We first developed the setup that allowed us to examine PEC formation under flow conditions. Our experimental setup consists of low-pressure pumps that continuously feed a microfluidic channel with PAA and PAH (Fig. ). The polymers were dissolved in water (with feeding concentrations [PAA]feed and [PAH]feed determined based on their monomer weight, 21 mM) and fed into the microfluidic channel using the syringes mounted on the pumps (see Methods). A 1:1 equivalent of PAA and PAH was used to create a charge balance between the polyelectrolytes. Additional syringes with hydrochloric acid ([HCl]feed, 10 mM) and potassium hydroxide ([KOH]feed, 10 mM) were used to control the pH in the microchannel. The outflow of the channel is connected to an absorbance detector, which we used to measure the transmittance of the solution at 500 nm. 27 Two distinct states were detected: The transmittance is i) low when the pH=7.0, and ii) high when the pH=9.0 (Fig. ). A low transmittance (thus, a turbid solution) is indicative of the appearance of PECs, which forms typically at a pH value within the range of an optimal charge density (6.0 < pH < 8.0, see SI 3.2). A choice for a pH value outside this range (in the case of pH=9.0) resulted in a high transmittance, indicating that a low amount of the PAH-PAA complex had formed. Fig. depicts an experiment wherein we monitored the trajectory of the assembly and disassembly of PECs as a function of the pH. The input pH is kept constant at 7.5 for 30 min to allow the signal to stabilize, after which the pH was changed abruptly to 9.0 and kept, again, constant for 30 min. We repeated these steps three times to determine the steady states at the two pH values, with the average of the observed transmittance depicted in Fig. . The screening of other initial conditions (i.e., feeding concentrations of PAA and PAH, salt concentration, and the flow rate) is appended to Supplementary Information, S4.