The flow cell was fixed atop an inverted microscope (LS620, Etaluma Inc., USA) with a 4x objective for the duration of the experiment. Transmission images were automatically captured each hour, thereby creating a time-lapse image stack. The captured images were analysed using open source software (ImageJ, NIH, USA). The first image (t=0) in each experiment was used as a background for calculating the x-y biofilm optical density (OD) maps, ODt(x,y), according to: