Next, we tested whether certain library members could assemble or phase separate. Short peptide sequences with multiple aromatic residues have been shown to phase separate into simple coacervate-based droplets, i.e., droplets with a high water content formed by a self-interacting building block. Even cationic sequences as short as dipeptides with aromatic residues could assemble into such droplets. Since cation-p interactions are one of the main driving forces for such phase-separations of peptides, we hypothesized that the ester products derived from Ac-YWRD-OH could also phase-separate upon fueling the peptide with the right amount of EDC. After fueling the peptide Ac-YRWD-OH with EDC (3 mM of EDC added to 10 mM of peptide or 10 mM of EDC added to 20 mM of peptide), the sample rapidly turned turbid, indicative of the formation of assemblies sufficiently large to scatter light (Fig. ). Noteworthy, the peptide Ac-YRFD-OH did not show any sign of self-assembly under the same fueling conditions likely due to weaker cation-p interactions between arginine and phenylalanine compared to tryptophan. The turbidity evolution was tracked by UV-Vis spectroscopy with a plate reader at a wavelength of 600 nm (Fig. ). The turbidity emerged and decayed after 10 hours when 10 mM peptide was fueled with 20 mM of EDC. Laser scanning confocal microscopy revealed the increased turbidity resulting from small amorphous assemblies (Fig. ). We tested the composition of the assemblies by spinning down the samples 90 minutes after fuel addition. We then analyzed the supernatant by HPLC and used these concentrations as a reference for all components that did not assemble. We also analyzed the composition of samples that had not been spun down, but instead, we dissolved all the assemblies by diluting the solution with 4M guanidinium chloride solution and rigorous mixing. We found that the assemblies were a co-assembly of peptide precursor, linear dimer, and cyclodimer product. To put that into the context of the dynamic combinatorial library members, 54% of the total linear dimer was in the assembly, whereas the rest remained in the solution. In contrast, 69% of the total cyclodimer concentration is taken up in the assemblies, indicating that the cyclodimer has a higher tendency to self-assemble (Fig. ). As we aimed for liquid assemblies through phase separation, we modified the YRWD-OH peptide sequence to create liquid assemblies. We added a 2-[2-(2-methoxyethoxy)ethoxy]acetic acid tail to the N-terminus of the peptide structure to increase its overall solubility without influencing the reactive core. This peptide also showed a fast increase in turbidity upon fueling with EDC (Fig. ). Confocal microscopy revealed micron-sized spherical assemblies that rapidly fused, from which we concluded they were liquid. Spinning down the droplets and analyzing the supernatant revealed that most cyclic dimer partitions into these droplets (75%). The liquid nature of the simple coacervate droplets was further investigated by FRAP analysis, which revealed a fast recovery of fluorescence intensity after bleaching (Fig. ). The corresponding diffusion coefficient was determined to D = 0.063 µm 2 .s -1 .