A camphor boat is a well-known example of a self-propelled particle that moves spontaneously on a water surface, driven by local differences in surface tension. Such boats have been widely studied as non-biological models of active matter. Furthermore, they serve as fascinating systems for investigating bifurcation phenomena, where behavior changes drastically at a specific parameter threshold. When an elliptical camphor disk is placed on water with its center of mass fixed to a rotational axis, it rotates spontaneously. In our study, we observed a bifurcation from a stable state to a rotational state by controlling the water depth. Interestingly, the type of bifurcation—either subcritical or supercritical—depended on the aspect ratio of the elliptical disk. To explain these findings, we proposed a phenomenological model that incorporates the frictional effect between the rotational axis and the edge of the disk's center pore. This model yields a subcritical bifurcation when friction is present, but switches to a supercritical bifurcation in the absence of friction. These theoretical results strongly suggest that the presence of friction determines the specific type of bifurcation. Finally, we discuss the underlying dynamics by comparing our experimental data with the theoretical results obtained from the phenomenological model. This study was conducted in collaboration with Prof. H. Kitahata (Chiba University) and Prof. H. Sakaguchi (Nihon University).
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