天然超疏水生物表面研究的新进展

文章简要地论述了表面浸润性的基本理论，介绍了几种天然超疏水生物表面最新的研究进展，包括荷叶、蝉翼、水稻叶以及水黾腿，特别介绍了作者的研究小组利用从壁虎脚高黏附力获得的灵感仿生制备出一种对水滴具有高黏附的超疏水阵列聚苯乙烯纳米管材料，最后对超疏水材料的研究趋势作了展望。     

The effect of riblets in rectangular duct flow

Much is known about the benefits of surface structures which mimic the riblets found on the skin of fast swimming sharks. Structures have been fabricated for study and application which replicate and improve upon the natural shape of the shark skin riblets, providing a maximum drag reduction of nearly 10% in external turbulent fluid flow. Mechanisms of fluid drag in turbulent flow and riblet-drag reduction theories from experiment and simulation are discussed. A brief review of riblet performance studies is given, and optimal riblet geometries are defined for external flow. A discussion of the structure of internal turbulent fluid flow is provided, and existing data for riblet drag reduction benefit is presented. A flow cell for studying riblet effects in internal rectangular duct flow is discussed, and data collected using several riblet surfaces fabricated for the flow cell is presented and analyzed. A discussion of the effects of the riblets on fluid flow is given, and conclusions are drawn about the possible benefits of riblets in internal fluid flow.     

二维地效翼及地效流动特性数值研究

用数值模拟的方法,对二维NACA0012翼型在地面效应下的空气动力特性和地效流动特性进行研究,得到地效翼的升力、阻力和翼型表面压力分布随攻角及相对飞行高度的变化情况.通过对计算结果的分析,可以看出,在一定的攻角,靠近地面飞行,机翼的升力得到提高;随着飞行高度的降低,地面效应增强,机翼的失速攻角减小;地面附近的粘性流动对机翼的空气动力特性影响很小;当相对飞行高度小于0.1时,应该考虑空气的可压缩性.     

Antifouling behaviour of silicon surfaces modified with self-assembled monolayers containing both ethylene glycol and charged moieties

Herein reported is the synthesis of functionalised oligoethylene glycol molecules, with an azido group at one end and an ionisable group at the other end, and their attachment onto alkyne-terminated silicon(100) surfaces using ‘click’ chemistry. The modified surfaces were characterised using X-ray photoelectron spectroscopy (XPS) and water contact angle goniometry. The antifouling behaviour of these surfaces was assessed and it was shown that while surfaces presenting both charged and ethylene glycol moieties are antifouling, the antifouling effectiveness is influenced by the surface charge as modulated via the pH of the solution.     

Facile fabrication of a lotus-effect composite coating via wrapping silica with polyurethane

A lotus-effect coating was fabricated by wrapping micro-silica and nano-silica with polyurethane (PU) and subsequent spraying. The coating shows the similar self-cleaning property as lotus leaves: the contact angle is as large as 168° and the sliding angle is as low as 0.5°. Surface morphology of the coating was studied with scanning electron microscopy and atomic force microscopy. The composite coating shows the similar structure as lotus leaves.     

Superhydrophobic wind turbine blade surfaces obtained by a simple deposition of silica nanoparticles embedded in epoxy

Samples of wind turbine blade surface have been covered with a superhydrophobic coating made of silica nanoparticles embedded in commercial epoxy paint. The superhydrophobic surfaces have a water contact angle around 152°, a hysteresis less than 2° and a water drop sliding angle around 0.5°. These surfaces are water repellent so that water drops cannot remain motionless on the surface. Examination of coated and uncoated surfaces with scanning electron microscopy and atomic force microscopy, together with measurements of water contact angles, indicates that the air trapped in the cavity enhances the water repellency similarly to the lotus leaf effect. Moreover, this new coating is stable under UVC irradiation and water pouring. The production of this nanoscale coating film being simple and low cost, it can be considered as a suitable candidate for water protection of different outdoor structures.     

Bioinspired fabrication of magneto-optic hierarchical architecture by hydrothermal process from butterfly wing

We developed a green solution to incorporate nano-Fe₃O₄ into the hierarchical architecture of a natural butterfly wing, thus obtaining unique magneto-optic nanocomposites with otherwise unavailable photonic features. Morphological characterization and Fourier Transform Infrared-Raman Spectroscope measurements indicate the assembly of Fe₃O₄ nanocrystallites. The magnetic and optical responses of Fe₃O₄/wing show a coupling effect between the biological structure and magnetic material. The saturation magnetization and coercivity values of the as-prepared magneto-optic architecture varied with change of subtle structure. Such a combination of nano-Fe₃O₄ and natural butterfly wing might create novel magneto-optic properties, and the relevant ideas could inspire the investigation of magneto-optical devices.     

基于N-S方程的支撑机翼高亚声速气动外型设计

相对常规悬臂梁布局飞机，支撑机翼飞机允许有更大的展弦比、更薄的机翼及较小的后掠角，从而可以减小诱导阻力、波阻，并增加层流范围，是未来飞机的一个可供选择方案.文章基于N-S方程对高亚声速支撑机翼构型进行了气动外型设计，在巡航Mach数为0.7，设计升力系数为0.6的条件下，支撑机翼构型相对无支撑构型升阻比仅减小6.3%，而初始无支撑翼身组合体构型相较常规悬臂梁翼身组合体构型最大升阻比提高了约35%，设计结果表明支撑机翼构型是可明显提高飞行性能的未来高亚声速飞机的一种新型外型.文章也对支撑外型、位置参数及机翼内翼下翼面外型修型对支撑机翼构型的干扰影响进行了研究，研究结果表明:支撑上翼面外型、支撑弦长、相对厚度、展向位置、扭转角分布及机翼下翼面外型对支撑机翼构型气动影响较大.      

Influence of surface roughness on superhydrophobicity

Superhydrophobic surfaces, with a liquid contact angle theta greater than 150 degrees , have important practical applications ranging from self-cleaning window glasses, paints, and fabrics to low-friction surfaces. Many biological surfaces, such as the lotus leaf, have a hierarchically structured surface roughness which is optimized for superhydrophobicity through natural selection. Here we present a molecular dynamics study of liquid droplets in contact with self-affine fractal surfaces. Our results indicate that the contact angle for nanodroplets depends strongly on the root-mean-square surface roughness amplitude but is nearly independent of the fractal dimension D(f) of the surface.     

疏水表面滑移流动及减阻特性的格子Boltzmann方法模拟

采用格子Boltzmann方法研究了固体壁面对流体的作用强度与其润湿性的关系,在此基础上进一步模拟了疏水表面微通道内的流体流动,获得了润湿性对疏水表面滑移流动及减阻特性的影响规律,证实了疏水表面表观滑移的存在性并揭示了其产生机理.结果表明,疏水性作用在疏水表面的近壁区诱导了一个低密度层,而表观滑移则发生在低密度层上.表观滑移是疏水表面具有减阻作用的直接原因,减阻效果随滑移长度的增大而增大.对于特定的流体系统,滑移长度是疏水表面的固有属性,仅是壁面润湿性的单一函数,而与流动本身的性质无关.     

A new model for the formation of contact angle and contact angle hysteresis

The formation mechanism of the contact angle and the sliding angle for a liquid drop on a solid surface plays an important role in producing hydrophobic surfaces. A new half soakage model is established in this paper as a substitute for Wenzel (complete soakage) and Cassie (no soakage) models. The model is suited to many solid surfaces, whether they are hydrophilic or hydrophobic, or even superhydrophobic. Based on the half soakage model, we analyse two surfaces resembling lotus, i.e. taper-like surface and corona-like surface. Furthermore, this new model is used to establish a quantitative relationship between the sliding angle and the parameters of surface morphology.     

Flow field measurements of leading-edge separation vortex formed on a delta wing with vortex flaps

Wind tunnel tests are carried out using a 70 delta wing model with leading-edge vortex flaps. The structure of the leading-edge separation vortex over the leading-edge vortex flap is measured by use of a 5 holes pitot probe, surface pressure measurement technique and oil flow visualization technique. Separation vortices formed on a plain delta wing, on a vortex flap and inboard the vortex flap hinge line are clearly visualized. Results indicate that the flow around the vortex flaps is classified into several different cross flow patterns. The streamwise flap deflection angle is defined to discuss the vortex flap performance. The optimum lift to drag ratio is attained when the amount of the wing angle of attack is not far different from that of the streamwise flap deflection angle, as long as the vortex flap is deflected modestly.     

Numerical investigation of relationship between water contact angle and drag reduction ratio of superhydrophobic surfaces

This paper proposes a novel bubble model to analyze drag reduction. The relationship between the slip length and air bubble height is discussed. The numerical relationship between the surface contact angle and slip length is obtained using the solid-liquid contact ratio in the Cassie equation. The surface drag reduction ratio increases by 40% at low velocities when the solid liquid contact ratio decreases from 90% to 10%. An experimental setup to study liquid/solid friction drag is reported. The drag reduction ratio for the superhydrophobic surface tested experimentally is 30%–35% at low velocities. These results are similar to the simulation results obtained at low velocities.     

表面张力对疏水微结构表面减阻的影响

通过构造具有棋盘状微结构的疏水表面,考虑表面张力的影响,利用定常与非定常结合的数值模拟方法,研究了疏水表面在湍流状态下的减阻特性以及微结构内气体封存的效果,其中Re=3000—30000.在低雷诺数下,疏水表面微结构内气体封存状态良好,减阻率最高约为30%;随着雷诺数的增大,压差阻力增大,减阻率有下降趋势.当来流速度过大时,水会大量进入微结构,疏水表面的减阻率变化剧烈,且已经不再减阻.结果表明,表面张力削弱了壁面切应力的影响,使得低雷诺数下微结构内气体能够有效封存,进而减小壁面阻力.     

FAAS测定嫩、老荷叶中7种营养元素含量

采用火焰原子吸收光谱法对嫩、老(成熟)荷叶中K、Ca、Mg、Mn、Fe、Zn、Cu7种营养元素的含量进行测定。结果表明,嫩、老荷叶中7种营养元素含量高低顺序一致,依次为KCaMgMnFeZnCu,但成熟荷叶中钙、锰含量明显高于嫩荷叶,约为嫩荷叶的两倍,铁含量稍高于嫩荷叶,钾、镁、锌、铜含量略低于嫩荷叶。其中成熟荷叶中锰的含量为738.04μg.g-1,比一般中药高出近20倍,荷叶的多种药用价值可能与富含微量元素锰有密切关系。     

升力系数不变时跨音速机翼阻力优化设计

基于共轭方程的优化设计理论,应用三维欧拉方程进行了升力系数不变时跨音速机翼阻力优化设计研究,根据给定的目标函数推导了在物理空间上表述的共轭方程及边界条件,研究了共轭方程的数值求解方法及目标函数对设计变量的敏感性导数求解问题,发展了一种跨音速机翼阻力优化设计方法,应用该设计方法进行了跨音速机翼阻力优化设计研究,优化后机翼表面的激波强度减弱很多,有效减少了波阻.     

Visualization of flapping wing of the drone beetle

Investigation of flapping wings of insect are focused on low Reynolds number effect and the unsteady aerodynamic properties. Interaction between flapping wing of insects and the air flow became one of important and fundamental research topics in micro air vehicle. The present work is aim to investigate the flow behavior of flapping wings of tethered scarab beetle. The generation mechanisms of velocity field and vortex formation are visualized with smoke-wire method. Tethered flight of the drone beetle shows the motion with elastic deformation of flapping wing. Measured flapping frequency is about 71 Hz and its frequency is higher than for dragonfly and butterfly. Beetle decreases negative lift by feathering motion in the upstroke process and increase positive lift by effect of wake capture in the downstroke process.     

Color generation in butterfly wings and fabrication of such structures

The wings of the morpho butterfly demonstrate an iridescent blue color over wide viewing angles. The mechanism that generates this blue color is studied. Optical and transmission electron microscopy of the butterfly wings reveal a complex wing structure with as many as 24 layers with periodic structures. The color generation is caused by interference of the multilayer structure as well as diffraction. It is possible to specially design grating structures so that a specific blue color can be generated and observed over wider angles. To demonstrate the grating concept, complex multigratings are designed and fabricated with electron-beam lithography. The light-diffraction properties of these gratings are presented.     

Replication of butterfly wing microstructures using molding lithography

This study employed a soft lithography technique to fabricate a polydimethylsiloxane (PDMS) replica of the multi-layered scales on the upper surface of a Morpho butterfly. The bionic photonic crystal microstructure of the replicated scales was examined using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The absorptivity, reflectivity and fluorescent characteristics of the wing were measured. The results showed that the microstructural and optical characteristics of the replicated wing qualitatively agree with those of the actual wing. The contact angle for the natural wing structure and the replicated wing were about 143° and 120°, respectively. As a result, it can be inferred that the soft lithography technique employed in this study represents a viable approach for the mass production of artificial photonic crystal structures for a variety of commercial applications, including optical elements for computing and communications purposes, photonic integrated circuits, anti-counterfeiting mechanisms, and so forth.     

Parameterized aeroelastic modeling and flutter analysis for a folding wing

To investigate the flutter characteristics of a folding wing with different configurations, a parameterized aeroelastic model is proposed. First, a parameterized structural model is established based on the substructure synthesis. Afterwards, the parameterized aerodynamic model is derived for each lifting surface using the so-called Doublet Lattice Method (DLM). The correctness of the resulting aeroelastic model is verified via NASTRAN. Finally, some aeroelastic simulations are performed using the proposed aeroelastic model. The results demonstrate that the flutter characteristics of the folding wing are very sensitive to the folding angle. With increasing folding angle, a transition between two unstable modes occurs. Such a transition results in a sudden change of flutter mode shapes and a jump of critical flutter frequency. Besides, there exists a region of folding angle, where the flutter behavior of the folding wing strongly depends on the structural damping.