Growth-induced phase changes in swimming bacteria at finite liquid interfaces

We introduce a system of bacteria confined to a finite 2D oil-water interface and driven on two distinct time scales by motility and by growth. The combined effect of activity on different time scales creates transitions between several common collective behaviors. These transitions are observed using time-lapse microscopy with high spatial and temporal resolution over eight hours. We sequentially observe an initial dilute state, a clustered state, an active turbulent state, and a glassy state, and we are able to directly observe and characterize the transitions between these states. This system allows the investigation of emergent effects surrounding transitions between phases, expanding on studies that have considered them in isolation. In particular, a peak in the velocity correlation length is observed at the turbulent-glassy transition, suggesting that this transition significantly increases the active turbulent length scale.