Phase transitions in growing bacteria colonies at liquid interfaces

Colonies of motile and growing bacteria can be considered active materials that display distinct collective behaviors based on parameters such as bacteria morphology, motility, and environmental confinement. Previously, efforts have focused on categorizing individual phases of distinct behavior, such as clustering or active turbulence. We present a growing colony of motile E. coli at an oil-water interface in which system evolution can be directly and continuously observed using time-lapse microscopy over eight hours. We use image analysis and particle image velocimetry to characterize multiple observed behaviors, which we label clustering, active turbulence, and glassy dynamics. The changes between behaviors are driven by increases in packing fraction due to bacteria growth. We believe that this is a useful model system for the investigation of continuous phase transitions in active materials, particularly the transitions into and away from the widely studied active turbulent phase.