Granulates are a large collection of macroscopic solid grains. Cohesive and non-cohesive granulates are vital in a large variety of industries, ranging from pharmaceutical to mining, from construction to agricultural. They also play an important role in many geological processes, such as land and mudslides, debris flows, erosion, particles’ resuspension by wind in humid regions, and dune formation that shape planets’ morphology including, but not limited to, Earth. Cohesive forces become relevant also when the size of the particles reduces to the nanoscale (e.g nanoparticles), due to Van Der Waals interactions.
The existence of a cohesive force between grains leads to fundamentally different dynamics in cohesive granulates compared to their dry (i.e., noncohesive) counterpart. Such differences include stability of granular piles and the location in a granular bed where incipient motion, either due to gravity or shearing, is first observed. While dry granulates break at top (e.g. sand resuspension by wind), cohesive granulates break at the bottom (see video). Our focus is to investigate the impact of destabilizing shear forces, exerted by a creeping flow, on the granulate instability. We employ both multiscale methods and experiments to study the instability onset of three-dimensional cohesive granulates.