The cryo-SEM images were taken using Apreo 2 S LoVac (Thermo Fisher Scientific). The cryo-SEM sample of the coacervates formed during evaporation was prepared by depositing a droplet (2 µL) of pep4 (10 mM, with 0.3 M NaCl) in acetate buffer (pH 4.8, 25 mM). The droplet was allowed to evaporate for 9 min before submerging in liquid nitrogen. The sample was then sublimated (85 K, 25 min) before imaging. The sample of the coacervates in solution was prepared by placing two cylinders with a solution of pep4 (7.5 mM, with 3 M NaCl) in phosphate buffer (pH 7.4, 25 mM). The solution was then submerged in liquid nitrogen and then the two cylinders were separated, fracturing the surface of the frozen droplet. The sample was sublimated before imaging (85 K, 10 min).