Collaborative Research: Mechanics of Granular Acoustic Meta-materials with Engineered Particles and Packings

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Acoustic meta-materials are engineered materials enabling the control of sound waves. Granular acoustic meta-materials are constructs of particles in periodic and disordered arrangements. This project seeks a fundamental understanding of the acoustic and mechanical response of granular packings where the particles possess specific engineered properties. These materials exhibit a key acoustical property, that is acoustic band gaps. Band gaps prevent sound of certain frequencies to propagate. Meta-materials exhibiting acoustic band gaps have applications for vibration isolation, sound wave communication, acoustic super-lenses, acoustic diodes, and acoustic cloaking devices. This project will combine engineered particle shapes and materials with specially designed spatial arrangements of the particles to allow detailed control over the acoustic properties of the material. This project will involve several undergraduates in research each year through partnerships between the collaborating institutions, one of which is minority-serving. Outreach initiatives include an annual lecture series on granular media. Teaching modules for use by the International Centre for Theoretical Physics will be developed for teaching basic computational research skills to graduate students from developing countries. In granular media, the discrete nature of the material allows optimization on both the grain and network scales. Also, the speed of sound in granular materials depends on the confining pressure due to changes in the particle-particle contact area. The objective of this research is to gain understanding of the acoustic response of engineered granular meta-materials, and to exploit their unique features to tune the mechanical response. Variations of the confining pressure will be employed to actively control the acoustic properties of granular meta-materials. Novel direct visualization techniques and discrete element simulations of individual particle motions and forces will enable close feedback between the predicted and measured mechanical response. The team will use 3D printing and other fabrication techniques to engineer particles with varied elastic properties, surface treatments, and complex shapes. In both experiments and simulations, the team will construct disordered and crystalline granular packings in both two- and three-dimensions using direct assembly methods. The team will also explore the effects of boundary conditions on the internal stress networks of granular packings with the goal of tuning their acoustic properties. Realistic inter-particle force laws obtained from direct measurements will be implemented into discrete element simulations of the mechanical response and compared to the results from experiments. Attainment of these goals will provide unprecedented insight into the acoustic properties of granular meta-materials.