Chung Ning Weng

Derivation of the generalized Young-Laplace equation of curved interfaces in nanoscaled solids

In nanoscaled solids, the mathematical behavior of a curved interface between two different phases with interface stress effects can be described by the generalized Young-Laplace equations [T. Young, Philos. Trans. R. Soc. London 95, 65 (1805); P. S. Laplace, Traite de Mechanique Celeste (Gauthier-Villars, Paris, 1805), Vol. 4, Supplements au Livre X]. Here we present a geometric illustration to prove the equations. By considering a small element of the curved thin interface, we model the interface stresses as in-plane stresses acting along its edges, while on the top and bottom faces of the interface the tractions are contributed from its three-dimensional bulk neighborhood. With this schematic illustration, simple force balance considerations will give the Young-Laplace equations across the interface. Similar procedures can be applied to conduction phenomena. This will allow us to reconstruct one type of imperfect interfaces, referred to as highly conducting interfaces.

Cloak for curvilinearly anisotropic media in conduction

We explore the possibility to cloak a region in curvilinearly anisotropic background materials in the context of conductivity. Materials with curvilinear anisotropy possess constant properties in specific curvilinear coordinate. For cylindrically and spherically anisotropic solids, the cloak center and the origin of material coordinate are generally not collocated. We show that in combination with a rigid-body translation from the cloak center to the material origin, the previous coordinate transformation procedure remains applicable. But now the transformed material specifications depend on the position of cloak center. The validity of the cloak parameters is verified by finite element simulations.

Materials with constant anisotropic conductivity as a thermal cloak or concentrator

An invisibility cloak based on transformation optics often requires material with inhomogeneous, anisotropic, and possibly extreme material parameters. In the present study, on the basis of the concept of neutral inclusion, we find that a spherical cloak can be achieved using a layer with finite constant anisotropic conductivity. We show that thermal localization can be tuned and controlled by anisotropy of the coating layer. A suitable balance of the degree of anisotropy of the cloaking layer and the layer thickness provides a cloaking effect. Additionally, by reversing the conductivities in two different directions, we find that a thermal concentrating effect can be simulated. This finding is of particular value in practical implementation as a material with constant material parameters is more feasible to fabricate. In addition to the theoretical analysis, we also demonstrate our solutions in numerical simulations based on finite element calculations to validate our results.

Invisibility cloak with a twin cavity

We study an invisibility cloak with a twin cavity, simulated by a plane algebraic curve-hippopede. The cloaked region, which looks like eight for some sets of geometric parameters, is expanded from one single point. Using a geometric transformation approach, we demonstrate that the material parameters of cloaking layer can be exactly determined. Numerical simulations show that the incoming rays pass in and out the cloaking region twice, and return to their original trajectory outside the curved cloak. A notable feature is that the cloaking region has two hollow regions in which two objects can be hidden at one time and that they could not perceive each other.

Design of Microstructures and Structures with Negative Linear Compressibility in Certain Directions

The linear compressibility of a solid is defined as the relative decrease in length of a line when the solid is subjected to unit hydrostatic pressure. Materials with a negative linear or area compressibility could have interesting technological applications. However, in the case of homogeneous materials only rare crystal phases exhibit this effect. In particular, for isotropic or cubic solids the linear compressibility is known to be isotropic and positive, namely a sphere of a cubic or isotropic crystal under hydrostatic pressure remains a sphere. For less symmetric solids, it generally varies with the direction n. Here we derive explicit expressions of the stationary values (maximum and minimum) of linear compressibility for single phase solids with monoclinic, orthotropic, tetragonal, trigonal, and hexagonal symmetry. A list of crystals that may exhibit negative linear compressibility in certain directions is outlined. Next, by assembling a two-component material, we propose microstructure networks to achieve such a property. Numerical simulations, based on a refined finite element method, are provided.

Equivalent configurations of optical transformation media

We demonstrate that a medium consisting of two adjoining distinct layers of transformation materials, corresponding respectively to two linear coordinate transformations, can behave effectively as that of the same region transformed by another linear transformation. The equivalence means that, irrespective of the direction of incident wave, the fields of the medium exterior to the transformed regions of the two configurations are exactly the same. This property can also apply to a domain that is transformed by a piecewise linear transformation function, and to a medium that is mapped by a general curved function. This proof is shown analytically based on a rigorous Fourier-Bessel analysis. The equivalence suggests that, for a given transformed domain, one can find an infinite number of complementary media that altogether can give a desired effective response of certain transformation path.