The Relationship between Steady Shear Viscosity and Complex Viscosity

Theoretical analysis on the relationship of complex viscosity (η*) and steady shear viscosity (η) indicates that viscous viscosity (η′) measured under linear viscoelastic region equates with steady shear viscosity at low shear rates, and the Cox‐Merz Rule applies only to the suspension whose viscous viscosity of the system dominates the whole response. From theoretical analysis, complex fluids with network structure such as hydrotalcite‐like compounds (HTlc)/montmorillonite (MT) suspensions are not suitable to the Cox‐Merz Rule, and departure from the rule has been found in all the HTlc/MT suspensions examined. The theoretical deduction and experimental results from HTlc/MT suspensions show that thixotropic types can be determined by the change of η′ in the linear viscoelastic region after the system has been presheared; the increase of η′ with time means positive thixotropy while the decrease of η′ with time means negative thixotropy. First increase and then decrease of η′ with increasing of time or vice versa means complex thixotropy. A very useful method to examine the thixotropic types in the undisturbed states has been found for the first time.