In this paper,a phenomenological continuum theory of surface piezoelectricity accounting for the linear superficial interplay between electricity and elasticity is formulated primarily for elastic dielectric materials.This theory is inspired by the physical idea that once completely relaxed,an insulating free dielectric surface will sustain a nontrivial spontaneous surface polarization in the normal direction together with a tangential self-equilibrated residual surface stress field.Under external loadings,the surface Helmholtz free energy density is identified as the characteristic function of such surfaces,with the in-plane strain tensor of surface and the surface free charge density as the independent state variables.New boundary conditions governing the surface piezoelectricity are derived through the variational method.The resulting concepts of charge-dependent surface stress and deformationdependent surface electric field reflect the linear electromechanical coupling behavior of nanodielectric surfaces.As an illustrative example,an infinite radially polarizable piezoelectric nanotube with both inner and outer surfaces grounded is investigated.The novel phenomenon of possible surface-induced polarity inversion is predicted for thin enough nanotubes.
The nonlinear atomistic interactions usually involve softening behavior. Instability resulting directly from this softening are called the material instability, while those unrelated to this softening are called the structural instability. We use the finite-deformation shell theory based on the interatomic potential to show that the tension instability of single-wall carbon nanotubes is the material instability, while the compression and torsion instabilities are structural instability.
A dragonfly wing consists of membranes and both longitudinal and cross veins.We observed the microstructure cross-section at several locations in the dragonfly wing using environmental scanning electron microscopy(ESEM).The organic nature of the junction between the vein and the membrane was clearly identifiable.The membrane was divided into two layers,the upper epidermis and the lower epidermis.These layers extend around the sandwich structure vein,and combine with the adjacent membrane at a symmetrical location along the vein.Thus,we defined this as an organic junction between the vein and the membranes. The organic junction is able to form a tight corrugation angle,which dramatically increases both the warping rigidity and the strength of the wing,but not the torsional rigidity.The torsional deformation is primarily controlled by the microstructure of the longitudinal veins,and is based on the relative rotation angle between the epidermal layer and the inner layer of the vein that forms the zigzag section.
Raman spectroscopy has been widely used to identify the physical properties of carbon nanotubes(CNTs),and to assess their functionalization as well as orientation.Recently,Raman spectroscopy has become a powerful tool to characterize the interfacial properties between CNTs and polymer matrices.This review provides an overview of micro-Raman spectroscopy of CNTs and its application in studying CNT reinforced polymer composites.Based on the specific Raman band shifts relating to the mechanical deformation of CNTs,Raman scattering can be used to evaluate the interactions between the CNTs and the surrounding polymer in the composites,and to detect the phase transitions of the polymer,and investigate the local stress state as well as the Young's modulus of the CNTs.Moreover,we also review the current progress of Raman spectroscopy in various CNT macroarchitectures(such as films,fibers as well as composite fibers).The microscale structural deformation of CNT macroarchitectures and strain transfer factors from macroscale architectures to microscale structures are inferred.Based on an in situ Raman-tensile test,we further predict the Young's modulus of the CNT macroarchitectures and reveal the dominating factors affecting the mechanical performances of the CNT macroarchitectures.
Thousands of plant and animal species have been observed to have superhydrophobic surfaces that lead to various novel behaviors. These observations have inspired attempts to create artificial superhydrophobic surfaces, given that such surfaces have multitudinous applications. Superhydrophobicity is an enhanced effect of surface roughness and there are known relationships that correlate surface roughness and superhydrophobicity, based on the underlying physics. However, while these examples demonstrate the level of roughness they tell us little about the independence of this effect in terms of its scale. Thus, they are not capable of explaining why such naturally occurring surfaces commonly have micron-submicron sizes. Here we report on the discovery of a new relation, its physical basis and its experimental verification. The results reveal that scaling-down roughness into the micro-submicron range is a unique and elegant strategy to not only achieve superhydrophobicity but also to increase its stability against environmental disturbances. This new relation takes into account the previously overlooked but key fact that the accumulated line energy arising from the numerous solid-water-air intersections that can be distributed over the apparent contact area, when air packets are trapped at small scales on the surface, can dramatically increase as the roughness scale shrinks. This term can in fact become the dominant contributor to the surface energy and so becomes crucial for accomplishing superhydrophobicity. These findings guide fabrication of stable super water-repellant surfaces.
