  The colonisation of a soft passive material by motile cells such as bacteria
is common in biology. The resulting colonies of the invading cells are often
observed to exhibit intricate patterns whose morphology and dynamics can depend
on a number of factors, particularly the mechanical properties of the substrate
and the motility of the individual cells. We use simulations of a minimal 2D
model of self-propelled rods moving through with a passive compliant medium
consisting of particles that offer elastic resistance before being plastically
displaced from their equilibrium positions. It is observed that the
motility-induced clustering of active (self-propelled) particles is crucial for
understanding the morphodynamics of colonisation. Clustering enables motile
colonies to spread faster than they would have as isolated particles. The
colonisation rate depends non-monotonically on substrate stiffness with a
distinct maximum at a non-zero value of substrate stiffness. This is observed
to be due to a change in the morphology of clusters. Furrow networks created by
the active particles have a fractal-like structure whose dimension varies
systematically with substrate stiffness but is less sensitive to particle
activity. The power-law growth exponent of the furrowed area is smaller than
unity, suggesting that, to sustain such extensive furrow networks, colonies
must regulate their overall growth rate.
