The mobility of sedimenting fibre suspensions is characterized here in three different, yet complementary studies. In the first study we present a simple mathematical analysis to define more
precisely the term sediment concentration. Through this analysis we correct the sediment concentration for compressibility effects and redefine this parameter as the gel concentration point. In the second study, we visualize the transient settling of radioactively labeled papermaking fibres using a new experimental technique, positron emission tomography (PET). In the third study, we measure the mass distribution of fibres (formation) in the sediment as a function of the initial suspension concentration. The results indicate that the gel concentration point occurs at a crowding number of approximately 16(±4). Two distinct regimes of settling were clearly identified with PET, depending upon the initial crowding number of the suspension (N). With N < 16, hindered settling was observed. With N > 16, fibres began to flocculate, starting with the long fibre fraction. Formation was found to be slightly dependent on N in the region N < 16 and then worsen significantly with N > 16. In summary, these findings indicate that within the suspension conditions found in papermaking 1 < N < 60, that there are two sub-regimes within these limits of differing levels of fibre mobility. These sub-regimes are delineated at N = 16.