Non-linear viscoelastic properties of concentrated suspensions

Non-linear viscoelastic properties and mechanisms for concentrated suspensions were reviewed using the disperse systems of carbon black in linseed oil vanish. Non-linear viscoelastic properties of concentrated suspensions are associated with the process of formation and breaking down of the flocculated network structure formed by dispersed particles which is sensitive to strain. The non-linear viscoelastic phenomena were discussed from the standpoints of the effects of strain, strain rate and strain history on viscoelastic properties, such as dynamic viscoelastic properties and relaxation modulus. The equilibrium modulus is observed in the frequency dependence of G' in lower frequency region and time dependence of relaxation modulus G(t) in longer time region. These equilibrium moduli exhibit a remarkable strain dependence. In the dynamic measurement of parallel superposition of steady shear and oscillation, the value of G' in the low frequency region decreases abruptly, and this region shifts to higher frequency with increasing rate of shear. On the other hand, the value of eta' at lower frequency region decreases remarkably and increases with increasing frequency, however, in higher frequency region the value of eta' and its frequency dependence close to those measured without superposition of steady shear flow. The effect of strain and rate of strain on viscoelastic properties can be explained with the box type relaxation spectrum. The longer time portion of the relaxation spectrum may be cut corresponding to rate of shear, while diminishing of the height of the box may be dependent on the magnitude of strain and/or shear stress application time. (C) 1997 Elsevier Science S.A.