Fracture and relaxation in dense cornstarch suspensions

We probe the fracture and relaxation characteristics of a dense cornstarch suspension, 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 that rise upwards under the action of buoyancy to sharp cracks that remain attached to the injection nozzle. Cracks form in suspensions with cornstarch mass fractions close to the jamming transition, while bubbles form in suspensions with lower mass fractions that can still discontinuously shear thicken. We further show that the shape and the relaxation dynamics of the air cavity are linked to the cornstarch rheology: sharp cracks relax into bubbles when the shear rate applied to the suspension by the bubble growth decreases below the critical shear rate for discontinuous shear thickening.