Characterization of a depletion-flocculated polydisperse emulsion II. Steady-state rheological investigations

Depletion flocculation of an alkane-in-water emulsion has previously been found to induce phase separation and, above certain polymer and oil concentrations, a delay period prior to the onset of creaming. In this paper the steady-state viscometry of an emulsion of 1-bromohexadecane-in-water has been investigated in the presence of hydroxyethylcellulose as the depletion flocculant. We have sought to characterize the flow behavior of this emulsion system and determine the characteristic rheology during the delay period. The use of this near-density-matched system eliminates the effect of creaming, and we have, therefore, characterized the delay period by reference to the behavior of a related system, differing only in oil composition. At oil and polymer concentrations conducive to the presence of a delay period, the emulsions were found to be shear thinning, whereas the emulsion system in the absence of polymer and the isolated continuous phases were Newtonian. The structure formation influencing the rheology of the emulsions at high polymer concentrations was attributed entirely to interdroplet depletion interactions. The concept of dynamic yield stress for such a system is discussed and the consequences for the structure of a buoyancy-resisting space-spanning phase or "transient gel." (C) 1998 Academic Press.