Spontaneous Motion of o-Toluidine Droplets: Repetitive Motion of Running and Squashing

We observed a unique spontaneous motion for an o-toluidine droplet containing iodine in an aqueous solution of cation surfactant. The droplet repeated a running motion and a squashing motion on the glass surface. The contact angle determines the motion of droplets on the glass surface. We analyzed this motion quantitatively using video capture. As the initial trigger of the running motion, the receding contact angle increased before the advancing contact angle increased. The predominant squashing span increased with time, although little change was found in the running span.