Drag force on a spherical intruder in a granular bed at low Froude number.

The drag force on an object, or "intruder," in a granular material arises from interparticle friction, as well as the cyclic creation and buckling of force chains within the material. In contrast to fluids, for which drag forces are well understood, there is no straightforward relationship between speed and mean drag force in granular materials. We investigate spherical intruder particles of varying radii moving at low speeds through granular beds. The system can be parametrized using the dimensionless Froude number Fr=2v/√[gR], for intruders of radius R moving at a speed v. For frictional systems, we find the drag force obeys a linear relationship with Fr for low Froude numbers above Fr>1. For Fr<1 we observe a deviation from this linear trend. This transition can be explained by considering the characteristic inertial and gravitational granular time scales of the system. We show that a suitably normalized measure of dissipated power obeys a linear relationship with the imposed intruder velocity, independent of the intruder dimensions. This is found to hold even for particles with no friction, identifying a relationship between the imposed motion of the intruder and the resistance of the granular material to purely geometric rearrangements.