Intermittent motion of a camphor float depending on the nature of the float surface on water

The periodic motion of a camphor float on water was investigated as a simple example of an autonomous motor. When a camphor disk that was connected to the center of a larger plastic disk was floated on water, intermittent motion (alternating between rest and rapid motion) was observed. The period of intermittent motion decreased with an increase in the hydrophobicity of the plastic film, which depended on the density of carbon powder that had been adhered to the plastic film with a laser printer. When a square plastic film with half-hydrophobic and half-hydrophilic surfaces was used as the camphor float, the direction of intermittent motion was mostly toward the hydrophilic side. These phenomena are discussed in relation to the diffusion of camphor molecules at the film/water interface and the surface tension around the film as the driving force.