Domain separation in bacterial monolayers on curved surfaces

Bacterial monolayers are ubiquitous in nature as the first step in a bacterial colony’s growth on a 2D substrate. These are most commonly studied on flat surfaces, but curved substrates are important in cases such as the growth of oil-degrading bacteria on the surface of oil droplets. A monolayer of rod-shaped bacteria behaves as an extensile active nematic. However, due to the particulate nature of the colonies at small length scales, they also display characteristics of granular materials such as pseudo-crystalline microdomains and grain boundaries. We study the interplay of nematic and granular behavior through hard-rod simulations of growing bacterial colonies confined to the surface of spherical droplets, varying the aspect ratio of the bacteria and the curvature of the surface. The size of the emergent microdomains sets a length scale for the system’s activity, matching the spatial correlation lengths of both pressure and velocity. This connection of length scales implies that the monolayer’s combined nematic and granular nature has direct consequences for its emergent behavior at larger scales and especially for the forces it exerts on its environment.