Dynamic properties of the metastable interface between two miscible solutions are investigated by the collision of two droplets. A clear interface is observed between the two colliding droplets. The interface moves in the colliding droplet toward the side where the original droplet has a lower surface tension. The interface is set to the middle of the colliding droplet by controlling the surface tension of the droplets to observe the chemical reactions at the droplet interface by cavity-enhanced Raman spectroscopy. This study provides a foundation for further research on the initial process of the chemical reactions of two miscible solutions.