Direct Observation of Flocculation/Coalescence of Metastable Oil Droplets in Surfactant-free Oil/Water Emulsion by Freeze-Fracture Electron Microscopy

Abstract

Direct imaging of flocculation/coalescence in surfactant-free oil/water emulsion was achieved by freeze-fracture electron microscopy (FFEM) on benzene oil droplets ultrasonically dispersed in water. Immediately after sonication, spherical droplets with diameters at 30−100 nm (S class) and aggregates consisting of S class droplets at 200−500 nm (M class) were observed. An hour later, we were able to obtain FFEM images, in which a number of S class droplets in flocculation coalesced into larger droplets. Additionally, two droplets of M class coalesced easily when these droplets were in contact with each other. These results support that M and L (≥1000 nm) class droplets are formed through flocculation of S class droplets and coalescence of M class droplets, respectively.