Abstract
Bilayer formation on solid substrates by unilamellar vesicle fusion in water has attracted considerable attention in biological, medical, and industrial fields. However, the actual dynamic process to spontaneously form highly ordered bilayer films occurs on the nanoscale and within a very short period of time such that it cannot be clearly observed even with the most advanced technology. Therefore, the essential mechanism remains hypothetical, and various models have been proposed. In this study, using high-speed atomic force microscopy, we succeeded for the first time in observing bilayer formation from N,N-dioctadecyl-N,N-dimethylammonium bromide (DODAB) unilamellar vesicles on a mica surface in situ with nanoscale 3D information. Furthermore, when another cationic surfactant, which was more cohesive in water than DODAB, was used, significant differences were observed in the initial process of domain formation on the surface. This study revealed that the spherical vesicles instantly transitioned from tetralayer to bilayer domains on the substrate, and then a uniform bilayer membrane was formed by fusion between the domains on the substrate.