Effect of chemical reaction causing contact angle variation, on the spontaneous motion of an I2-containing nitrobenzene droplet

• We analyze running distance and duration of droplet motion.
• We determine iodine in oil droplet and iodic ions in aqueous solution.
• Iodic ions are produced from the disproportionate reaction under alkaline conditions.
• Competition between TSA+ adsorption and ion aggregation controls contact angle.
• Disruption of interfacial tension balance induces spontaneous droplet motion.

Spontaneous motion of nitrobenzene droplet including only iodine (potassium iodide-free) in cation surfactant aqueous solution was investigated. For quantitative clarification of the motion mechanism, the contact angle of droplet and the concentrations of iodic components in this system, i.e. I2, I−, IO3−, and I3−, were measured. A disproportionate reaction of iodine in the alkaline conditions to produce iodide and iodate ions was identified, inducing the reduction of the droplet contact angle. A significant relation between running distance of spontaneous droplet motion and the droplet contact angle was found. The mechanism of motion was discussed based on these results.

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