General experimental conditions

The oil phase consisted of mineral oil (Nacalai Tesque) containing 0.5% Span80 (Tokyo Chemical). Aqueous phases 1 and 2 were water for Fig. 2e,f. For Figs 2g and 4b–e, aqueous phase 1 was 0.2-mM Fl–Na (Sigma-Aldrich) and aqueous phase 2 was water. The aqueous phases used in the other cases are described in the Methods section reporting the BSF pH oscillation. F_aq1=F_aq2=10 μl min⁻¹ for all experiments. F_oil=15 μl min⁻¹ (that is, w=0.99 s) for Figs 3, ​,4,4, ​,5.5. F_oil was varied for each experiment in Fig. 2. The a.c. voltage applied to effect droplet fusion was V_pp=300 V (peak to peak; 1 kHz).

All the experiments were carried out using a fluorescence microscope (IX71 or IX81, Olympus). For the observations in Fig. 2e,f, a high-speed complementary metal-oxide-semiconductor camera (FASTCAM SA3 120K, Photron) was used. For the experiments in Figs 2g, ​,3,3, ​,4,4, ​,5,5, a digital camera (EOS 60D, Canon) was used.

The reproducibility of all experimental results was confirmed by performing at least two or three experiments.