Mode Selection in the Spontaneous Motion of an Alcohol Droplet

An alcohol (pentanol) droplet exhibits spontaneous agitation on an aqueous solution, driven by a solutal Marangoni effect. We found that the droplet's mode of motion is controlled by its volume. A droplet with a volume of less than $0.1 \mu\rm{l}$ shows irregular translational motion, whereas intermediate-sized droplets of $0.1-200 \mu\rm{l}$ show vectorial motion. When the volume is above $300 \mu\rm{l}$, the droplet splits into smaller drops. These experimental results regarding mode selection are interpreted in terms of the wave number selection depending on the droplet volume.