  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.
