pH effects on shear thickening behaviors of polystyrene-ethylacrylate colloidal dispersions

In this study, the effects of pH on shear thickening behaviors were investigated based on concentrated dispersions consisting of polystyrene-ethylacrylate (PSt-EA) copolymer nanospheres. The pH values of the above shear thickening fluids were controlled by adding different kinds of acid and alkali solutions, and the rheological testing indicated that the shear thickening behaviors of PSt-EA colloidal dispersions is significantly dependent on the system's pH. Zeta potential of this system tends to be close to zero at a pH of 3 or 9. Since the surface characteristics of copolymer nanospheres were changed, the isoelectric point is 3 in the acid circumstance, while the isoelectric point turns to be 9 by adding alkali. Both decreasing and increasing the pH value near to the isoelectric point can lower the critical shear rate of suspensions and make it easier for the ST behavior to take place. The effect of pH can be qualitatively explained by using a clustering mechanism, which attributes the phenomena to the formation of temporary, hydrodynamic clusters. (papers)