Fig. 3. The differential viscosity of Na-montmorillonite suspensions as a function of the shear stress at various humic acid (HA) concentrations and pH values: (A) pH = 8; and (B) pH = 10. Figures (a) and (b) present an enlargement of the area outlined in figures (A) and (B). A curve related to Ca-montmorillonite is individually indicated.