Ultrasonic Fluidization of Highly Concentrated Suspensions

The flow of dense suspensions is hindered when the solid volume fraction is high, particularly in frictional systems where particle contacts dominate the mechanical response. We investigate the role of mechanical vibrations, ranging from megahertz to tens of hertz, on dense granular suspensions with solid volume fractions between 10% and 50 %, in a frictional regime below the jamming transition yet below random close packing. Vibrations are shown to significantly fluidify these suspensions, strongly modifying their rheological properties. Surprisingly—and contrary to common intuition—we find that the dramatic drop in viscosity is not due to a weakening of frictional contacts, but rather to the aggregation of particles under flow. Crucially, this structural reorganization concentrates the flow in low-density regions, where resistance is minimal, while dense aggregates are displaced away from the primary flow paths. In addition, pronounced wall slip further facilitates the flow. Together, these effects lead to an unexpectedly large reduction in macroscopic viscosity—one that friction-based models completely fail to predict particulate systems.