Complexation between oppositely charged nanoparticles (NPs) and polyelectrolytes (PEs) on aqueous two-phase systems (ATPSs) provides a scalable route to assemble functional, stimuli-responsive membranes. The aim of this thesis is to identify the membrane self-assembly mechanism of such systems. We study two PE/NP systems in a dextran/PEG ATPS and find very different growth behaviors. In one case, PE/NP membranes can grow continuously to thicknesses approaching several millimeters and display high tunability via modification of ionic strength. For the other PE/NP pair, only thin layers form, suggesting a distinct mechanism for membrane growth. To probe the assembly mechanism, we devise a microfluidic platform that allows direct imaging of the membrane growth over prolonged times, avoiding depletion effects and limited finite growth. The first system studied has silica (SiO2) NPs suspended in the PEG phase that undergo interfacial complexation with poly(diallyldimethylammonium chloride) (PDADMAC) supplied in the dextran phase. To probe the self-assembly mechanism, we exploit the microfluidic platform that facilitates sequential insertion of fluorescent and non-fluorescent PDADMAC. We observe a transition in the membrane growth mechanism with ionic strength. In the absence of salt [NaCl=0mM] the PDADMAC chains permeate through the membrane to complex with NPs, leading to well-stratified structures. At elevated salt [NaCl =500mM], this permeation mechanism is lost. Rather, the complexing species incorporate uniformly across the membrane. We attribute this transition to a rapid exchange of PE-counterion, NP-counterion and PE/NP binding sites facilitated by an increase in extrinsically compensated charged groups. In our second system, poly (L-lysine) (PLL) complexes with SiO2 NPs at the dextran/PEG interfaces. The resulting membrane growth rates are significantly slower than the PDADMAC/SiO2 NP system. To understand the assembly mechanism, membranes were formed via sequential insertion of PLL and fluorescently labeled PLL. No stratification in membrane growth could be discerned. We infer that the primary amine chemistry of PLL induces a high degree of binding upon complexation, inhibiting growth. While stratified membranes were not formed from alternating PLL and fluorescent-PLL streams, hybrid membranes containing PDADMAC and PLL do exhibit strong stratification. We attributed this stratification to PDADMAC’s capacity to quickly diffuse and generate a thick, porous membranes and to PLL’s strong complexation. PE/NP membranes have tremendous potential for the formation of functional membranes, offering ample control over the internal structure of the membrane. Future work centers around gauging the interdiffusion capacities of the PE species, by employing methods like fluorescent recovery after photobleaching the motility of the species can be empirically measured.