Viscosity of emulsions of polydisperse droplets with a thick adsorbed layer

Three oil-in-water dispersions stabilized by the anionic surfactant Alkanol-XC and adsorbed gelatin have been isolated by centrifugation, resuspended in water, and characterized with respect to particle size and gelatin binding. The viscosities of these resuspended dispersions were measured as a function of applied shear rate and dilution with the aim of understanding the rheological role of the adsorbed gelatin shell. The emulsions are polydisperse (weight:number mean size similar to 3) with the number-mean diameter similar to 50-90 nm. The low-shear behavior (from the Newtonian plateau to the critical shear stress) can be described using a simple hard-sphere model using an estimated length-mean size for the oil droplets with an adsorbed gelatin layer of effective thickness from 25 to 39 nm, increasing with mean oil particle size. At high shear the model breaks down because there is no second Newtonian plateau. Instead, power-law thinning continues to the highest rates (similar to 10(5) s(-1)) and stresses (similar to 500 Pa) measured. We attribute the excess thinning to the deformation of the gelatin shell. We extract an interparticle pair potential from the flow curves, using an effective hard-sphere model based on that due to Buscall [Buscall, R. Colloids Surf. A 1994, 83, 33]. A self-consistent, physically reasonable picture of the rheology is then apparent for the different dispersions at different concentrations.