Understanding coalescence of phase separated droplets and verification via growing a single suspended crystal

Liquid-liquid phase separation (LLPS) is a ubiquitous process found in a variety of fields. It is of particular importance in biological sciences since it plays essential and vital roles in a number of physiological and pathological processes in biological systems. After LLPS, the dense droplets can grow in size via incorporating solutes from surrounding environment and in some cases coalescing with other droplets. Interestingly, the size of the dense droplets seems to have an upper limit but the mechanism remains to be explored. Since the droplet size can be essential for biological functioning, it is important to understand the size evolution of the phase separated droplets. Here we propose a physical mechanism with consideration of impurities on the surface of the dense liquid droplets. The theoretical predictions can be used to explain the observations on the size evolution. Based on the coalescence mechanism, we succeeded for the first time performing a challenging task, i.e., growing a single suspended protein crystal via merging phase-separated droplets in a fully non-contact manner. The mechanism observed may be considered as a basic model for researches in much broader fields involving phase separation, such as in biology, materials science, physics, chemistry, and meteorology.