Multiscale Experimental Mechanics of Hierarchical Carbon‐Based Materials

Investigation of the mechanics of natural materials, such as spider silk, abalone shells, and bone, has provided great insight into the design of materials that can simultaneously achieve high specific strength and toughness. Research has shown that their emergent mechanical properties are owed in part to their specific self‐organization in hierarchical molecular structures, from nanoscale to macroscale, as well as their mixing and bonding. To apply these findings to manmade materials, researchers have devoted significant efforts in developing a fundamental understanding of multiscale mechanics of materials and its application to the design of novel materials with superior mechanical performance. These efforts included the utilization of some of the most promising carbon‐based nanomaterials, such as carbon nanotubes, carbon nanofibers, and graphene, together with a variety of matrix materials. At the core of these efforts lies the need to characterize material mechanical behavior across multiple length scales starting from nanoscale characterization of constituents and their interactions to emerging micro‐ and macroscale properties. In this report, progress made in experimental tools and methods currently used for material characterization across multiple length scales is reviewed, as well as a discussion of how they have impacted our current understanding of the mechanics of hierarchical carbon‐based materials. In addition, insight is provided into strategies for bridging experiments across length scales, which are essential in establishing a multiscale characterization approach. While the focus of this progress report is in experimental methods, their concerted use with theoretical‐computational approaches towards the establishment of a robust material by design methodology is also discussed, which can pave the way for the development of novel materials possessing unprecedented mechanical properties.     

Bacterially Produced,Nacre‐Inspired Composite Materials

The impressive mechanical properties of natural composites, such as nacre, arise from their multiscale hierarchical structures, which span from nano‐ to macroscale and lead to effective energy dissipation. While some synthetic bioinspired materials have achieved the toughness of natural nacre, current production methods are complex and typically involve toxic chemicals, extreme temperatures, and/or high pressures. Here, the exclusive use of bacteria to produce nacre‐inspired layered calcium carbonate‐polyglutamate composite materials that reach and exceed the toughness of natural nacre, while additionally exhibiting high extensibility and maintaining high stiffness, is introduced. The extensive diversity of bacterial metabolic abilities and the possibility of genetic engineering allows for the creation of a library of bacterially produced, cost‐effective, and eco‐friendly composite materials.     

Nano measurements with micro-devices: mechanical properties of hydrated collagen fibrils.

The mechanical response of a biological material to applied forces reflects deformation mechanisms occurring within a hierarchical architecture extending over several distinct length scales. Characterizing and in turn predicting the behaviour of such a material requires an understanding of the mechanical properties of the substructures within the hierarchy, the interaction between the substructures, and the relative influence of each substructure on the overall behaviour. While significant progress has been made in mechanical testing of micrometre to millimetre sized biological specimens, quantitative reproducible experimental techniques for making mechanical measurements on specimens with characteristic dimensions in the smaller range of 10-1000 nm are lacking. Filling this void in experimentation is a necessary step towards the development of realistic multiscale computational models useful to predict and mitigate the risk of bone fracture, design improved synthetic replacements for bones, tendons and ligaments, and engineer bioinspired efficient and environmentally friendly structures. Here, we describe a microelectromechanical systems device for directly measuring the tensile strength, stiffness and fatigue behaviour of nanoscale fibres. We used the device to obtain the first stress-strain curve of an isolated collagen fibril producing the modulus and some fatigue properties of this soft nanofibril.     

自相似多级纳米蜂窝铝结构力学性能的分子动力学模拟

首次采用分子动力学方法预测了自相似多级纳米蜂窝铝面内和面外（轴向）的压缩力学性能（弹性模量和压缩强度）。重点研究了相对密度、层级数和长度比对自相似多级纳米蜂窝铝结构力学性能的影响。在Gibson模型中引入了表面效应因子,结果表明修正的Gibson-Ashby模型与分子动力学计算结果更加吻合。此外,通过比较一级、二级和三级纳米蜂窝铝结构的变形机制发现,二级和三级纳米蜂窝铝结构由于分别在单级蜂窝和二级蜂窝的角点处接入六边形,在压缩过程中,多级纳米蜂窝铝结构激发的位错远高于单级蜂窝铝结构。也就是说,在压缩载荷下,多级蜂窝铝结构可以更好地利用结构的承载能力,吸收更多的能量。但是,自相似纳米蜂窝铝结构的力学性能无法通过增加级数的方法来无限增强,在相对密度和长度比不变的情况下,当纳米蜂窝铝结构的级数达到二级时,其综合力学性能最佳。研究结果还表明,相对密度不变时,二级纳米蜂窝铝结构长度比分别在0.3和0.4附近时,二级蜂窝铝结构具有最佳的面内和面外力学性能。研究成果对自相似多级纳米蜂窝结构的优化设计具有重要的指导作用。      

Combining Bottom‐Up Self‐Assembly with Top‐Down Microfabrication to Create Hierarchical Inverse Opals with High Structural Order

Colloidal particles can assemble into ordered crystals, creating periodically structured materials at the nanoscale without relying on expensive equipment. The combination of small size and high order leads to strong interaction with visible light, which induces macroscopic, iridescent structural coloration. To increase the complexity and functionality, it is important to control the organization of such materials in hierarchical structures with high degrees of order spanning multiple length scales. Here, a bottom‐up assembly of polystyrene particles in the presence of a silica sol–gel precursor material (tetraethylorthosilicate, TEOS), which creates crack‐free inverse opal films with high positional order and uniform crystal alignment along the (110) crystal plane, is combined with top‐down microfabrication techniques. Micrometer scale hierarchical superstructures having a highly regular internal nanostructure with precisely controlled crystal orientation and wall profiles are produced. The ability to combine structural order at the nano‐ and microscale enables the fabrication of materials with complex optical properties resulting from light–matter interactions at different length scales. As an example, a hierarchical diffraction grating, which combines Bragg reflection arising from the nanoscale periodicity of the inverse opal crystal with grating diffraction resulting from a micrometer scale periodicity, is demonstrated.     

高强度水凝胶纳米复合材料的研究进展

水凝胶是化学或物理交联而成的具有三维网络结构的高分子材料,其高分子网络中含有大量的水并能保持一定的形状,是一种特殊的半固体材料。水凝胶由于具有许多优异的性质,在工业、农业、生物医学领域得到广泛重视,然而传统水凝胶的力学性能差,限制了其应用。因此提高水凝胶力学强度的研究吸引了国内外众多研究者的关注。总结了近年来几种主要类别的高强度水凝胶纳米复合材料的实验及理论研究工作,重点分析了纳米复合凝胶在力学性能方面的研究结果,并对其未来的发展进行了展望。     

Strong Anisotropy and Ultralow Percolation Threshold in Multiscale Composites Modified by Carbon Nanotubes Coated Hollow Glass Fiber

Inspired by biological materials, the use of combined fillers of different types and sizes has led to multiscale, hierarchical composites which are considered to be the multifunctional materials of the next generation. However, the effects of hierarchical architecture on the electrical properties and percolation behavior remain poorly understood. Here, a multiscale polymer‐based micro‐/nano‐composite with hollow glass fibers coated by carbon nanotubes (CNTs) has been produced based on a simple dip‐coating approach. Besides a significant increase in electrical performance, the composites exhibit a very strong anisotropy of electrical properties with the difference of 2–5 orders of magnitude in different directions. In the longitudinal direction of composites, an ultralow percolation threshold is found. These unique properties are shown to be related to the hierarchical morphology, which gives rise to the existence of two percolation levels with different thresholds: a local threshold in the nanoscale 2D CNT networks at the fiber‐polymer interfaces and a global threshold in 3D network formed by the fibers. This study helps to deeper understand the macroscopic electrical performance of the hierarchical composites, potentially opening up new ways for designing novel materials via flexible tailoring the orientation of fiber and the morphology of interfaces.

     

Biological materials: Functional adaptations and bioinspired designs

Biological materials are typically multifunctional but many have evolved to optimize a chief mechanical function. These functions include impact or fracture resistance, armor and protection, sharp and cutting components, light weight for flight, or special nanomechanical/chemical extremities for reversible adhesive purposes. We illustrate these principles through examples from our own research as well as selected literature sources. We conduct this analysis connecting the structure (nano, micro, meso, and macro) to the mechanical properties important for a specific function. In particular, we address how biological systems respond and adapt to external mechanical stimuli. Biological materials can essentially be divided into mineralized and non-mineralized. In mineralized biological materials, the ceramics impart compressive strength, sharpness (cutting edges), and stiffness while the organic components impart tensile strength, toughness and ductility. Non-mineralized biological materials in general have higher tensile than compressive strength, since they are fibrous. Thus, the mineralized components operate optimally in compression and the organic components in tension. There is a trade-off between strength and toughness and the stiffness and density, with optimization. Mineralization provides load bearing capability (strength and stiffness) whereas the biopolymer constituents provide viscoelastic damping and toughness. The most important component of the nascent field of Biological Materials Science is the development of bioinspired materials and structures and understanding of the structure–property relationships across various length scales, from the macro-down to the molecular level. The most successful efforts at developing bioinspired materials that attempt to duplicate some of the outstanding properties are presented.     

Bulk gold with hierarchical macro-, micro- and nano-porosity

Millimeter-sized, free-standing gold structures were created with three levels of multiscale porosity. First, macro- and microporosity, which are useful for mass and heat transport within the structure, are formed within an Ag-19 at.% Au alloy by salt powder replication during powder densification and by entrapped gas expansion during sintering, respectively. Nanoporosity, which provides high surface area, is then produced by silver dealloying of these Ag-19 at.% Au foams. The resulting hierarchical gold structures are annealed at 100-800 °C, thus coarsening the ligaments, increasing relative density, and healing cracks produced during dealloying. The first effect weakens the structure, while the other two make it stronger. A bulk Au sample with hierarchical porosity annealed at 600 °C shows good compressive ductility and a strength in agreement with models.     

Multi-length scale micromorphic process zone model

The prediction of fracture toughness for hierarchical materials remains a challenging research issue because it involves different physical phenomena at multiple length scales. In this work, we propose a multiscale process zone model based on linear elastic fracture mechanics and a multiscale micromorphic theory. By computing the stress intensity factor in a K-dominant region while maintaining the mechanism of failure in the process zone, this model allows the evaluation of the fracture toughness of hierarchical materials as a function of their microstructural properties. After introducing a multi-length scale finite element formulation, an application is presented for high strength alloys, whose microstructure typically contains two populations of particles at different length scales. For this material, the design parameters comprise of the strength of the matrix–particle interface, the particle volume fraction and the strain-hardening of the matrix. Using the proposed framework, trends in the fracture toughness are computed as a function of design parameters, showing potential applications in computational materials design.     

Multilevel Hierarchically Ordered Artificial Biomineral

Living organisms are known for creating complex organic–inorganic hybrid materials such as bone, teeth, and shells, which possess outstanding functions as compared to their simple mineral forms. This has inspired many attempts to mimic such structures, but has yielded few practical advances. In this study, a multilevel hierarchically ordered artificial biomineral (a composite of hydroxyapatite and gelatine) with favorable nanomechanical properties is reported. A typical optimized HAp/gelatin hybrid material in the perpendicular direction of the HAp c‐axis has a modulus of 25.91 + 1.78 GPa and hardness of 0.90 + 0.10 GPa, which well matches that of human cortical bone (modulus 24.3 + 1.4 GPa, hardness 0.69 + 0.05 GPa). The bottom‐up crystal constructions (from nano‐ to micro‐ to macroscale) of this material are achieved through a hard template approach by the phase transformation from DCP to HAp. The structural biomimetic material shows another way to mimic the complex hierarchical designs of sclerous tissues which have potential value for application in hard tissue engineering.     

25th Anniversary Article: Scalable Multiscale Patterned Structures Inspired by Nature: the Role of Hierarchy

Multiscale, hierarchically patterned surfaces, such as lotus leaves, butterfly wings, adhesion pads of gecko lizards are abundantly found in nature, where microstructures are usually used to strengthen the mechanical stability while nanostructures offer the main functionality, i.e., wettability, structural color, or dry adhesion. To emulate such hierarchical structures in nature, multiscale, multilevel patterning has been extensively utilized for the last few decades towards various applications ranging from wetting control, structural colors, to tissue scaffolds. In this review, we highlight recent advances in scalable multiscale patterning to bring about improved functions that can even surpass those found in nature, with particular focus on the analogy between natural and synthetic architectures in terms of the role of different length scales. This review is organized into four sections. First, the role and importance of multiscale, hierarchical structures is described with four representative examples. Second, recent achievements in multiscale patterning are introduced with their strengths and weaknesses. Third, four application areas of wetting control, dry adhesives, selectively filtrating membranes, and multiscale tissue scaffolds are overviewed by stressing out how and why multiscale structures need to be incorporated to carry out their performances. Finally, we present future directions and challenges for scalable, multiscale patterned surfaces.     

Ultrahigh Energy Absorption Multifunctional Spinodal Nanoarchitectures

Nanolattices are promoted as next‐generation multifunctional high‐performance materials, but their mechanical response is limited to extreme strength yet brittleness, or extreme deformability but low strength and stiffness. Ideal impact protection systems require high‐stress plateaus over long deformation ranges to maximize energy absorption. Here, glassy carbon nanospinodals, i.e., nanoarchitectures with spinodal shell topology, combining ultrahigh energy absorption and exceptional strength and stiffness at low weight are presented. Noncatastrophic deformation up to 80% strain, and energy absorption up to one order of magnitude higher than for other nano‐, micro‐, macro‐architectures and solids, and state‐of‐the‐art impact protection structures are shown. At the same time, the strength and stiffness are on par with the most advanced yet brittle nanolattices, demonstrating true multifunctionality. Finite element simulations show that optimized shell thickness‐to‐curvature‐radius ratios suppress catastrophic failure by impeding propagation of dangerously oriented cracks. In contrast to most micro‐ and nano‐architected materials, spinodal architectures may be easily manufacturable on an industrial scale, and may become the next generation of superior cellular materials for structural applications.     

多级结构的金属材料强韧化机理研究进展

强度和塑韧性是金属结构材料主要的性能指标,然而通常会出现强度与塑韧性倒置的现象,即传统的固溶强化、纳米晶强化、弥散强化和加工硬化在追求强度的同时会不可避免地牺牲金属材料的塑韧性.根据多级多尺度仿生结构可协同提高强度和韧性的思路,系统介绍了两级Ti-TiBw/Ti复合材料、不锈钢复合板、多层复合钢、层/网耦合结构钢和超细...     

Superhydrophobic Coatings for Technical Applications

Water repellent coatings developed in this work showed that two superimposed rough structures are required to generate superhydrophoby with WCA >150°. A micro rough surface overcoated with a submicro or nano rough hydrophobic material behave similar to the hierarchical structures found on leafs of some plants which generate the highly mobile Cassie/Baxter droplets. Coatings of inorganic and organic materials were performed by thermal spraying (inorganic materials) and spraycoating (organic materials) on sandblasted metal, mainly stainless steel, surfaces. Hierarchical roughness structures were developed by thermal treatment of pure and composite fluoropolymeric layers. Such hierarchical superhydrophobic coating systems showed water contact angles between 160° and 170°. The coatings have good mechanical stability and can be applied in numerous technical applications.     

A variational framework for fiber‐reinforced viscoelastic soft tissues

The mechanical properties of soft biological tissues vary depending on how the internal structure is organized. Classical examples of tissues are ligaments, tendons, skin, arteries, and annulus fibrous. The main element of such tissues is the fibers which are responsible for the tissue resistance and the main mechanical characteristic is their viscoelastic anisotropic behavior. The objective of this paper is to extend an existing model for isotropic viscoelastic materials in order to include anisotropy provided by fiber reinforcement. The incorporation of the fiber allows the mechanical behavior of these tissues to be simulated. The model is based on a variational framework in which its mechanical behavior is described by a free energy incremental potential whose local minimization provides the constraints for the internal variable updates for each load increment. The main advantage of this variational approach is the ability to represent different material models depending on the choice of suitable potential functions. Finally, the model is implemented in a finite‐element code in order to perform numerical tests to show the ability of the proposed model to represent fiber‐reinforced materials. The material parameters used in the tests were obtained through parameter identification using experimental data available in the literature. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis of hierarchical porous materials with ZSM-5 structures via template-free sol–gel method

Interests are focused on preparation of hierarchical porous materials with zeolite structures by using soft or rigid templates in order to solve diffusion and mass transfer limitations resulting from the small pore sizes of zeolites. Here we develop a convenient template-free sol–gel method to synthesize hierarchical porous materials with ZSM-5 structures. This method involves hydrothermal recrystallization of the xerogel converted from uniform ZSM-5 sol by a vacuum drying process. By utilizing this method we can manipulate the size of zeolite nanocrystals as building units of porous structures based on controlling temperature of recrystallization, consequently obtain hierarchical porous materials with different intercrystalline pore sizes and ZSM-5 structures.     

Superductile,Wavy Silica Nanostructures Inspired by Diatom Algae

Biology implements intriguing structural design principles that allow for attractive mechanical properties—such as high strength, toughness, and extensibility despite being made of weak and brittle constituents, as observed in biomineralized structures. For example, diatom algae contain nanoporous hierarchical silicified shells, called frustules, which provide mechanical protection from predators and virus penetration. These frustules generally have a morphology resembling honeycombs within honeycombs, meshes, or wavy shapes, and are surprisingly tough when compared to bulk silica, which is one of the most brittle materials known. However, the reason for its extreme extensibility has not been explained from a molecular level upwards. By carrying out a series of molecular dynamics simulations with the first principles‐based reactive force field ReaxFF, the mechanical response of the structures is elucidated and correlated with underlying deformation mechanisms. Specifically, it is shown that for wavy silica, unfolding mechanisms are achieved for increasing amplitude and allow for greater ductility of up to 270% strain. This mechanism is reminiscent to the uncoiling of hidden length from proteins that allows for enhanced energy dissipation capacity and, as a result, toughness. We report the development of an analytical continuum model that captures the results from atomistic simulations and can be used in multiscale models to bridge to larger scales. Our results demonstrate that tuning the geometric parameters of amplitude and width in wavy silica nanostructures are beneficial in improving the mechanical properties, including enhanced deformability, effectively overcoming the intrinsic shortcomings of the base material that features extreme brittleness.     

纳米细菌纤维素/聚乙烯醇复合水凝胶在模拟体液中的疲劳行为

聚乙烯醇(PVA)复合水凝胶作为半月板及软骨等长期承重植入体,在生理环境中的疲劳行为关系到植入体的持久性和稳定性。采用弥散增强的方法将纳米细菌纤维素(BC)均匀分散在PVA水凝胶基体中,制备了纳米BC/PVA复合水凝胶。在模拟体液(SBF)环境中,采用压缩疲劳过程分析、疲劳前后刚度变化分析及疲劳前后尺寸稳定性分析3种方法,测试和评价了复合水凝胶的抗疲劳性能和力学稳定性。结果表明:纳米BC/PVA复合水凝胶在模拟人体环境中具有良好的抗疲劳性能,能够满足体内植入物的抗疲劳性能需求;纳米BC的加入可以有效提升复合水凝胶的力学稳定性和抗疲劳性能,但随着纳米BC含量的进一步升高,复合水凝胶的抗疲劳性能有所减弱,当PVA与纳米BC质量比为30∶1时,纳米BC/PVA复合水凝胶疲劳前期与后期最大位移变化量最小(0.002mm),疲劳前后刚度变化最小(5.41%),且疲劳前后尺寸稳定性最强,变形量仅为0.427mm,抗疲劳性能达到最佳。     

Switching Colloidal Superstructures by Critical Casimir Forces

Recent breakthroughs in colloidal synthesis promise the bottom‐up assembly of superstructures on nano‐ and micrometer length scales, offering molecular analogues on the colloidal scale. However, a structural control similar to that in supramolecular chemistry remains very challenging. Here, colloidal superstructures are built and controlled using critical Casimir forces on patchy colloidal particles. These solvent‐mediated forces offer direct analogues of molecular bonds, allowing patch‐to‐patch binding with exquisite temperature control of bond strength and stiffness. Particles with two patches are shown to form linear chains undergoing morphological changes with temperature, resembling a polymer collapse under poor‐solvent conditions. This reversible temperature switching carries over to particles with higher valency, exhibiting a variety of patch‐to‐patch bonded structures. Using Monte Carlo simulations, it is shown that the collapse results from the growing interaction range favoring close‐packed configurations. These results offer new opportunities for the active control of complex structures at the nano and micrometer scale, paving the way to novel temperature‐switchable materials.