  Bacillus subtilis biofilms were grown in laminar channel flow at wall shear
stress spanning one order of magnitude ($\tau_w = 0.068$ Pa to $\tau_w = 0.67$
Pa). We monitor, non-invasively, the evolution of the three-dimensional
distribution of biofilm over seven days using optical coherence tomography
(OCT). The obtained biofilms consist of many microcolonies where the
characteristic colony has a base structure in the form of a leaning pillar and
a streamer in the form of a thin filament that originates near the tip of the
pillar. While the shape, size and distribution of these microcolonies depend on
the imposed shear stress, the same structural features appear consistently for
all shear stress values. The formation of streamers seems to occur after the
development of a base structure, suggesting that the latter induces a curved
secondary flow that triggers the formation of the streamers. Moreover, we
observe that the biofilm volume grows approximately linearly over seven days
for all the shear stress values, with a growth rate that is inversely
proportional to the wall shear stress. We develop a simple model of
friction-limited growth, which agrees with the experimental observations. The
model provides physical insight into growth mechanisms and can be used to
develop accurate continuum models of bacterial biofilm growth.
