Theory of strains in auxetic materials

Auxetic materials, or negative-Poisson's-ratio materials, are important technologically and fascinating theoretically. When loaded by external stresses, their internal strains are governed by correlated motion of internal structural degrees of freedom. The modelling of such materials is mainly based on ordered structures, despite existence of auxetic behaviour in disordered structures and the advantage in manufacturing disordered structures for most applications. We describe here a first-principles expression for strains in disordered such materials, based on insight from a family of 'iso-auxetic' structures. These are structures, consisting of internal structural elements, which we name 'auxetons', whose inter-element forces can be computed from statics alone. Iso-auxetic structures make it possible not only to identify the mechanisms that give rise to auxeticity, but also to write down the explicit dependence of the strain rate on the local structure, which is valid to all auxetic materials. It is argued that stresses give rise to strains via two mechanisms: auxeton rotations and auxeton expansion / contraction. The former depends on the stress via a local fabric tensor, which we define explicitly for 2D systems. The latter depends on the stress via an expansion tensor. Whether a material exhibits auxetic behaviour or not depends on the interplay between these two fields. This description has two major advantages: it applies to any auxeton-based system, however disordered, and it goes beyond conventional elasticity theory, providing an explicit expression for general auxetic strains and outlining the relevant equations.Comment: 7 pages, 4 figures, Appeared in Journal of Superconductivity and Novel Magnetism Volume 25, 565-571 (2012

Zika, Pregnancy, and the Law

The public health emergency surrounding the spread of the Zika virus has resurrected and brought into sharp relief some of the most vexing questions surrounding the relationship between pregnancy and law: the appropriate circumstances, if any, in which fetal tissue research is permissible; when and how the government may sponsor statements intended to influence reproductive decisions; and how to balance the health and rights of both women and their unborn children when health threats target both

On the Integration of Emerging Stock Markets in the Middle East

Results from the Johansen-Juselius test suggest that the Middle East emerging stock markets are segmented globally, but appear highly integrated within the region. Moreover, the Gonzalo- Granger test, in conjunction with error-correction models, indicates that the market in Egypt is a dominant force driving other markets in the region. The apparent segmentation of the markets in the Middle East from the global market implies that these emerging markets provide international investors with potential diversification gains.

Three-dimensional structure of basal body triplet revealed by electron cryo-tomography.

Basal bodies and centrioles play central roles in microtubule (MT)-organizing centres within many eukaryotes. They share a barrel-shaped cylindrical structure composed of nine MT triplet blades. Here, we report the structure of the basal body triplet at 33 Å resolution obtained by electron cryo-tomography and 3D subtomogram averaging. By fitting the atomic structure of tubulin into the EM density, we built a pseudo-atomic model of the tubulin protofilaments at the core of the triplet. The 3D density map reveals additional densities that represent non-tubulin proteins attached to the triplet, including a large inner circular structure in the basal body lumen, which functions as a scaffold to stabilize the entire basal body barrel. We found clear longitudinal structural variations along the basal body, suggesting a sequential and coordinated assembly mechanism. We propose a model in which δ-tubulin and other components participate in the assembly of the basal body

Da Vinci Fluids, catch-up dynamics and dense granular flow

We introduce and study a da Vinci Fluid, a fluid whose dissipation is dominated by solid friction. We analyse the flow rheology of a discrete model and then coarse-grain it to the continuum. We find that the model gives rise to behaviour that is characteristic of dense granular fluids. In particular, it leads to plug flow. We analyse the nucleation mechanism of plugs and their development. We find that plug boundaries generically expand and we calculate the growth rate of plug regions. In systems whose internal effective friction coefficient is relatively uniform we find that the linear size of plug regions grows as (time)$^{1/3}$. The suitability of the model to granular materials is discussed.Comment: 5 pages, 3 figures, edited for clarifications and added reference