Strains of PPF-1 from Pseudomonas aeruginosa isolated from dental unit water lines were studied in microchannels using time-lapse imaging, and the results were compared to those from a reference strain of PAO1. Using height-calibrated OD maps, three-dimensional biofilm topographies were created, and the applied shear stresses were modelled using computational fluid dynamics simulations. While marked differences existed between biofilms from PPF-1 and the reference PAO1, PPF-1 exposure to Mg 2+ ions changed its mechanical properties and became similar to those of the reference strain. These properties included an increased ability to resist sloughing until the applied shear stresses surpassed a critical value. The resulting catastrophic sloughing showed evidence of viscoplastic failure, an understudied mechanical regime in the biofilm literature. The extreme differences in the Mg 2+ -exposed PPF-1 biofilm shape after sloughing points to highly distinctive sloughing and post-sloughing growth states. Conversely, in a low-ionic-strength environment, shear stresses applied before and after PPF-1 biofilm sloughing were not significantly different. For the latter case, this indicates that the sloughing and post-sloughing growth states were highly intertwined, which can result in a nearly continuous process of bacterial ejection and downstream colonization. Therefore, Mg 2+ addition could play a role in future strategies for limiting Pseudomonas aeruginosa contamination of dental unit water lines and other engineered water systems.