  We observe the formation of heaps of repulsive microspheres, created by
flowing a colloidal microsphere suspension towards a flat-topped ridge placed
within a quasi two-dimensional microfluidic channel. This configuration allows
for both shear and normal forces on the microspheres in contact with the ridge.
The heaps, which form against the ridge, are characterized by two distinct
phases: a solid-like bulk phase in the interior, and a highly-fluctuating,
liquid-like state which exists along its leading edge. We observe that heaps
only form above a critical flow velocity, (v_c), and that they are destroyed by
thermal rearrangements when the flow ceases. We monitor the dynamics of heap
formation using fluorescence video microscopy, measuring the heap volume and
the angle of repose in response to microsphere deposition and erosion
processes. We find that the steady-state angle of repose, (\theta_f), increases
as a function of inflow velocity, (v_\infty), with a functional form (\theta_f
\propto \sqrt{v_\infty - v_c}).
