  We investigate the dynamics of a partially saturated grain-liquid mixture
with a rotating drum apparatus. The drum is partially filled with the mixture
and then rotated about its horizontal axis. We focus on the continous
avalanching regime and measure the impact of volume fraction and viscosity of
the liquid on the dynamic surface angle. The inclination angle of the surface
is observed to increase sharply to a peak and then decrease as a function of
liquid volume fraction. The height of the peak is observed to increase with
rotation rate. For higher liquid volume fractions, the inclination angle of the
surface can decrease with viscosity before increasing. The viscosity where the
minima occurs decreases with the rotation rate of the drum. Limited
measurements of the flow depth were made, and these were observed to show only
fractional changes with volume fraction and rotation speeds. We show that the
qualitative features of our observations can be understood by analyzing the
effect of lubrication forces on the timescale over which particles come in
contact.
