Using the phase diagram of the PLL/ATP system, we established a model system to reproduce the droplet formation/disappearance in a chip (Figure and). We filled a small chamber with a PLL/ATP solution mixture containing no droplet and then added an ATP solution gently from one side of the chamber. Concentration gradients and a boundary between the one-phase and two-phase regions formed naturally. Observation of the boundary helps predict the droplet formation/disappearance in the chamber.