Direct observation of flocculation/coalescence of metastable oil droplets in surfactant-free oil/water emulsion by freeze-fracture electron microscopy

Direct imaging of flocculation/coalescence in surfactant-free oil/water emu lsion was achieved by freeze-fracture electron microscopy (FFEM) on benzene oil droplets ultrasonically dispersed in water. Immediately after sonicati on, 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 h our 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 w ith each other. These results support that M and L (greater than or equal t o 1000 nm) class droplets are formed through flocculation of S class drople ts and coalescence of M class droplets, respectively.