  The influence of controlled vibrations on the granular rheology is
investigated in a specifically designed experiment. We study experimentally a
thin granular film spreading under the action of horizontal vibrations. A
nonlinear diffusion equation is derived theoretically that describes the
evolution of the deposit shape. A self-similar parabolic shape (the``granular
droplet'') and a spreading dynamics are predicted that both agree
quantitatively with the experimental results. The theoretical analysis is used
to extract effective dynamic friction coefficients between the base and the
granular layer under sustained and controlled vibrations. We derive an
empirical friction law involving external driving parameters and the layer
height to represent our data.
