Flow experiments. Flow experiments were performed using a flow setup that consists of low-pressure pumps connected to a microfluidic channel. HCl (1 mM), NaOH (1 mM), and phenol red (0.45 mM) were prepared and transferred into glass syringes and mounted on low-pressure pumps. CETONI Elements so�ware was used to program low-pressure pumps in the flow experiments shown in Fig. , and 5 (see Supplementary Informa�on S6 for further informa�on on the sequences used in each experiment). The same so�ware was used to analyze the absorbance data (see Supplementary Informa�on S3 for details of the experimental setup).