  The sessile microbial communities known as biofilms exhibit varying
architectures as environmental factors are varied, which for immersed biofilms
includes the shear rate of the surrounding flow. Here we modify an established
agent-based biofilm model to include affine flow, and employ it to analyse the
growth of surface roughness of single-species, three-dimensional biofilms. We
find linear growth laws for surface geometry in both horizontal and vertical
directions, and measure the thickness of the active surface layer, which is
shown to anti-correlate with roughness. Flow is shown to monotonically reduce
surface roughness without affecting the thickness of the active layer. We argue
that the rapid roughening is due to non-local surface interactions mediated by
the nutrient field, which are curtailed when advection competes with diffusion.
We further argue the need for simplified models to elucidate the underlying
mechanisms coupling flow to growth.
