A coarse-grained model developed by Marrink et al. [J. Phys. Chem. B 111, 7812, 2007] is applied to investigate vesiculation of lipid dipalmitoylphosphatidylcholine DPPC droplets in water. Three kinds of morphologies of micelles are found with increasing lipid droplet size. When the initial lipid droplet is smaller, the equilibrium structure of the droplet is a spherical micelle. When the initial lipid droplet is larger, the lipid ball starts to transform into a disk micelle or vesicle. The mechanism of vesicle formation from a lipid ball is analyzed from the self-assembly of DPPC on the molecular level, and the morphological transition from disk to vesicle with increasing droplet size is demonstrated. Importantly, we discover that the transition point is not very sharp, and for a fixed-size lipid ball, the disk and vesicle appear with certain probabilities. The splitting phenomenon, i.e., the formation of a disk/vesicle structure from a lipid droplet, is explained by
applying a hybrid model of the Helfrich membrane theory. The elastic module of the DPPC bilayer and the smallest size of a lipid droplet for certain formation of a vesicle are successfully predicted.