Two-dimensional granular flow in a vibrated small-angle funnel

We have studied the flow of a single layer of uniform balls in a small-angle funnel when it is vibrated parallel to the flow. Generally, we measured the flow rate as a function of a dimensionless acceleration Gamma. However, for sufficiently small outlet widths, the flow can jam so we also measured the elapsed times between balls and their correlations to study jam dynamics. In particular, we found that when the funnel angle beta was larger than approximately 4 degrees, a stable jam always formed for Gamma<1 and the flow stopped. For Gamma approximately 1-4, jams still occurred, but now they broke and reformed, although they could last approximately 100 s, resulting in long-time correlations in the flow. The elapsed time distributions in this case show distinct, possibly algebraic, tails. Beyond Gamma approximately 4, the flow no longer jammed and the flow rate became constant. The general behavior has been mapped out in a rough phase diagram.