Therefore, at a FRR of 0.5 in Class II fibers, the consumption of Fe(II) in the large RD zone can likely bottleneck the [Fe(Htrz)2(trz)]BF4 formation and growth. At higher FRRs of 4 and 9, the insufficient Fe(II) injected in the main channel (1:20 and 1:45 Fe:Htrz) becomes the limiting factor, as [Fe(Htrz)2(trz)]BF4 cannot grow larger. Furthermore, the small diffusion coefficient of Fe(II) results in a similar width of the RD zone at these FRRs (Figures ), consistent with the similarity in particle sizes. Consequently, in Class II fibers, the limited Fe(II) injection combined with the RD zone dimensions lead to particles of similar size across the different FRRs. These effects are not observed in Class I fibers due to the excess Fe(II) provided to the microfluidic device at higher FRRs of 4 and 9 (1:1.25 and 1:0.5 Fe:Htrz), allowing consistent formation of [Fe(Htrz)2(trz)]BF4. Here, however, the reduction of the RD zone becomes the limiting growth factor as explained above. At a low FRR of 0.5, although the amount of Fe(II) is very low (1:10 Fe:Htrz), the limited diffusion of Fe(II) plays in its favor by constraining the RD zone and leaving a significant amount of unreacted Htrz towards the center of the microfluidic channel (Figures and), effectively balancing the reagent stoichiometry within the RD zone and enabling particle growth.