VISCOSITY OF EMULSIONS OF POLYDISPERSE DROPLETS WITH A THICK ADSORBEDLAYER

Three oil-in-water dispersions stabilized by the anionic surfactant Al kanol-XC and adsorbed gelatin have been isolated by centrifugation, re suspended in water, and characterized with respect to particle size an d gelatin binding. The viscosities of these resuspended dispersions we re measured as a function of applied shear rate and dilution with the aim of understanding the rheological role of the adsorbed gelatin shel l. The emulsions are polydisperse (weight:number mean size similar to 3) with the number-mean diameter similar to 50-90 nm. The low-shear be havior (from the Newtonian plateau to the critical shear stress) can b e described using a simple hard-sphere model using an estimated length -mean size for the oil droplets with an adsorbed gelatin layer of effe ctive thickness from 25 to 39 nm, increasing with mean oil particle si ze. At high shear the model breaks down because there is no second New tonian plateau. Instead, power-law thinning continues to the highest r ates (similar to 10(5) s(-1)) and stresses (similar to 500 Pa) measure d. 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 [B uscall, R. Colloids Surf. A 1994, 83, 33]. A self-consistent, physical ly reasonable picture of the rheology is then apparent for the differe nt dispersions at different concentrations.