Granular drag induced by oblique impact

Considering granular medium as a complex fluid with a finite yield stress, an object moving inside has to locally unjam and mobilize the surrounding particles in order to step forward. Consequently, granular drag depends strongly on the local rheological behavior. Using a recently developed bi-static radar system capable of tracking a metallic object with a diameter down to a few millimeters, we monitor the trajectory of a projectile penetrating obliquely into a granular medium and characterize the velocity dependent granular drag in both vertical (along gravity) and horizontal directions. Recent advances in this particle tracking technique and the possibility of using it as a local rheometer for granular media will be discussed.