Supplemental Information:

Movie 1 shows the onset phase of flow. Blue colors indicate a flow to the right, while a red tint shows flow with a main direction to the left (recirculation). The white bars represent the approximate direction of flow at a certain location. The flow rate rises during the first 200 ms from 0.1 to 3.5 ml/s and remains constant thereafter. A steady state is reached 300 ms after the onset of flow. The development of the primary eddy below the sudden expansion, and the secondary eddy at the top downstream of the reattachment point, can clearly be seen. Both the increase of flow and the growth of the primary eddy cause strong temporal gradients of shear stress at the bottom plate.