Next, Fig. depicts an experiment wherein we gradually changed the pH from 5.0 to 9.0, and vice versa. Similar to the previous experiments, we kept each pH value constant for 30 min to ensure that a steady state could be reached before we increased (or decreased) the pH by 0.5 units. Hence, the pH gradient in this experiment is ∆pH/∆t = 1 h -1 . Fig. shows that the transmittance remained at approximately 70% between 0-120 min (corresponding to a pH change from 5.0 to 7.0) before it decreased to 5% between 120-210 min. That is, an increase in the pH led to a significant decrease in the transmittance, with the minimum reached at pH 8.0. The high transmittance (~70%) was restored upon a further increase to pH 9.0. The direction of the gradient was changed at t=300 min (returning from pH 9.0 to 5.0), and the same pattern with a minimum in the transmittance around pH=8.0 as well as the recovery of the signal at pH=5.0 was observed. The appearance of both minima in the transmittance confirms that PECs are formed under a pH range wherein both polymers are highly charged. The observation of a robust output demonstrates that the experimental setup can provide a modular approach to examine the formation of PECs in flow.