Synchronized motion of two camphor disks on a water droplet levitated under microgravity

Two camphor disks, as self-propelled objects, were placed on a water droplet levitated under microgravity to understand the nature of their self-propelled motion on a sphere field without a boundary. When a single camphor disk was placed on the droplet, unidirectional rotation with a constant speed was observed. When two camphor disks were placed on the droplet, synchronized motion occurred; that is, the two disks exhibited unidirectional motion while the distance between them was maintained. The characteristic features of the selfpropelled motion are discussed in relation to the camphor molecules developed from the disk as the source of both the driving force of motion and the repulsive force between the disks. The present study suggests that selfpropelled objects can exhibit cooperative motion in a field without boundary.