Two-step yielding in surfactant suspension pastes

In this work, we investigate two-step yielding behavior of pastes containing anionic surfactants, clay, and abrasive particles of calcite. Such pastes are commercially used as hand dishwash paste. The surfactant suspension pastes have soft solid-like consistency with finite elastic modulus. The total volume fraction of the particulate matter in the same is around 36 %; consequently, we attribute the elastic modulus of the same to the bonding between abrasive particles leading to attractive gel phase. We observe that the surfactant suspension pastes undergo physical aging during which evolution of relaxation time takes place. In dynamic strain sweep experiments, the pastes demonstrate clear signature of two-step yielding. It is observed that during the first yielding event, elastic modulus decreases while viscous modulus shows a maximum. For the second yielding event, however, elastic modulus demonstrates plateau before decreasing. While the first yielding event is observed to remain unaffected by the frequency of oscillations, the plateau values of elastic modulus, before the second yielding process, is observed to increase with frequency. We attribute the first yielding event in surfactant suspension pastes to rupture of network, while the second yielding event to breakage of the aggregates.