UC Merced

Diatoms are microscopic algae featuring porous cell wall structures (frustules). Due to the wide variety of diatom shapes and intricate architectures, diatom frustules are viable prototypes to guide the design and fabrication of nanodevices for applications ranging from sensors to nanotemplates. The porous structure and mechanical behavior of biosilica diatom frustules were evaluated through ambient nanoindentation, AFM-based nanoindentation and finite element method (FEM) analysis. Two diatom species, Coscinodiscus sp. (centric) and Synedra sp. (pennate), were investigated through ambient nanoindentation and FEM simulation. High-resolution microscopy unveiled the diatom species structures. These diatom frustule dimensions varied largely depending on diatom species (diameters from 70-250 μm and lengths from 100-300 μm) with pore diameters ranging from 0.1-3.0 μm. Young’s modulus E and hardness H of the diatom frustules were obtained via ambient nanoindentation. These values varied also depending on diatom species: E from 1.1-10.6 GPa, H from 0.10-1.03 GPa for the Coscinodiscus sp.; and E from 13.7-18.6 GPa, H from 0.85-1.41 GPa for the Synedra sp. Predictive FEM simulations were performed on well-validated 3D frustule models, to correlate the mechanical response with specific morphology variables such as pore sizes. A correlation between mechanical properties and porosity was established for selected frustules. Furthermore, AFM investigation and FEM simulation involving 3D frustule models with hierarchical geometries were achieved to investigate the structure-property relationship upon compressive loads. AFM imaging and AFM-based nanoindentation of the centric and pennate frustules provided surface morphology and associated load-displacement curves, which were used to further characterize the diatoms. The calculated average E was 21.5 GPa and 26 GPa for the Coscinodiscus sp. and Synedra sp. respectively, which are higher than the values obtained from ambient nanoindentation. FEM compression simulations were also performed on centric models only to study the role of the hierarchical structures on their mechanical response. Displacement and stress distributions of a three-layered hierarchical frustule model under varied loads were systematically investigated. Results were compared with a stacking of three porous layers for further discussion. Suggestions for future experiments and modeling efforts are provided to enhance the ability to map strategies for the fabrication of 3D nanostructures and nanodevices.