No abstract

Fish scales from Arapaima Gigas have been studied in terms of their structure (morphology, collagen content) and mechanical properties. A strong mechanical gradient was revealed in the scale, hardness and reduced modulus being reduced by a factor of three. Correlations between the hardness and reduced modulus and the mineral content have been found. The external mineral rich layers were determined to be hard and stiff while the inner surface, being collagen rich, was less hard and less stiff. The inside of the scales showed a progressive decrease of mechanical properties, with variations attributed to the plywood-like structure of the scale.

Density gradients across reaction fronts propagating vertically can lead to Rayleigh–Taylor instabilities. Reaction fronts can also become unstable due to diffusive instabilities, regardless the presence of a mass density gradient. In this paper, we study the interaction between density driven convection and fronts with diffusive instabilities. We focus in fluids confined in Hele–Shaw cells or porous media, with the hydrodynamics modeled by Brinkman’s equation. The time evolution of the front is described with a Kuramoto–Sivashinsky (KS) equation coupled to the fluid velocity. A linear stability analysis shows a transition to convection that depends on the density differences between reacted and unreacted fluids. A stabilizing density gradient can surpress the effects of diffusive instabilities. The two-dimensional numerical solutions of the nonlinear equations show an increase of speed due to convection. Brinkman’s equation lead to the same results as Darcy’s laws for narrow gap Hele–Shaw cells. For large gaps, modeling the hydrodynamics using Stokes’ flow lead to the same results.