Experimental study of the break-up of starch suspension droplets in step-up shear flow

Abstract : The deformation and break-up of a droplet of suspension of swollen-in-water starch granules placed in an immiscible fluid, silicon oil, are investigated. The study was carried out on a physically modified waxy maize starch suspension with a theoretical granule volume fraction of 100%. Granules were swollen to their maximum; they were highly deformable. Measurements were carried out using a counter rotating shear cell. The starch suspension was shear thinning above a yield stress of about 90-100 Pa showing elasticity (first normal stress difference) above 300 Pa. The rheo-optical experiments were carried out by stepping up the stress from zero to a constant value and deformation of suspension droplets of various sizes was observed with time. Critical break-up capillary numbers Ca-* were calculated and correlated with droplet-to-matrix viscosity ratio p. At low p values Ca-* were found to be smaller or close to those determined for Newtonian fluids. No break-up was observed for viscosity ratios above 0.1, a limit that is roughly 40 times lower than that for the Newtonian fluids. This result was assumed to be due to the fact that internal stress in the droplet is lower than the mean one applied, because of inter-particle interactions and friction, thus shifting suspension from low to extremely high viscosity fluid if applied stress is close to the yield transition.