Rhythmic Motion of Self-Propelled Particles Sliding on a Water Surface

Abstract

Variety of non-living self-propelled particles have been reported, for example, metal particles moving in hydrogen peroxide solution, oil droplets swimming in surfactant aqueous solution, and solid particles sliding on water. The self-propelled particles change the environmental condition around them and spontaneously break spacial symmetry by themselves resulting in stably generating driving force. In addition, the self-propelled particles interact each other, and thus, induce transition of collective behaviors depending on number density. Recently, mode change of self-propelled motion sliding on water, that is camphor disk, was observed with increase in the number density. The circular camphor disks showed continuous motion with low number density. With overcoming threshold value, the disks motion change to oscillatory motion, where the disks alternate rest and rapid motion. The mode-bifurcation was explained using mathematical model, where the essential factor was the coupling of slow dynamics of mean camphor concentration field and fast process of disk motion.