(418d) The Impact of Shear Thickening Fluid Volume Fraction On the Rheological Response of a Suspension Emulsion

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 that makes them well suited for impact resistant material applications. Suspoemulsions containing volume fraction of STF as low as 10-20% show significant shear thickening, but the mechanism for this behavior is not currently well understood. The magnitude of the shear thickening response is sensitive to the composition of the shear thickening fluid and is therefore tunable.  The bulk suspoemulsion experiences an onset of shear thickening at approximately the same shear stress as the neat STF.  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. Rheo-microscopy is coupled with standard rheology experiments to show that there are fundamental changes in droplet 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 verified experimentally. The composition of the STF and the bulk suspoemulsion can be used in conjunction with the processing conditions to completely tune the response of the final material across a broad range of shear stresses.