The spreading of bacterial colonies at solid-air interfaces is determined by
the physico-chemical properties of the involved interfaces. The production of
surfactant molecules by bacteria is a widespread strategy that allows the
colony to efficiently expand over the substrate. On the one hand, surfactant
molecules lower the surface tension of the colony, effectively increasing the
wettability of the substrate, which facilitates spreading. On the other hand,
gradients in the surface concentration of surfactant molecules result in
Marangoni flows that drive spreading. These flows may cause an instability of
the circular colony shape and the subsequent formation of fingers. In this
work, we study the effect of bacterial surfactant production and substrate
wettability on colony growth and shape within the framework of a hydrodynamic
thin film model. We show that variations in the wettability and surfactant
production are sufficient to reproduce four different types of colony growth,
which have been described in the literature, namely, arrested and continuous
spreading of circular colonies, slightly modulated front lines and the
formation of pronounced fingers
