Under Pressure: Fracture and Relaxation in Bulk Cornstarch Suspensions

We probe the fracture and relaxation characteristics of dense cornstarch suspensions, a complex fluid that exhibits discontinuous shear-thickening behavior. We inject air at a constant pressure into suspensions of different mass fractions of cornstarch in water placed in an open three-dimensional container. Because the suspension is opaque, fast X-ray radiography is required to image the growth of the air cavity upon air injection. The X-ray images reveal shapes ranging from smooth bubbles to sharp cracks. By measuring the air thickness, we find that both bubbles and cracks grow at a constant volumetric flow rate that increases linearly with the applied pressure, a remarkably simple behavior for a complex fluid. Surprisingly, the flow rate is not affected by an increase in the cornstarch mass fraction from 58% to 60%, despite major changes in the rheology of the suspension. Conversely, the air cavity shape and relaxation dynamics strongly depend on the mass fraction: sharp cracks relax into bubbles with a relaxation time scale that depends on the mass fraction. This relaxation process under constant stress will yield new insights into the transition of the material from the discontinuous shear-thickening state to the fluid state.