Vibration Assisted Self-Assembly of Colloidal Crystals
Dr. Green (Committee Chair)
Dr. Floro (MSE)
Dr. DuBay (ChE)
Dr. Deshpande (University of Cambridge)
Dr. Wadley (Advisor)
Colloidal crystals are a metamaterial composed of monodisperse particles with dimensions in the nanometer to micrometer range arranged in periodic, crystal-like arrays. Most commonly, and in this work, the colloidal crystal particles are spherical, with less than 5% variance in their radii, and can be assembled into amorphous or close packed face centered cubic (FCC) arrangements. Colloidal crystals are of interest because the periodic arrangement of their particles enables diffraction of photons and phonons in a manner that can be manipulated through rotating the colloidal crystal or by varying the particle size (and thus spacing compared to photon or phonon wavelength) and/or composition (to manipulate Young’s modulus and density which dictate phonon velocities or the refractive index which dictates photon scattering). Consequently, colloidal crystals could be used to inhibit or enhance spontaneous photon emission, as high reflection omnidirectional mirrors, low loss light guides, and much more. Additionally, inverse colloidal crystals, produced through the infiltration of the colloidal crystal with a second material followed by the removal of the initial colloidal particles, have been shown to exhibit high specific strength and modulus.
While colloidal crystals and their inverses have promising properties, producing colloidal crystals of sufficient size and perfection for these applications has been challenging. For perspective, a 10 mm wide spherical colloidal crystal comprised of 500 nm diameter spheres requires controlling the assembly of about six trillion spheres. Commonly, colloidal crystals are produced through the sedimentation of particles from a colloidal suspension. In this process as particles sediment out of suspension the particles naturally order due to greater entropic freedom of the particles in ordered arrays. However, with no fine control over particle motion, particles can easily fail to fill crystal lattice sites and become jammed resulting in high concentrations of defects (vacancies, stacking faults, and voids), small grains and even amorphous arrangements. Therefore, this dissertation seeks to model (in collaboration with colleagues at Cambridge University) the assembly process and explore vibration assisted growth of colloidal crystals to better control colloidal crystal growth.
Experimental studies are investigating the use of vibration during the sedimentation of particles on a flat plate or templated seed surfaces to order particles into a thermodynamically favorable periodic array to produce large, defect free colloidal crystals. In this process vibration behaves much like thermal energy in Bridgman crystal growth. The dissertation proposes to test and determine optimal vibration parameters that impart enough energy to loose particles, enabling their ordering while not disordering previously ordered crystalline regions. This is being conducted though modeling and the systematic analysis of colloidal crystals experimentally produced from particles ranging in diameter from 30 μm down through 300 nm under vibration conditions with independently and systematically varied frequency and amplitude. The measured grain size of these samples will be used to produce three-dimensional phase diagrams for particles of varying size comparing grain size to vibration frequency and amplitude. These results will then be used to test and refine the kinetic assembly model of the system which can then be used to predict optimal vibration conditions for systems not explicitly explored.