Reciprocating Motion of a Self-Propelled Object on a Molecular Layer

The mode change of a simple autonomous motor depending on the nature of a monolayer on water is investigated. A camphor disk is floated on a molecular layer of N-stearoyl-p-nitroaniline (C(18)ANA), which gives a surface-pressure (pi)-area per molecule (A) isotherm with a local maximum and a local minimum. The nature of the camphor motion changes depending on A, and in particular, reciprocating motion is observed at a lower A while cutting out its own trajectory of motion. The characteristic motion of a camphor disk depending on A is discussed in relation to the pi-A isotherm of C(18)ANA and the influence of the molecular interaction between molecules on the driving force of motion.