Fig. 2

Fluorescence intensity of 6-carboxyfluorescein in dependency to pH measured for single droplets in flow, indicating the inter-droplet transport of pH mediating molecules between droplet populations. Histograms depict the abundance distribution of fluorescence intensity per single droplet for droplet populations (bin width 0.01 V). The abundance was normalized to the total count for each population. A minimum of 2880 droplets was analyzed per population. a Abundance distribution for a droplet population of pH 7.5 during generation and during immediate reinjection to a microfluidic chip. The variation between both distributions underlined the susceptibility of the pH monitoring method to small changes in the setup (i.e. chip or position on the microscope). b Two droplet populations of pH 6 and pH 7.5 were generated separately and are distinguished by their fluorescence intensity distribution. When the populations were mixed and incubated together for 5 min, the pH started to equilibrate over the droplets exemplified by the intermediate fluorescence intensities. c pH equilibration proceeds until the populations were not distinguishable anymore. d pH decrease in a droplet population was specifically induced by adding perfluorinated oil in which a minute amount of acetic acid was dissolved