MechE PhD Dissertation Defense: Aobo Li


MechE PhD Dissertation Defense: Aobo Li



ABSTRACT: Diatoms are incredibly diverse, photosynthetic microalgae, which are widely distributed in aquatic environments. The wide array and intricate morphology of the micro- and nanostructured silica exoskeletons (also known as frustules) encasing the single-celled diatoms offer a tremendous opportunity for myriad applications. Diatom frustules has been used as template for biomedical, energy harvesting, filtering, surface enhanced Raman spectroscopy, photonics crystal applications, among many others. In this thesis work, diatom frustules, as biosilica templates, are further leveraged to develop into advanced materials, with the aim that their advanced functionalities could benefit a broad range of applications.

In many of the applications based on diatom frustules, individual diatom manipulation is required for device fabrication, which has greatly limited their application in batch fabrication. The lack of effective methods for arranging diatoms into large area, uniformly oriented monolayers has greatly hindered their applications in scale. Therefore, this work first explores the method to develop large-scale compact monolayers of diatom frustules. Specifically, frustule assembly at the water-air interface are discussed with experimental and numerical efforts to demonstrate the mechanisms to form nearly uniformly oriented frustule monolayers.

The intrinsic micro/nanostructures of the frustules result in their large surface area. However, these properties can be further improved with the frustules as templates. Therefore, in the second part of the work, the material properties of diatom frustules are enhanced by synthesizing nanowire structures on diatom frustule templates. A detailed analysis reveals that the nanowire growth is governed by the silicon dioxide precipitation at their root region and tip region, and enhancements in the light scattering and surface area have been achieved with the composite materials.

The hierarchical pore structures of diatom frustules can be used as “molds” for the replication of frustules’ 3D structures, or in a 2D fashion, used as templates for the “stencil” fabrication of hierarchical nanoparticle patterns. Inspired by the hierarchical arrangement of such nanoparticle patterns, similar designs are applied to electromagnetic metamaterial absorbers. It has been revealed that hierarchy introduced an addition resonance state to the device, broadening the absorption spectrum. It is further demonstrated that the resonance peaks shift linearly with the inter-unit-cell spacing, which is attributed to the effective dipole resonances due to the collective resonances in the grouped nanoparticles.

By developing batch manipulation method to assemble diatom frustules, enhancing the intrinsic frustule material properties, and using diatom frustule pore designs as a source of inspiration in metamaterial design and nano-fabrications, this thesis work aims to use diatom frustules as basic templates for the development of engineering tool sets, with the aim of transforming and translating the mesoporous bio-silica into advanced functional materials.

COMMITTEE: ADVISOR Professor Xin Zhang, ME/ECE/BME/MSE; CHAIR Professor Michael Gevelber, ME/MSE/SE; Professor Stephan Anderson BUSM/ME; Professor Thomas Bifano, ME/MSE/BME; Professor Keith Brown, ME/MSE/Physics; Professor Chuanhua Duan, ME/MSE


10:00am on Thursday, November 7th 2019

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