Biofilm growth has a strong effect on the available channel free space and in turn can affect the local hydrodynamic conditions applied to the biofilm (specifically shear stress). We define the critical shear stress (τc) as the maximum shear stress experienced by the biofilm as it reached its critical volume, just before sloughing. To quantify τc, we used a computational fluid dynamic model to simulate the shear stress based on a threedimensional representation of the biofilm shape in advance of each sloughing event. Estimates of the biofilm threedimensional shape were obtained as described in the Material and Methods section. In this way, we quantified the restrictions in channel free space as well as the corresponding changes in shear stress. As explained in the Electronic Supplementary Information (Section S3), the quantity of biofilm distributed on the top and bottom walls did not affect the calculated shear stress if the total biofilm volume was conserved along the channel cross-section. Therefore, for simplicity of visualisation and reduction of computational time, we represented the totality of the biofilm as adhered to the bottom wall. Visual representations of channels containing the PPF-1 or PAO1 strain at maximum overgrowth just before a sloughing event and the same channels immediately after sloughing are shown in Figure . In this case, the τc values generated due to flow of 0.3 mL•h -1 across the headspace, just before sloughing, were τc = 2.6 and 6.1 Pa for PPF-1 and PAO1, respectively. The shear stresses applied to the residual surface adhered biofilms layers after the sloughing events shown in Figure were 1.2 and 1.9 Pa for PPF-1 and PAO1, respectively. We conclude that biofilms from the PPF-1 strain are more likely to slough than those from the PAO1 strain. This is the result of three characteristics of the PPF-1 biofilm: (i) lower attachment strength to the microchannel, (ii) rapid and nearly continuous regeneration of new growth biofilms, and (iii) lower τc required for sloughing. Each factor is linked to the biofilm mechanical properties and is evidenced by the higher number of sloughing events observed for PPF-1 compared to PAO1.