Creep motion and fluidization in a granular pile

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We investigated experimentally granular piles exhibiting steady surface flow. Below the surface flow, even the particles in layers deep in the bulk exhibit very slow flow and that such motion can be detected at an arbitrary depth. The mean velocity of the creep motion decays exponentially with depth, and the characteristic decay length is approximately equal to the particle-size and independent of the flow rate. We analyzed particle diffusion in deep layer in rescaled time scale, and also observed the surface flow using highspeed camera to obtain the velocity profile.Evaporation rate of water from several soil types were measured under controlled conditions. When the layer of soil is sufficiently thin, the evaporation efficiency is well described by a function of the mean volume fraction of water. The function has a common form for several types of soil and can be parameterized by a single parameter. The value depends on the soil type and also on the wind speed. The latter dependence can be robustly described in terms of the evaporation resistance for the watered surface. These findings are expected to hold commonly for granular assemblies and to become a basis for calculating the evaporation efficiency for the soil with an arbitrary depth in a systematic way.