  Recent theories predict that discontinuous shear-thickening (DST) involves an
instability, the nature of which remains elusive. Here, we explore unsteady
dynamics in a dense cornstarch suspension by coupling long rheological
measurements under constant shear stresses to ultrasound imaging. We
demonstrate that unsteadiness in DST results from localized bands that travel
along the vorticity direction with a specific signature on the global shear
rate response. These propagating events coexist with quiescent phases for
stresses slightly above DST onset, resulting in intermittent, turbulent-like
dynamics. Deeper into DST, events proliferate, leading to simpler, Gaussian
dynamics. We interpret our results in terms of unstable vorticity bands as
inferred from recent model and numerical simulations.
