Aqueous dispersions of polystyrene particles were prepared containing sodium carboxymethylcellulose (CMC) and sodium chloride (NaCl). The destruction process of the aggregates at a time scale less than one second was observed directly using a parallel disk geometry with startup rotation. The shear history differs with vertical positions under this unsteady shear flow and affects the time variation of aggregate size. It is found that the destruction process of aggregates at any position was correlated well by the introduction of cumulative shear strain. The effects of the additives on the fragility and stable size of aggregates under the unsteady shear flows have been investigated. It was revealed that the dominant factor of the fragility was changed from electrostatic repulsion to steric repulsion as increasing polymer concentration. In the electrostatic dominant regime, aggregates were easily destroyed at lower ionic strength. In contrast, stable aggregate size was decreased monotonically by increasing CMC concentration even in the electrostatic dominant regime.
