The coarsening of emulsions and foams due to a diffusive flux of dissolved disperse phase between droplets and bubbles is considered, and the effects of trapping an extra species within droplets /bubbles are studied. It is demonstrated that the extra species may provide an osmotic pressure to counteract the effects of surface tension and "osmotically stabilise" an emulsion/foam. For dilute emulsions a rigorous condition to prevent coarsening by a diffusive flux of disperse phase is derived, which remains valid with polydispersity in droplet size and number of trapped species. The coarsening of dilute, insufficiently stabilised emulsions was found to proceed as when no trapped species were present, but with a reduction in the volume fraction of the growing droplets due to the volume fraction now residing in stable, shrunken droplets. Foams are studied by considering the osmotic compression of previously dilute foam bubbles by an osmotic pressure II. Careful arguments are given for the dependence of bubble pressure on II, which are confirmed for a monodisperse 2D model. The arguments are believed to be valid for sufficiently dry and monodisperse foams, for which the stability requirement is shown to be of the same order of magnitude as for dilute foam bubbles, regardless of the magnitude of II. In the absence of bubble rearrangements, the elastic energy from the necessary deformation of surrounding bubbles is also found to stabilise a foam. Sources of dissipation in coarsening nondi-lute foams are considered, and for given parameters enable prediction of the rate limiting mechanism and the associated coarsening rate. Since an osmotically stabilised emulsion may be destabilised by a rapid transport of the trapped species between droplets, the extent to which micelles may affect the rate of coarsening by transporting oil between droplets is considered. Different mechanisms of micelle-mediated exchange are considered, and the applicability of the different mechanisms indicated.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:663595 |
| Date | January 2000 |
| Creators | Webster, Anthony James |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13216 |
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