Mode selection in the spontaneous motion of an alcohol droplet

An alcohol (pentanol) droplet exhibits spontaneous motion 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 l shows irregular translational motion, whereas intermediate-sized droplets of 0.1-200 mu l show vectorial motion. When the volume is above 300 mu 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.