From insect scales to sensor design: modelling the mechanochromic properties of bicontinuous cubic structures.

Many of the three-dimensional photonic crystals occurring in the scales of insects have bicontinuous cubic structures. Their optical properties have been studied extensively, however little is known about their mechanical properties and their optical response under deformation. We demonstrated a mechanochromic effect by deforming the scales of a weevil and calculated the elastic, optical and mechanochromic (assuming homogeneous deformation) properties of the three types of bicontinuous cubic structures occurring in nature: P-structure (primitive), G-structure (gyroid) and D-structure (diamond). The results show that all investigated properties of these three structure types strongly depend on their geometry, structural parameters such as volume fractions of the two constituting phases and the directions of the incident light or applied stress, respectively. Interestingly, the mechanochromic simulation results predict that these structures may show blue-shift or even red-shift under compression along certain directions. Our results provide design guidelines for mechanochromic sensing materials operating in the elastic regime, including parameters such as sensitivity and direction of spectral shift.

Self-Assembled Monolayers: Star-Shaped Crystallographic Cracking of Localized Nanoporous Defects (Adv. Mater. 33/2015).

On page 4877, F. U. Renner, A. Bashir, M. Valtiner, and co-workers describe a star-like dealloying corrosion morphology that appears during the localized attack of smooth well-prepared Cu-Au surfaces. The surfaces are initially protected by thiol or selenol inhibitior films. Localized dealloying of Cu-Au produces nanoporous gold under stress and crystallographic cracks - thereby opening a new approach combining surface science with nanoscale mechanical testing.

Star-Shaped Crystallographic Cracking of Localized Nanoporous Defects.

On self-assembled monolayer-covered Cu-Au substrates, localized volume shrinkage at initial dealloying sites leads to cracks within the attacked regions. It is started from well-controlled surface structures to gain fundamental insights in the driving mechanisms of localized corrosion and crack formation. Both the crack density and the crack morphology are critically dependent on surface orientation, crystallography, and inhibitor molecule species.

Ab initio identified design principles of solid-solution strengthening in Al.

Solid-solution strengthening in six Al-X binary systems is investigated using first-principle methods. The volumetric mismatch parameter and the solubility enthalpy per solute were calculated. We derive three rules for designing solid-solution strengthened alloys: (i) the solubility enthalpy per solute is related to the volumetric mismatch by a power law; (ii) for each annealing temperature, there exists an optimal solute-volume mismatch to achieve maximum strength; and (iii) the strengthening potential of high volumetric mismatch solutes is severely limited by their low solubility. Our results thus show that the thermodynamic properties of the system (here Al-X alloys) set clear upper bounds to the achievable strengthening effects owing to the reduced solubility with increasing volume mismatch.

The use of flat punch indentation to determine the viscoelastic properties in the time and frequency domains of a soft layer bonded to a rigid substrate.

This paper reports a computational study of the indentation of a flat punch into a compressible elastic layer (with Poisson's ratio varying from 0 to 0.49) bonded to a rigid substrate. Based on the computational results and using Sneddon's solution [Sneddon IN. The relation between load and penetration in the axisymmetric Boussinesq problem for a punch of arbitrary profile. Int J Eng Sci 1965;3:47] and the asymptotic solution [Jaffar MJ. A general solution to the axisymmetric frictional contact problem of a thin bonded elastic layer. Proc Inst Mech Eng C 1997;211:549; Yang FQ. Asymptotic solution to axisymmetric indentation of a compressible elastic thin film. Thin Solid Films 2006;515:2274] as the two limits, a simple expression of the load-depth curve valid for an arbitrary ratio of the indenter radius to the thickness of the layer is obtained. Further, a correlation between indentation load and depth for a rigid flat punch indenting into linearly viscoelastic layers bonded to a rigid substrate is proposed by using the correspondence principle. Several procedures are suggested based on the results reported in this study to determine the viscoelastic properties of the layer in the time or frequency domains. The findings are verified by numerical examples. The results may facilitate the use of depth-sensing indentation tests to characterize the mechanical properties of polymeric films or functional coatings on hard substrates, and some biological materials, e.g. articular cartilage.