Clustering, jamming, and topological defects in growing bacterial colonies at liquid interfaces

Active nematic matter encompasses a broad variety of systems including confined vibrating rods, flocks of birds, and colonies of bacteria. Here, we present experimental results on growing colonies of rod-shaped bacteria confined in two dimensions by adsorption at an oil-water interface. Using microfluidics and time-lapse microscopy, we investigate the roles of motility and growth on cluster formation, topological defects, and jamming. As colonies grow on finite liquid interfaces, we observe cluster formation where regions of tightly packed bacteria that display notably low swimming velocities increase in number and size over time. As the surface coverage increases, a densely packed monolayer of bacteria with long-range orientation order is observed and topological defects are quantified. This setup functions as a useful model system for low-friction confinement of a growing active nematic comprised of discrete particles as well as carrying important implications for the study of biofilm formation at liquid interfaces.