Chemical reaction in a liquid–liquid phase-separated multiple droplet: Synchronization of color change dynamics with droplet movement

Liquid–liquid phase separation in a biotic cell system organizes complicated biochemical reactions and functions by forming membraneless compartments that allow a substrate to move across the phase boundary. On the other hand, liquid–liquid phase separation in an abiotic system gives rise to an emulsion and/or multiple droplets that hardly undergo chemical reactions. We have developed a method for the formation of phase-separated multiple droplet in a ternary mixture with a 3D-printed microchannel and demonstrated the occurrence of the iron(III) thiocyanate ligand exchange reaction in the multiple droplet. The reaction proceeded differently in the outer- and the inner-droplet phases, giving a different iron(III) complex that was identified on the basis of its color change. Surprisingly, the color change was dynamic, enabling visualization of the interphase mass transfer. At the same time, the color change dynamics synchronized with the multiple-droplet movement.