Mechanics of Tunable Helices and Geometric Frustration in Biomimetic Seashells

Helical structures are ubiquitous in nature and engineering, ranging from DNA molecules to plant tendrils, from sea snail shells to nanoribbons. While the helical shapes in natural and engineered systems often exhibit nearly uniform radius and pitch, helical shell structures with changing radius and pitch, such as seashells and some plant tendrils, adds to the variety of this family of aesthetic beauty. Here we develop a comprehensive theoretical framework for tunable helical morphologies, and report the first biomimetic seashell-like structure resulting from mechanics of geometric frustration. In previous studies, the total potential energy is everywhere minimized when the system achieves equilibrium. In this work, however, the local energy minimization cannot be realized because of the geometric incompatibility, and hence the whole system deforms into a shape with a global energy minimum whereby the energy in each segment may not necessarily be locally optimized. This novel approach can be applied to develop materials and devices of tunable geometries with a range of applications in nano/biotechnology

Nonlinear geometric effects in mechanical bistable morphing structures

Bistable structures associated with non-linear deformation behavior, exemplified by the Venus flytrap and slap bracelet, can switch between different functional shapes upon actuation. Despite numerous efforts in modeling such large deformation behavior of shells, the roles of mechanical and nonlinear geometric effects on bistability remain elusive. We demonstrate, through both theoretical analysis and table-top experiments, that two dimensionless parameters control bistability. Our work classifies the conditions for bistability, and extends the large deformation theory of plates and shells.Comment: 3 figure

Evolution of interfacial structure of the joints between a tungsten-copper composite and austenitic stainless steel

Joining of CuW70 composite and 0Cr18Ni9 stainless steel was successfully achieved at temperatures 1150 °C with durations from 15 to 90 min after adding a pure copper intermediate layer. Microstructure, phase composition and elemental distribution at the interfacial region were studied. The optimum joining interface with a dense microstructure was obtained at a joining temperature of 1150 °C with a duration of 60 min. Interdiffusion of atoms occurred at the interface between the tungsten-copper composite and stainless steel, resulting in formation of various compounds such as W _0.6 Cu _0.4 , Cu _3.8 Ni and Fe _0.946 Ni _0.054 . An iron chromium-rich layer was formed at the interface near the tungsten-copper composite side, and an island structure containing Cr, Cu, and Ni was formed near the stainless steel side

Face Normals “in-the-wild” using Fully Convolutional Networks

Leftward twisting of the embryonic chick brain subjected to surface tension in an abnormal chick embryo with a leftward looped hear

Supplementary Figure S1 from How the embryonic chick brain twists

Measurement of the torsional angle from the OCT image

Erratum: Mechanics of tunable helices and geometric frustration in biomimetic seashells

Original article: EPL, 105 (2014) 6400