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