The aim of this paper is to analyse the role of hydrodynamics in flocculation. The effects of a hydrodynamic sequencing (flocculation–break-up–reflocculation–break-up–reflocculation) on the evolution of aluminium–bentonite floc size distributions and structure are investigated by image analysis in a Taylor–Couette reactor. The flocculation phenomena analysed in this study mainly occur in the viscous subrange, with floc size below the Kolmogorov micro-scale. The high sensitivity of steady-state floc size distribution to initial floc size distribution (elementary particles or flocs formed issuing break-up stages) is highlighted. Reversibility or irreversibility of agglomeration and break-up phenomena are discussed in terms of floc history and hydrodynamic stress. Finally, the hydrodynamic conditioning for aluminium–bentonite flocs is examined.

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