Coupling Between Rotational And Translational Motions Of A Vibrated Polygonal Disk

We investigate experimentally the dynamics of a single polygonal disk (regular n-gon with 3 <= n <= 8) confined in a closed container under vertical vibrations against gravity. The disks tend to precess continuously upon vibrations, transferring mechanical energy into rotational and lateral translational degrees of freedom (DoF). An analysis of the velocity distribution functions in both DoF suggests that the mobility in both DoF are coupled with each other, exhibiting a characteristic angular velocity that depends on confinement and disk shape. The characteristic angular velocity can be captured with an analytical model considering sustainable precession due to continuous driving. Depending on confinement, translational and rotational kinetic energy fluctuations within one vibration cycle can be synchronized with each other and there exists a regime where injected energy is equally distributed in different DoF. Depending on n, the tendency for the disk to precess varies and there exists a regime (n <= 6) where persistent rotation of the disk rarely lasts longer than one vibration period. Our results suggest the possibility of tuning energy injection into different DoF in vibrated granular disk mono-layers via shape design and confinement.