In bulk, the double emulsion was first produced at high pH as described above, and the pH of the sample was further decreased by adding some HCl (1M, 50 µL in the emulsion) to reprotonate the PNIPAM-COO -derivative (we used the same volume of HCl than that of NaOH initially added to increase the pH, both at the same concentration of 1M). The resulting double emulsion showed droplets with a very bright shell and a single dark core, suggesting that a transition from a multiple core to a single core double emulsion occurred when decreasing the pH (figure ). This feature was also nicely evidenced by decreasing the pH by adding a HCl (1M) drop on the glass microscopy slide when observing the multiple core double emulsion initially prepared at basic pH (see movies S2-3). In the first movie (S2) prepared using 2 mL PVA, 50 µL NaOH and 200 µL oil (that also contained PEG and PNIPAM-COOH (2%) as described above), the decrease of the pH induced fast transition of many multiple core droplets into single core ones, whereas for others, the transition occurred slowly, affording to observe the coalescence of internal aqueous droplets. In the second movie (S3), the initial basic multiple core double emulsion was prepared by pouring 2 mL oil (that also contained PEG and PNIPAM-COOH as described above) into 2 mL PVA, 50 µL NaOH. This yielded viscous multiple core double emulsion, with droplets in close contact, so that the decrease of the pH occurred more progressively when adding HCl on the microscopy slide (from right to left on the film).