Laser speckle correlation applied to flow field measurements in granular media

A laser speckle correlation method is developed to measure slow macroscopic flow fields in a randomly packed granular medium. The method is useful for planar geometry configurations like the one considered here, namely a sample that is subjected to an indentation type test. The main feature revealed by the method in this system, is the formation of shear bands of well-defined shapes, proper of a Coulomb-like material. We characterize these structures by showing that they concentrate vorticity and shear deformation. In addition, the method provides high-precision measurements of tiny displacements occurring far below shear bands.