  A paraffin droplet containing camphor and oil red O (dye) floating on the
water surface shows spontaneous motion and deformation generated by the surface
tension gradient around the droplet. We focused on the intermittent motion with
a pronounced deformation into a crescent shape observed at specific
concentrations of camphor and oil red O. We quantitatively analyzed the time
changes in the droplet deformation and investigated the role of the oil red O
by measuring the time-dependent paraffin/water interfacial tension with the
pendant drop method. The observed effect can be explained by the active role of
the oil red O molecules at the paraffin/water interface. The interfacial
tension decreases gradually after the interface formation, allowing for the
dynamic deformation of the droplet. The combination of the decrease in
interfacial tension and the reduction in driving force related to camphor
outflow generates intermittent motion with dynamic deformation into a crescent
shape.
