The dramatic impact of the suspending fluid chemistry on cornstarch suspension rheology
Suspensions of cornstarch in water can exhibit fracturing [Roche et al, 2013] or formation of stable holes in an oscillating layer [Merkt et al, 2004]. These unusual dynamical behaviour cannot be explained by classical rheology measurements. Numerical works [e.g. Mari et al, 2015] recently pointed out that particle-particle interactions might be at the origin of this atypical rheology. In order to investigate this hypothesis, we study cornstarch suspension in different fluids combining macroscopic dynamical and classical rheology with microscopic probing of particle-particle interactions.
We first perform experiments of sphere settling into suspensions of cornstarch grains in mixtures of water and ethanol, from pure water to pure ethanol. Our observations range from dynamic shear thickening for pure water to nearly Newtonian behaviour for pure ethanol. For intermediate mixtures we also observe yield stress fluid behaviour.
We then use atomic force microscopy (AFM) to probe the interactions between cornstarch grains in different water-ethanol mixtures. We show that the particle-particle interactions vary as the suspending fluid is changed. Finally, we have evidence that the surface of cornstarch grains could be covered by polymer brushes which might be at the origin of such peculiar rheology.