Channel-fouling is a pervasive problem in continuous flow chemistry, causing poor product
control and reactor failure. Droplet chemistry, in which the reaction mixture flows as discrete
droplets inside an immiscible carrier liquid, prevents fouling by isolating the reaction from the
channel walls. Unfortunately, the difficulty of controllably adding new reagents to an existing
droplet stream has largely restricted droplet chemistry to simple reactions in which all
reagents are supplied at the time of droplet formation. Here we describe an effective method
for repeatedly adding controlled quantities of reagents to droplets. The reagents are injected
into a multiphase fluid stream, comprising the carrier liquid, droplets of the reaction mixture
and an inert gas that maintains a uniform droplet spacing and suppresses new droplet
formation. The method, which is suited to many multistep reactions, is applied to a five-stage
quantum dot synthesis wherein particle growth is sustained by repeatedly adding fresh
feedstock.