Laser Ultrasonic Characterization of Contact Dynamics in Single- and Few-Layer Self-Assembled Microscale Granular Crystals

Thesis (Ph.D.)--University of Washington, 2018Materials with designed structural discreteness and local resonances have garnered significant interest in recent years due their ability to manipulate waves in new ways. A number of studies over the past decade have explored mechanical wave phenomena in one such system, composed of ordered, close-packed arrays of elastic particles in contact, referred to as ‘granular crystals’. Proposed applications for such acoustic wave tailoring designer materials include vibration isolation, frequency-selective acoustic wave filtering, and acoustic wave guiding and focusing. Besides serving as a platform to gain new insight into the complex dynamic behavior of granular media, macroscale granular crystals (composed of millimeter- to centimeter-sized units) have also shown potential for use as such a designed composite material. Granular crystals also exhibit additional capabilities over some other types of other types of wave-tailoring designer materials, in that they exhibit a tunable dynamic response in the linear, weakly nonlinear and strongly nonlinear regimes to tailor acoustic waves. While macroscale granular crystals are designed to affect sonic frequency acoustic waves, extending granular crystals to the micro- and nanoscale has the potential to enable granular-based devices that operate at megahertz and gigahertz frequencies. In addition, microscale granular crystals also serve as a platform for exploring and developing a more general class of high, frequency, and micro- to nanostructured designer wave-tailoring materials, wherein large-scale fabrication presents significant challenges. Micro- and nanoscale granular crystals, however, cannot be thought of as simply scaled down versions of their macroscale counterparts since effects such as adhesion between particles, which are negligible at the macroscale, become significant at reduced length scales, and can drastically alter the granular crystal dynamics. This thesis focuses on addressing open questions relating to the contact-based dynamics of self-assembled, single- and few-layer-thick microscale granular crystals using an experimentally-driven approach. Convective colloidal self-assembly tech- niques are used to fabricate mono- and multilayer granular crystals comprised of micron- and sub-micron-sized particles. The vibrational dynamics of the microparticle arrays and the interaction of the contact resonances of the particles with surface, bulk and Lamb waves are studied experimentally using photoacoustic techniques. The applicability of adhesive contact models at the microscale, including the effects of adhesion-induced plasticity, is also discussed. Novel mechanisms to tune the interparticle and particle-substrate contact stiffness via nanoscale solid bridges and local ablation of the contact zone are also explored. This work sheds new light on our understanding of the contact-based dynamics of micro- to nanoscale particles, and particle assemblies, and opens avenues for developing a new class of locally-resonant granular acoustic metamaterials with applications such as signal processing and ultrasonic wave imaging, and offers new insights into the mechanical properties of self-assembled materials