Dynamic ordering caused by a source-sink relation between two droplets.

Two droplets composed of different chemicals, 1-decanol and liquid paraffin, floating on the water surface show characteristic co-responsive behavior. The presence of two different types of droplets in the system imposes an asymmetry that would not be possible with single droplets alone. The self-propulsion and interactions between droplets appear because surface active 1-decanol molecules provided by the source are absorbed by the paraffin sink thus generating an asymmetric surface tension gradient. This source-sink relation between droplets stabilizes and enhances the self-propulsion, and leads to a variety of dynamic structures including oscillations in the inter-droplet distance. We found that the character of time evolution also depends on the concentration of dye, Sudan Black B, initially used just to stain the decanol droplet. A simple mathematical model explains the transition between the stationary state and the oscillations as a Hopf bifurcation.