These data indicate that phase diagrams can be used to design droplet-based systems. In this example of IgG embracing, we had to utilize the originally one-phase condition. By contrast, the two-phase condition in the phase diagram only formed the aggregation of IgG (Figure ). They indicated that a previous grasp of a phase diagram and focused on one phase region upper the phase diagram is profitable to generate droplets with additional proteins. This is a good example to tell us the significance of depicting the phase diagram carefully before applying an LLPS system to advanced applications. LLPS droplets are related not only to biophysics but also to synthetic biology and chemical biology for material production, analysis of droplet-enzyme formation, and medicinal applications. Gaining insight into the formation/disappearance and behavior of droplets is essential for the rational design of systems related to LLPS droplets. Other few examples of experimental phase diagram observation for the study of LLPS also have shown informative insights. In this study, we developed a phase-diagram-based approach to understand and predict LLPS systems for biophysical and biochemical applications. Using a classical PLL/ATP system, we constructed a LLPS phase diagram and investigated the concentration dependency of the salt resistance of PLL/ATP droplets for the first time. Then, we applied the phase diagram to the development of a model to predict the droplet formation/disappearance dynamics, which has potential application in the fields of biology and physics. We also revealed that the antibody incorporation into droplets can be predicted according to the phase diagram. For instance, the conditions of the upper area of the two-phase region in the phase diagram of the PLL/ATP system were suitable for IgG incorporation.