Swirling Motion in the System of Vibrated Elongated Particles

We study large-scale collective motion emerging in a monolayer of vertically vibrated elongated particles. The motion is characterized by recurring swirls with the characteristic scale exceeding several times the size of individual particle. Our experiments identified small horizontal component of the oscillatory acceleration of the vibrating plate in a combination with orientation-dependent bottom friction as a source for the swirls formation. We developed a continuum model operating with velocity field and local alignment tensor which is in a qualitative agreement with the experiment.