Particle Shape and Dynamics of Granular Matter: Swarming to Swirling

We will discuss a series of experiments performed with granular rods, dimers, and flexible chains on a vibrated plate to illustrate the effect of particle shape on self-organization. A non-spherical shape is shown to lead to not only states which resemble nematic and smectic phases but also causes novel dynamics [1]. The ratchet mechanism which leads to vortex motion in a collection of rods on a vibrated plate and drift motion in a bouncing dimer will be discussed [2, 3]. The friction at the point of contact between particle and the substrate, and the coupling about the center of mass of a non-spherical is proposed to lead to observed motion. Exploiting this mechanism we construct mechanical self-propelled particles (SPP) using rods with asymmetric mass distributions. We then investigate the SSP number fluctuations, flow fields, and orientation order inside a container as a function of number density and excitation, and compare their statistics with recent models of active nematic particles and living cells.\\ 1. ``Vortices in vibrated granular rods," D.L. Blair, T. Neicu, and A. Kudrolli, Phys. Rev. E 67, 031303 (2003).\\ 2. ``Anisotropy driven dynamics in vibrated granular rods," D. Volfson, A. Kudrolli, and L.S. Tsimring, Phys. Rev. E 70, 051312 (2004).\\ 3. ``Dynamics of a bouncing dimer," S. Dorbolo, D. Volfson, L. Tsimring, and A. Kudrolli, Phys. Rev. Lett. 95, 044101 (2005).