Dilatancy in the flow and fracture of stretched colloidal suspensions

Concentrated particulate suspensions, commonplace in the pharmaceutical, cosmetic and food industries, display intriguing rheology. In particular, the dramatic increase in viscosity with strain rate (shear thickening and jamming) which is often observed at high volume fractions, is of strong practical and fundamental importance. Yet manufacture of these products and their subsequent dispensing often involves flow geometries substantially different from that of simple shear flow experiments. Here we show that the elongation and breakage of a filament of a colloidal fluid under tensile loading is closely related to the jamming transition seen in its shear rheology. However, the modified flow geometry reveals important additional effects. Using a model system with nearly hard-core interactions, we provide evidence of surprisingly strong viscoelasticity in such a colloidal fluid under tension. With high speed photography we also directly observe dilatancy and granulation effects, which lead to fracture above a critical elongation rate.