Driving force fluctuations in a driven 2D granular array

We have performed an experimental study of a two-dimensional array of cylinders driven at constant velocity along a level surface. During a typical run of 10^7 data points, the driving force undergoes irregular fluctuations characterized by stick-slip behavior. The magnitudes of these stick-slip events roughly follows a power law distribution whose exponent is correlated with driving speed. Furthermore, we find that the analysis of the data is very sensitive to the amount of data smoothing used. We also report on how varying the number of cylinders and their geometrical arrangement effect the size and distribution of events.