Aging of cornstarch particles suspended in aqueous solvents at room temperature.

Starch suspensions are often used as model systems to demonstrate extreme shear-thickening effects. We study the aging of cornstarch particles in aqueous suspensions at room temperature by granulometry and rheological measurements. When starch is diluted in glycerol, no long-term changes are observed. The situation differs when water is used as solvent. For volume fractions up to 20 vol %, when the cornstarch suspensions in water are stored under continual agitation, we observe an increase in viscosity. When the cornstarch suspension is aged under quiescent conditions, no evolution of the particle size is observed. In the concentrated situation, the rheological properties vary independent of the storage condition. We show that the increase in viscosity is related to air trapped in the pore space and to the swelling of the granules and leakage of the amylopectin component of the starch into the surrounding water. The relative importance of the two processes depends upon the particle concentration and upon the energy brought to the system.

Stress Field inside the Bath Determines Dip Coating with Yield-Stress Fluids in Cylindrical Geometry.

We study experimentally and theoretically the thickness of the coating obtained by pulling out a rod from a reservoir of yield-stress fluid. Opposite to Newtonian fluids, the coating thickness for a fluid of large enough yield stress is determined solely by the flow inside the reservoir and not by the flow inside the meniscus. The stress field inside the reservoir determines the thickness of the coating layer. The thickness is observed to increase nonlinearly with the sizes of the rod and of the reservoir. We develop a theoretical framework that describes this behavior and allows us to precisely predict the coating thickness.