Breakup and strength of polystyrene latex flocs subjected to a converging flow

Breakup of flocs (aggregates of colloidal particles) in converging flows is encountered in many industrial processes such as filtration, pipetting for sampling of flocs, and mechanical dispersion. The experiment of breakup of flocs by a converging flow opens the possibility to obtain the strength of floc against breakup. The quantity is crucial for controlling flocculator in water treatments and predicting transport of colloidal particles in aqueous environments. Therefore, the mechanism of disaggregation of flocs subjected to a converging flow is emerging as an interesting theme from engineering and scientific points of view. Recently, Blaser evaluated the hydrodynamic disrupting force on a floc in an axisymmetrical straining flow. In the present study, to confirm the validity of the ellipsoidal model proposed by Blaser and to obtain the strength of latex flocs in a simple electrolyte solution, polystyrene sulfate latex (PSL) flocs formed in a simple KCl solution were disrupted by a converging flow of the solution. Experimental data of relationships between floc size and volumetric flow rate were analyzed by assuming flocs hydrodynamically behave like solid ellipsoids to evaluate strength of flocs. The obtained floc strength was 2 nN in average and did not depend on floc size. When the values of floc strength of this study were compared with those of previous researches, they were found to be 1–2 orders smaller than strengths of flocs formed by the addition of polymer flocculants and/or precipitated ferric or aluminum hydroxide coagulants evaluated by similar methods. This result is reasonable because flocculants and coagulants are usually used to increase floc strength in water and wastewater treatments. The value of obtained floc strength was also comparable to the pull-off force between latex particles in water by means of colloid probe atomic force microscopy (AFM) by Hodges et al. However, the obtained floc strength was 1–2 orders of magnitude smaller than the pull-off force between latex particles as found with colloid probe AFM by Li et al.