(181z) Rheo-Physics of Shear Thickening Fluid Suspoemulsions
Shear thickening fluid (STF) suspension emulsions are immiscible polymer blends where a shear thickening fluid (Φ=0.42-0.55 silica in 200 g/mol polyethylene glycol) is dispersed in a Newtonian homopolymer, in this case PDMS. These so called suspoemulsions exhibit unique rheological behavior due to complex multiphase flow. Suspoemulsions containing volume fraction of STF as low as 10-20% show significant bulk shear thickening, but the mechanism for this behavior is not currently well understood. The dispersed STF droplets display different flow behavior, size and deformability depending on the stress being applied to them via shear flow. As volume fraction of shear thickening fluid increases, shear induced phase inversion becomes an important consideration in approaching a mechanism for bulk suspoemulsion shear thickening. A novel Rheo-microscope was developed to enable simultaneous observation of droplet microstructure and rheological response. Rheo-microscopy is used to show that there are fundamental changes in droplet size, deformability and distribution at the onset of shear thickening. The Utracki model gives a reasonable approximation for the point of phase co-continuity for suspoemulsions under shear flow and is evaluated experimentally. The composition of the STF and the bulk suspoemulsion can be manipulated in conjunction with the flow conditions to completely tune the response of the final material across a broad range of shear stresses.