Visualization of the breakdown of dilute particles gels during steady-shear

A feature of models concerning the rheology of coagulated suspensions is the development of shear planes. An experimental set-up was developed in which we investigated, by visual analysis, whether shear planes really develop in such systems during steady-shear. A transparent coagulated PTFE dispersion was used, in which the refractive indices of the continuous and dispersed phases were matched, for the formation of a gel. Coagulation was effected by adding NaCl to a concentration of 0.5 M. Polystyrene particles were built into the gel structure as tracer particles. During steady-shear the velocities and trajectories of the tracer particles were analyzed by Particle Tracking Velocimetry (PTV). Indeed layers with approximately the same velocity were observed for a coagulated PTFE-gel, during steady-shear. Deviations from rectilinear motion of the tracer particles were observed. These observations correspond with the assumptions of the giant floc model in which the shear is not distributed homogeneously, but limited to certain shear planes. Deviations from rectilinear motion of the tracer particles in a dilute gel correspond with the results found by Folkersma et al. (1998) in which the distance by which a moving particle entrains its neighbours was assumed to be larger at low volume fractions (≈0.1) than at high volume fractions (≈0.4).