STABILIZATION OF EMULSIONS BY TRAPPED SPECIES

We consider an emulsion whose droplets contain a trapped species (inso luble in the continuous phase) and study the emulsion's stability agai nst coarsening via Lifshitz-Slyozov dynamics (Ostwald ripening). Exten ding an earlier treatment by Kabalnov et al. (Colloids Surf:, 1987, 24 , 19-32), we derive a general condition on the mean initial droplet vo lume which ensures stability, even when arbitrary polydispersity is pr esent in both size and composition of the initial droplets. We disting uish ''nucleated'' coarsening, which requires either fluctuations abou t the mean Geld equations or a tail in the initial droplet size distri bution, from ''spinodal'' coarsening in which a typical droplet is loc ally unstable. A weaker condition for stability, previously suggested by Kabalnov et al., is sufficient only to prevent ''spinodal'' coarsen ing and is best viewed as a condition for metastability. The coarsenin g of unstable emulsions is considered and shown at long times to resem ble that of ordinary emulsions (with no trapped species), but with a r educed value of the initial volume fraction of dispersed phase. We dis cuss the physical principles relevant to the stability of emulsions wi th trapped species, describing how these may be exploited to restabili ze partially coarsened emulsions and to ''shrink'' previously formed e mulsion droplets to form ''miniemulsions''.