensity of 1000 W m^–2 (1 Sun). Moreover, TiO_x nanoparticles have inherent advantages over other materials used for solar water desalination, such as their tunable light absorption, low‐cost, low‐toxicity, superhydrophobicity, and chemical stability.     

Wetting Characteristics of Insect Wing Surfaces

Biological tiny structures have been observed on many kinds of surfaces such as lotus leaves,which have an effect on thecoloration of Morpho butterflies and enhance the hydrophobicity of natural surfaces.We investigated the micro-scale andnano-scale structures on the wing surfaces of insects and found that the hierarchical multiple roughness structures help in enhancingthe hydrophobicity.After examining 10 orders and 24 species of flying Pterygotan insects,we found that micro-scaleand nano-scale structures typically exist on both the upper and lower wing surfaces of flying insects.The tiny structures such asdenticle or setae on the insect wings enhance the hydrophobicity,thereby enabling the wings to be cleaned more easily.And thehydrophobic insect wings undergo a transition from Cassie to Wenzel states at pitch/size ratio of about 20.In order to examinethe wetting characteristics on a rough surface,a biomimetic surface with micro-scale pillars is fabricated on a silicon wafer,which exhibits the same behavior as the insect wing,with the Cassie-Wenzel transition occurring consistently around apitch/width value of 20.     

Brochosomal coats turn leafhopper (Insecta,Hemiptera, Cicadellidae) integument to superhydrophobic state

Leafhoppers (Insecta, Hemiptera, Cicadellidae) actively coat their integuments with brochosomes, hollow proteinaceous spheres of usually 200–700 nm in diameter, with honeycombed walls. The coats have been previously suggested to act as a water-repellent and anti-adhesive protective barrier against the insect''s own exudates. We estimated their wettability through contact angle (CA) measurements of water, diiodomethane, ethylene glycol and ethanol on detached wings of the leafhoppers Alnetoidia alneti, Athysanus argentarius and Cicadella viridis. Intact brochosome-coated integuments were repellent to all test liquids, except ethanol, and exhibited superhydrophobicity, with the average water CAs of 165–172°, and the apparent surface free energy (SFE) estimates not exceeding 0.74 mN m⁻¹. By contrast, the integuments from which brochosomes were removed with a peeling technique using fluid polyvinylsiloxane displayed water CAs of only 103–129° and SFEs above 20 mN m⁻¹. Observations of water-sprayed wings in a cryo-scanning electron microscope confirmed that brochosomal coats prevented water from contacting the integument. Their superhydrophobic properties appear to result from fractal roughness, which dramatically reduces the area of contact with high-surface-tension liquids, including, presumably, leafhopper exudates.     

Leaf gas films,underwater photosynthesis and plant species distributions in a flood gradient

Traits for survival during flooding of terrestrial plants include stimulation or inhibition of shoot elongation, aerenchyma formation and efficient gas exchange. Leaf gas films form on superhydrophobic cuticles during submergence and enhance underwater gas exchange. The main hypothesis tested was that the presence of leaf gas films influences the distribution of plant species along a natural flood gradient. We conducted laboratory experiments and field observations on species distributed along a natural flood gradient. We measured presence or absence of leaf gas films and specific leaf area of 95 species. We also measured, gas film retention time during submergence and underwater net photosynthesis and dark respiration of 25 target species. The presence of a leaf gas film was inversely correlated to flood frequency and duration and reached a maximum value of 80% of the species in the rarely flooded locations. This relationship was primarily driven by grasses that all, independently of their field location along the flood gradient, possess gas films when submerged. Although the present study and earlier experiments have shown that leaf gas films enhance gas exchange of submerged plants, the ability of species to form leaf gas films did not show the hypothesized relationship with species composition along the flood gradient.     

网粒体超疏水性在茶小绿叶蝉防杀虫剂渗透中的屏障效应

【目的】茶小绿叶蝉Empoasca onukii体表覆盖的网粒体具有超疏水性，杀虫剂喷雾触碰虫体后药滴动态是否受网粒体影响尚未完全清楚。本研究旨在明确网粒体在茶小绿叶蝉成虫抵御杀虫剂雾滴渗透的屏障作用。【方法】以罗丹明B(RhB)作为指示剂添加到测试的杀虫剂(联苯菊酯和茚虫威)中，利用可拍照显微镜观察记录联苯菊酯(1.25 mg/L和0.05 mg/L)和茚虫威(0.006 mg/L和0.0009 mg/L)喷雾处理后24 h，茶小绿叶蝉成虫翅面药滴滚落、蒸发、被抖动扫除等行为动态，分析翅面药滴大小与蒸发后固化形态的关系；测定网粒体移除前后药滴与翅面的接触角，统计不同疏水性翅面上的网粒体分布密度；收集并利用扫描电镜分析叶蝉体表抖落的药滴及药剂颗粒是否含有网粒体，同时观察网粒体与翅面残留溶质接触的显微形态。【结果】药滴动态观察显示，圆球状药滴在茶小绿叶蝉成虫翅面不会自行滚落，72.0%成虫静止等待翅面药滴蒸发，蒸发后形成药剂颗粒或不规则药斑与药滴大小无关，而与虫体翅面的疏水类型有关，蒸发后24 h内翅面的药剂颗粒都被叶蝉抖动扫除；在叶蝉疏水性强翅面上，药滴的静态接触角为141.63±8.06°，药滴蒸发后形成药剂颗粒，网粒体分布密度为6.1±1.2粒/μm²，而疏水性弱的翅面上药滴蒸发形成药斑，网粒体分布密度为2.2±0.9粒/μm²；SEM图片显示被茶小绿叶蝉抖落的药滴和药剂颗粒表面均带有网粒体，药斑和药剂颗粒的显微结构显示网粒体出现聚集并与残留溶质相融合。【结论】超疏水性网粒体的均匀分布决定药滴触碰茶小绿叶蝉成虫翅面后形成圆球状，网粒体的亲油性及团聚性促使药滴蒸发后形成药剂颗粒，网粒体的脱落性使药剂颗粒可被茶小绿叶蝉成虫抖动扫除。