  We experimentally study the frictional behavior of a two-dimensional slider
pulled slowly over a granular substrate comprised of photoelastic disks. The
slider is vibrated at frequencies ranging from 0 to 30 Hz in a direction
parallel to sliding. The applied vibrations have constant peak acceleration,
which results in constant average friction levels. Surprisingly, we find that
stick-slip behavior, where stress slowly builds up and is released in
intermittent slips, is enhanced as the frequency of vibration is increased. Our
results suggest that increasing the frequency of vibration may help to combine
many smaller rearrangements into fewer, but larger, avalanche-like slips, a
mechanism unique to granular systems. We also examine the manner in which the
self affine character of the force curves evolves with frequency, and we find
additional support for this interpretation.
