Self-motion of an oil droplet was investigated on a sodium dodecyl sulfate (SDS) aqueous phase. With an increase in the concentration of SDS, the nature of self-motion of a butyl salicylate (BS) droplet as the oil droplet was changed, i.e., no motion, reciprocation with a small amplitude, and reciprocation with a large amplitude, which was a value close to the half-length of the chamber. The interfacial tension, contact angle, and convective flow around the droplet were measured to clarify the driving force of reciprocation. The mechanisms of two types of reciprocation and mode-change were discussed in terms of the adsorption of SDS molecules at the BS/water interface and the dissolution of a mixture of BS and SDS into the bulk phase, the convective flow, and the Young's equation. The features of reciprocation and mode-change depending on the concentration of SDS were qualitatively reproduced by numerical calculation based on an equation of motion and the kinetics of SDS and BS at the air/aqueous interface.

We studied rotational motion of a symmetric self-propelled object on water under periodic halt and release operations with an external force. We propose a novel system in which the direction of rotation inverts after each halt-and-release operation. The considered self-propelled object was composed of a hexagonal plastic plate with a small orifice in the center. Six camphor disks were glued to one side of the plate at the corners. The plate was placed on the water surface and could rotate around a vertical axis located in the center. The initial direction of rotation, either clockwise or counterclockwise, depended on initial conditions. We discovered that, after a temporal halt of the rotor by the external force and next release, the direction of rotation inverted spontaneously. The probability of such inversion was studied as a function of the halt time, release time, area of the plastic plate, and stirring rate of the water phase. The distribution of camphor molecules around a camphor disk was visualized. We explain the mechanism of inversion by the coupling between the camphor distribution on water and the inertial water flow.

We investigated self-propelled motions of thin filaments atop water, where we focused on understanding pendulum-type oscillations and synchronization. The filaments were produced from a commercial adhesive (consisting mainly of nitrocellulose and acetone), and exhibited deformable motions. One end of each filament was held on the edge of a quadrangular water chamber while the other was left free. Acetone and other organic molecules from the nitrocellulose filament develop on the water surface and decrease the surface tension. The difference in the surface tension around the filament becomes the driving force of the self-propelled motions. When a single filament was placed in the water chamber, a pendulum-type oscillation in the deformation of the filament was observed. When two filaments were placed in parallel in the chamber, in-phase, out-of-phase, and no-synchronization motions were observed. It was found that the class of motions depends on the distance between the two fixed points of the filaments. Mathematical modeling and numerical simulations were also used in order to further understand the dynamics of the surface active molecules and the filament motions. We propose a mathematical model equation and reproduce various behaviors exhibited by soft self-propelled matters through numerical simulation.

The response of a traveling pulse to a local external stimulus is considered numerically for a modified three-component Oregonator, which is a model system for the photosensitive Belousov-Zhabotinsky (BZ) reaction. The traveling pulse is traced and constantly stimulated, with the distance between the pulse and the stimulus being kept constant. We are interested in the minimal strength of the spatially localized stimulus in order to eliminate the pulse. The use of a stimulus of small width allows us to detect the point in the pulse most sensitive to the external stimulus, referred to as the "Achilles' heel" of the traveling pulse, at which minimal strength of stimulus causes a collapse of the pulse. Our findings are demonstrated experimentally as well with the photosensitive BZ reaction.