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We report on grain dynamics versus depth for steady-state gravity-driven flowof grains along a heap formed between two parallel sidewalls. Near the surfacethe flow is steady and fast, while far below there is no flow whatsoever;therefore, a jamming transition occurs where depth is an effective controlparameter for setting the distance from the transition. As seen previously, thetime-averaged velocity profile along the sidewall exhibits a nearly exponentialdecay vs depth. Using speckle-visiblility spectroscopy (SVS), we find thatvelocity fluctuations grow relative to the average on approach to jamming.Introducing an image-based order parameter and the variance in its temporaldecay at a given depth, we also characterize the spatially heterogeneous natureof the dynamics and find that it increases on approach to jamming. Inparticular, the important time and length scales for dynamical heterogeneitiesare found to grow almost exponentially as a function of depth. These dynamicalchanges occur without noticeable change in structure, and are compared tobehavior found by experiment and simulation elsewhere in order to test theuniversality of the jamming concept. In particular we find that the size of theheterogeneities scales as the inertia number raised to the -1/3 power, inagreement with two recent simulations.

Jamming and growth of dynamical heterogeneities versus depth for granular heap flow