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Selection of droplets of different pH using SIFT. Droplets enter from bottom left of image. White droplets at lower pH (pH = 6.90) of higher interfacial tension follow the rail upwards. Clear droplets at higher pH (pH = 7.47) are only slightly deflected by the rail. A small amount of fluorescein is added to the droplets at pH = 6.90 to identify the two droplet populations.
