Memory in two-dimensional heap experiments

The measurement of force distributions in sandpiles provides a useful way to test concepts and models of the way forces propagate within noncohesive granular materials. Recent theory [J.-P. Bouchaud, M.E. Cates, and P. Claudin, J. Phys. I 5, 639 (1995); M. E. Cates, J. P. Wittmer, J.-P. Bouchaud, and P. Claudin, Phil. Trans. Roy. Soc. 356, 2535 (1998)] by Bouchaud et al. implies that the internal structure of a heap (and therefore the force pathway) is a strong function of the construction history. In general, it is difficult to obtain information that could test this idea from three-dimensional granular experiments except at boundaries. However, two-dimensional systems, such as those used here, can yield information on forces and particle arrangements in the interior of a sample. We obtain position and force information through the use of photoelastic particles. These experiments show that the history of the heap formation has a dramatic effect on the arrangement of particles (texture) and a weaker but clear effect on the forces within the sample. Specifically, heaps prepared by pouring from a point source show strong anisotropy in the contact angle distribution. Depending on additional details, they show a stress dip near the center. Heaps formed from a broad source show relatively little contact angle anisotropy and no indication of a stress dip.