Rheology of Complex Fluids

Structural Transitions and the Rheology of Soft Sphere Suspensions


The book deals with experimental and theoretical studies on the flow dynamics of concentrated colloidal suspensions. The experimental investigations show that in concentrated colloidal suspensions (from a volume fractions above approx. 45%) the viscosity can increase abruptly with increasing shear rate. This effect is called discontinuous shear thickening, dilatancy or Shear Thickening . In order to better understand this effect, experiments were carried out with concentrated suspensions of electrically-stabilized colloidal particles in fluids whose refractive indices were very close to that of the fluid. This made it possible to study the movement of the particles at defined shear rates using laser light scattering experiments in a suitable setup. It was found that the particles arrange in a periodic structure at low shear rates, combined with
, i.e. a decrease in viscosity with increasing shear rate. However, this effect only occurs during the first measurement. Since colloidal particles in very highly concentrated suspensions can not relax into a disordered state by Brownian motion, shear thinning no longer occurs in subsequent measurements.

The investigations show that the occurrence of an abrupt increase in viscosity (discontinuous shear thickening) is associated with the disappearance of the internal, local-periodic structure. Therefore, both effects must be causally connected. Dilatancy in this case can be interpreted as a structural (phase) transition. It occurs when the inner ordered periodic structure changes into a disordered structure with increasing shear rate. The theoretical interpretation of this structural instability was made applying a Landau theory. Since a sheared suspension is not a system in thermodynamic equilibrium, the abrupt increase in viscosity with increasing shear rate (dilatancy) is therefore a nonequilibrium phase transition and can be interpreted as a shear-induced phase transition. This result is consistent with other non-equilibrium phase transitions, such as those described in synergetics .


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