Complex contact-based dynamics of microsphere monolayers revealed by resonant attenuation of surface acoustic waves

Contact-based vibrations play a critical role in the dynamics of granular materials. Significant insights into vibrational granular dynamics have been obtained with reduced-dimensional systems containing macroscale particles. We study contact-based vibrations of a two-dimensional monolayer of micron-sized spheres on a solid substrate. Measurements of the resonant attenuation of laser-generated surface acoustic waves reveal three collective vibrational modes involving both displacements and rotations of the microspheres. To identify the modes, we tune the interparticle stiffness, which shifts the frequency of the horizontal-rotational resonances while leaving the vertical resonance unaffected. From the measured contact resonance frequencies we determine both particle-substrate and interparticle contact stiffnesses and find that the former is an order of magnitude larger than the latter. This study paves the way for investigating complex contact-based dynamics of microgranular media, demonstrates a novel acoustic metamaterial, and yields a new approach to studying micro- to nanoscale contact mechanics in multiparticle networks.