Yielding in Surfactant Suspension Pastes: Effect of Surfactant Type

In this work, we study the rheological properties and the yielding behavior of cleaning pastes containing surfactant and abrasive particles, for three different types of surfactants, namely linear alkyl benzene sulfonate (LAS), alpha olefin sulfonate (AOS) and sodium lauryl ether sulfonate (SLES). All the pastes were observed to have soft solid-like consistency with their elastic modulus significantly greater than the viscous modulus. With around 36 volume percent of particulate matter, the high stiffness of the pastes suggests that particles form a space spanning network. Interestingly, when subjected to oscillatory shear deformation with increasing strain amplitude, the elastic modulus undergoes a decrease in two steps thereby showing a two-step yielding behavior. It is observed that the first yield stress does not show frequency dependence, and for LAS-containing paste was the largest followed by AOS- and SLES-containing pastes, respectively. The second yield stress, on the other hand, for all the three pastes is observed to increase with frequency. Careful assessment of the experimental data suggests that the first yielding event is due to rupture of the network which leads to formation of particulate aggregates. The second yielding event is attributed to breakage of aggregates. In both yielding phenomena, surfactants play an important role. Since, the phase behavior of surfactant in water determines the inter-particle interaction and network density, the nature of surfactant has a pivotal influence on both the yielding phenomena in surfactant suspension pastes used for cleaning purposes.