Superductile, Wavy Silica Nanostructures Inspired by Diatom Algae
AP Garcia and N Pugno and MJ Buehler, ADVANCED ENGINEERING MATERIALS, 13, B405-B414 (2011).
Biology implements intriguing structural design principles that allow for attractive mechanical properties-such as high strength, toughness, and extensibility despite being made of weak and brittle constituents, as observed in biomineralized structures. For example, diatom algae contain nanoporous hierarchical silicified shells, called frustules, which provide mechanical protection from predators and virus penetration. These frustules generally have a morphology resembling honeycombs within honeycombs, meshes, or wavy shapes, and are surprisingly tough when compared to bulk silica, which is one of the most brittle materials known. However, the reason for its extreme extensibility has not been explained from a molecular level upwards. By carrying out a series of molecular dynamics simulations with the first principles-based reactive force field ReaxFF, the mechanical response of the structures is elucidated and correlated with underlying deformation mechanisms. Specifically, it is shown that for wavy silica, unfolding mechanisms are achieved for increasing amplitude and allow for greater ductility of up to 270% strain. This mechanism is reminiscent to the uncoiling of hidden length from proteins that allows for enhanced energy dissipation capacity and, as a result, toughness. We report the development of an analytical continuum model that captures the results from atomistic simulations and can be used in multiscale models to bridge to larger scales. Our results demonstrate that tuning the geometric parameters of amplitude and width in wavy silica nanostructures are beneficial in improving the mechanical properties, including enhanced deformability, effectively overcoming the intrinsic shortcomings of the base material that features extreme brittleness.
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