Pull-out Experiment in Granular Material

Two-dimensional impact experiments by Clark et al. identified the source of inertial drag to be caused by ‘collisions’ with a latent force network, leading to large fluctuations of the force experienced by the impactor. These collisions provided the major drag on an impacting intruder until the intruder was nearly at rest. As a complement, we consider controlled pull-out experiments where a buried intruder is pulled out of a material, starting from rest. This provides a means to better understand the non-inertial part of the drag force, and to explore the mechanisms associated with the force fluctuations. The pull out process is a time reversed version of the impact process. In order to visualize this pulling process, we use 2D photoelastic disks from which circular intruders of different radii are pulled out. We check the effect of the initial depth of the intruder, as well as the widths and friction of boundaries. We present results about the dynamics of the intruder and the structures of the force chains inside the granular system as captured by high speed imaging. Before conducting the pull-out dynamic experiments, we first measured the critical pulling force that is needed to pull the intruder out. Under gradually increasing upward pulling force, a steadily strengthening force network forms in response to small displacements of intruder, then eventually fails and the intruder exits the material in a rapid event. We find that just before failure, the force chains bend in a way that is consistent with recent predictions by Blumenfeld and Ma. We found the boundary width together with friction plays an important role in this static pre-failure experiment. However, the system boundary does not have much effect on the dynamics of the intruder once the pull-out process starts.