Selection of the Rotation Direction for a Camphor Disk Resulting from Chiral Asymmetry of a Water Chamber

Self-motion of a camphor disk rotating inside a water chamber composed of two half-disks was investigated. The half-disks were joined along their diameter segments, and the distance between their midpoints (d_s) was considered as the control parameter. Various types of camphor disk motions were observed depending on d_s. When d_s = 0, the chamber had a circular shape, so it was symmetric. A camphor disk showed either a clockwise (CW) or counterclockwise (CCW) rotation with the direction determined by its initial state. The symmetry of the chamber was broken for d_s > 0. For moderate distances between the midpoints, a unidirectional orbital motion of the disk was observed. The preferred rotation direction was determined by the shape of the chamber, and it did not depend on the initial rotation direction. For yet larger d_s, the unidirectional circular motion was no longer observed and the trajectory became irregular. A mathematical model coupling the camphor disk motion with the dynamics of the developed camphor molecular layer on water was constructed, and the numerical results were compared with the experimental results. The selection of motion type can be explained by considering the influence of camphor concentration on the disk trajectory through the surface tension gradient.