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Query: UMLS:C0851184 (thinning)
 11,252 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

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." Copyright 1998 Academic Press.
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PMID:II. Steady-State Rheological Investigations. 979 72

We have investigated the combined effect of ionic calcium and ethanol on the visual creaming behavior and rheology of sodium caseinate-stabilized emulsions (4 wt% protein, 30 vol% oil, pH 6.8, mean droplet diameter 0.4 microm). A range of ionic calcium concentrations, expressed as a calcium/caseinate molar ratio R, was adjusted prior to homogenization and varying concentrations of ethanol were added shortly after homogenization. A stability map was produced on the basis of visual creaming behavior over a minimum period of 8 h for different calcium/caseinate/ethanol emulsion compositions. A single narrow stable (noncreaming) region was identified, indicating limited cooperation between calcium ions and ethanol. The shear-thinning behavior of the caseinate-stabilized emulsions is typical of systems undergoing depletion flocculation. Addition of calcium ions and/or ethanol was found to lead to a pronounced reduction in viscosity and the onset of Newtonian flow. The state of aggregation was correlated with emulsion microstructure from confocal laser scanning microscopy. Time-dependent rheology (18 h) with a density-matched oil phase (1-bromohexadecane) revealed that the visually stable emulsions were time-independent low-viscosity fluids. Surface coverage data showed that increasing amounts of caseinate were associated with the oil-water interface with increasing R and ethanol content. A decrease in free calcium ions in the aqueous phase with moderate increases in R and ethanol content was observed, which is consistent with greater calcium-caseinate binding (aggregation). Ostwald ripening occurred at the high-ethanol emulsion compositions that were stable to depletion flocculation. While the coarsening rate was low, this can account for the cream plug formation observed during gravity creaming experiments. The caseinate emulsion with no ionic calcium or ethanol exhibits depletion flocculation from excess nonadsorbed caseinate submicelles. Addition of calcium ions reduces the submicelle number density via specific calcium-binding in the aqueous phase (fewer, larger calcium-caseinate aggregates) and at the droplet surface (increased surface coverage). Nonspecific ethanol-induced (calcium-dependent) caseinate submicelle aggregation in the bulk phase and on the droplet surface (increased surface coverage) culminates in a reduction in the number density of caseinate submicelles. A narrow window of inhibition of depletion flocculation occurs in systems containing both calcium ions and ethanol, both species combining to aggregate the protein and so reduce the density of free submicelles.
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PMID:Stability and rheology of emulsions containing sodium caseinate: combined effects of ionic calcium and alcohol. 1514 44