Slow drag in polydisperse granular mixtures under high pressure.

The behavior of polydisperse granular materials, composed of mixtures of particles of different sizes, is studied under conditions of high pressure and confinement. Two types of experiments are performed. In the first type, granular mixtures are compressed, with the resulting force-displacement curve used to calculate density and volume modulus. In the second set of experiments, the drag force is measured by pulling a cylinder, horizontally, through a compressed granular mixture. The density, volume modulus, and drag forces for the mixtures are quantified in terms of the mixture composition. The results show that the behavior of these mixtures depends strongly on the mass fractions of the different sized particles, with density, volume modulus, and drag force all reaching values significantly higher than observed in the monodisperse granular materials. Furthermore, the trends for density and drag force show strong correlation, suggesting that drag resistance of confined granular media could be directly related to packing effects. These results should prove useful in understanding the physics of drag in granular materials under high pressure, such as ballistic penetration of soils or ceramic armors.