Huiling Duan

Eshelby formalism for nano-inhomogeneities

The Eshelby formalism for inclusion/inhomogeneity problems is extended to the nano-scale at which surface/interface effects become important. The interior and exterior Eshelby tensors for a spherical inhomogeneous inclusion with the interface stress effect subjected to an arbitrary uniform eigenstrain embedded in an infinite alien matrix, and the stress concentration tensors for a spherical inhomogeneity subjected to an arbitrary remote uniform stress field are obtained. Unlike their counterparts at the macro-scale, the Eshelby and stress concentration tensors are, in general, not uniform inside the inhomogeneity but are position-dependent. They have the property of radial transverse isotropy. It is also shown that the size-dependence of the Eshelby tensors and the stress concentration tensors follow very simple scaling laws. Finally, the Eshelby formula to calculate the strain energy in the presence of the interface effect is given.

Theory of Elasticity at the Nanoscale

Abstract We have shown in a series of recent papers that the classical theory of elasticity can be extended to the nanoscale by supplementing the equations of elasticity for the bulk material with the generalized Young–Laplace equations of surface elasticity. This review article shows how this has been done in order to capture the often unusual mechanical and physical properties of nanostructured particulate and porous materials. It begins with a description of the generalized Young–Laplace equations. It then generalizes the classical Eshelby formalism for nano-inhomogeneities; the Eshelby tensor now depends on the size of the inhomogeneity and the location of the material point in it. Then the stress concentration factor of a spherical nanovoid is calculated, as well as the strain fields in quantum dots (QDs) with multi-shell structures and in alloyed QDs induced by the mismatch in the lattice constants of the atomic species. This is followed by a generalization of the micromechanical framework for determining the effective elastic properties and effective coefficients of thermal expansion of heterogeneous solids containing nano-inhomogeneities. It is shown, for example, that the elastic constants of nanochannel-array materials with a large surface area can be made to exceed those of the nonporous matrices through pore surface modification or coating. Finally, the scaling laws governing the properties of nanostructured materials are derived. The underlying cause of the size dependence of these properties at the nanoscale is the competition between surface and bulk energies. These laws provide a yardstick for checking the accuracy of experimentally measured or numerically computed properties of nanostructured materials over a broad size range and can thus help replace repeated and exhaustive testing by one or a few tests.

Surface stress effect in mechanics of nanostructured materials

This review article summarizes the advances in the surface stress effect in mechanics of nanostructured elements, including nanoparticles, nanowires, nanobeams, and nanofilms, and heterogeneous materials containing nanoscale inhomogeneities. It begins with the fundamental formulations of surface mechanics of solids, including the definition of surface stress as a surface excess quantity, the surface constitutive relations, and the surface equilibrium equations. Then, it depicts some theoretical and experimental studies of the mechanical properties of nanostructured elements, as well as the static and dynamic behaviour of cantilever sensors caused by the surface stress which is influenced by adsorption. Afterwards, the article gives a summary of the analytical elasto-static and dynamic solutions of a single as well as multiple inhomogeneities embedded in a matrix with the interface stress prevailing. The effect of surface elasticity on the diffraction of elastic waves is elucidated. Due to the difficulties in the analytical solution of inhomogeneities of complex shapes and configurations, finite element approaches have been developed for heterogeneous materials with the surface stress. Surface stress and surface energy are inherently related to crack propagation and the stress field in the vicinity of crack tips. The solutions of crack problems taking into account surface stress effects are also included. Predicting the effective elastic and plastic responses of heterogeneous materials while taking into account surface and interface stresses has received much attention. The advances in this topic are inevitably delineated. Mechanics of rough surfaces appears to deserve special attention due to its theoretical and practical implications. Some most recent work is reviewed. Finally, some challenges are pointed out. They include the characterization of surfaces and interfaces of real nanomaterials, experimental measurements and verification of mechanical parameters of complex surfaces, and the effects of the physical and chemical processes on the surface properties, etc.

A scaling law for properties of nano-structured materials

In this brief communication, we identify intrinsic length scales of several physical properties at the nano-scale and show that, for nano-structures whose characteristic sizes are much larger than these scales, the properties obey a simple scaling law. The underlying cause of the size-dependence of these properties at the nano-scale is the competition between surface and bulk energies. This law provides a yardstick for checking the accuracy of experimentally measured or numerically computed properties of nano-structured materials over a broad size range and can thus help replace repeated and exhaustive testing by one or a few tests.

Novel Planar-Structure Electrochemical Devices for Highly Flexible Semitransparent Power Generation/Storage Sources

Flexible and transparent power sources are highly desirable in realizing next-generation all-in-one bendable, implantable, and wearable electronic systems. The developed power sources are either flexible but opaque or semitransparent but lack of flexibility. Therefore, there is increasing recognition of the need for a new concept of electrochemical device structure design that allows both high flexibility and transparency. In this paper, we present a new concept for electrochemical device design--a two-dimensional planar comb-teeth architecture on PET substrate--to achieve both high mechanical flexibility and light transparency. Two types of prototypes--dye-sensitized solar cells and supercapacitors--have been fabricated as planar devices and demonstrated excellent device performance, such as good light transparency, excellent flexibility, outstanding multiple large bending tolerance, light weight, effective prevention of short circuits during bending, and high device integration with up-date microelectronics, compared to conventional sandwich structure devices. Our planar design provides an attractive strategy toward the development of flexible, semitransparent electrochemical devices for fully all-in-one elegant and wearable energy management.

Importance of hierarchical structures in wetting stability on submersed superhydrophobic surfaces.

Submersed superhydrophobic surfaces exhibit great potential for reducing flow resistance in microchannels and drag of submersed bodies. However, the low stability of liquid-air interfaces on those surfaces limits the scope of their application, especially under high liquid pressure. In this paper, we first investigate the wetting states on submersed hydrophobic surfaces with one-level structure under hydrostatic pressure. Different equilibrium states based on free-energy minimization are formulated, and their stabilities are analyzed as well. Then, by comparison with the existing numerical and experimental studies, we confirm that a new metastable state, which happens after depinning of the three-phase contact line (TCL), exists. Finally, we show that a strategy of using hierarchical structures can strengthen the TCL pinning of the liquid-air interface in the metastable state. Therefore, the hierarchical structure on submersed surfaces is important to further improve the stability of superhydrophobicity under high liquid pressure.

Hierarchical, multilayered cell walls reinforced by recycled silk cocoons enhance the structural integrity of honeybee combs

We reveal the sophisticated and hierarchical structure of honeybee combs and measure the elastic properties of fresh and old natural honeycombs at different scales by optical microscope, environmental scanning electron microscope, nano/microindentation, and by tension and shear tests. We demonstrate that the comb walls are continuously strengthened and stiffened without becoming fragile by the addition of thin wax layers reinforced by recycled silk cocoons reminiscent of modern fiber-reinforced composite laminates. This is done to increase its margin of safety against collapse due to a temperature increase. Artificial engineering honeycombs mimic only the macroscopic geometry of natural honeycombs, but have yet to achieve the microstructural sophistication of their natural counterparts. The natural honeycombs serve as a prototype of truly biomimetic cellular materials with hitherto unattainable improvement in stiffness, strength, toughness, and thermal stability.

PERSPECTIVES IN MECHANICS OF HETEROGENEOUS SOLIDS

The Micro- and Nano-mechanics Working Group of the Chinese Society of Theoretical and Applied Mechanics organized a forum to discuss the perspectives, trends, and directions in mechanics of heterogeneous materials in January 2010. The international journal, Acta Mechanica Solida Sinica, is devoted to all fields of solid mechanics and relevant disciplines in science, technology, and engineering, with a balanced coverage on analytical, experimental, numerical and applied investigations. On the occasion of the 30th anniversary of Acta Mechanica Solida Sinica, its editor-in-chief, Professor Q.S. Zheng invited some of the forum participants to review the state-of-the-art of mechanics of heterogeneous solids, with a particular emphasis on the recent research development results of Chinese scientists. Their reviews are organized into five research areas as reported in different sections of this paper. §I firstly brings in focus on micro- and nano-mechanics, with regards to several selective topics, including multiscale coupled models and computational methods, nanocrystal superlattices, surface effects, micromechanical damage mechanics, and microstructural evolution of metals and shape memory alloys. §II shows discussions on multifield coupled mechanical phenomena, e.g., multi-fields actuations of liquid crystal polymer networks, mechanical behavior of materials under radiations, and micromechanics of heterogeneous materials. In § III, we mainly address the multiscale mechanics of biological nanocomposites, biological adhesive surface mechanics, wetting and dewetting phenomena on microstructured solid surfaces. The phononic crystals and manipulation of elastic waves were elaborated in § IV. Finally, we conclude with a series of perspectives on solid mechanics. This review will set a primary goal of future science research and engineering application on solid mechanics with the effort of social and economic development.

Microstructures and mechanical properties of silks of silkworm and honeybee

Natural silks as important biomaterials have wide applications in scaffolds for tissues, biocompatible coatings and drug delivery, etc. In this paper, we report on the microstructures and mechanical properties of honeybee and silkworm silks. The microstructures and mechanical properties of these natural silks are measured by environment scanning electron microscopy (ESEM), scanning probe microscopy (SPM), tensile tests, and nanoindentation. It is found that the honeybee silk appears as a single fiber with a circular cross-section and has a much finer and smoother texture than the silkworm silk. The honeybee silk exhibits a distinct linear and brittle elastic feature with an elastic modulus of 6.5GPa and a breaking strain of 3.8%, whereas the silkworm silk possesses a nonlinear and hierarchical failure character with an initial elastic modulus of 8.9GPa and a breaking strain of 15.7%. Moreover, the nanoindentation measurements show that the honeybee silk exhibits much less anisotropy than the silkworm silk. The ratio of the longitudinal modulus to the transverse modulus of the honeybee silk is 2.0, whereas that of the silkworm silk is 18.9. The different structural and mechanical properties of the honeybee and silkworm silks are likely a result of their specific biological functions.

Tuning graphene morphology by substrate towards wrinkle-free devices: Experiment and simulation

Graphene grown by chemical vapor deposition can be used as the conductive channel in metal oxide semiconductor field effect transistors, metallic electrodes in capacitors, etc. However, substrate-induced corrugations and strain-related wrinkles formed on the graphene layer impoverish the properties of these devices by lowering the conductance and increasing their variability. Using the scanning electron microscopy, Auger electron spectroscopy, scanning tunneling microscopy, and atomic force microscopy, we investigated the morphology of as-grown and transferred graphene sheets on different substrates. We show that while the compressive strain (from the growth process) in the graphene sheet on flat substrates is minimized by generating wrinkles, and on rough substrates, it can be minimized by improving the graphene-substrate adhesion, leading to lower densities of wrinkles. This method paves the way to the design of wrinkle-free graphene based devices.