Unified understanding of physics on granular drag force

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Due to its discreteness, the behavior of bulk granular matter is much different from usual fluids or solids. Therefore, the fundamental understanding of the unique behavior for granular matter has not been enough yet. Particularly, we focus on the granular drag force to understand the fundamentals of the granular physics, in this research project. Recently, some research groups have independently performed granular drag force experiments, and obtained various results. However, there has not been any unifying concept. We carried out two kinds of experiments in order to solve this desperate situation. The one is the impact between a granular layer and a liquid droplet. The other one is the simultaneous measurement of drag force and wall pressure by a solid penetration into a granular column. From the former experiment, we have found some novel crater shapes that have never observed in other impact experiments. In addition, we have revealed the scaling relation for the crater radius using a dimensionless parameter. From the latter one, on the other hand, we have found nonlinear scaling laws for slow granular drag force and induced wall pressures. By detailed analyses of the experimental results, we have concluded that the origin of nonlinearity of the scaling relates to the attenuation at grains boundaries. Through these experimental results, we have now obtained some fundamental understandings for the granular drag force.