Cycloid Motions of Aggregates in a Dust Plasma

Hypocycloid and epicycloid motions of aggregates consisted of one large and one small grains are experimentally observed in an rf dust plasma.The cycloid motions are regarded as combination of a primary circle and a secondary circle.Measurements with high spatiotemporal resolution show that the secondary circle is determined by the initial angle velocity of the dropped aggregate.The primary circle originates from the asymmetry of the aggregate.The small grain in the aggregate always leads the large one as they travelling,which results from the difference of the natural frequency of the two grains.Comparative experiments with regular microspheres show that the cycloid motions are distinctive features of aggregates immersed in a plasma.