Flow-induced structure in a system of nuclear waste simulant slurries

Thixotropy and antithixotropy were characterized in nuclear waste simulant slurries. For the neutralized current acid nuclear waste (NCAW) simulant slurries, NCAW with glycolic acid (NCAW+GA), and NCAW with nitric acid (NCAW+NA) (pH 4), a pre-shear flow at constant shear rate destroys the aggregates in the suspension and reduces viscosity. For the NCAW+NA (pH7–9), a pre-shear enhances the aggregates in the suspension and increases viscosity. With the addition of silica to the NCAW+GA and NCAW+NA slurries, the pre-shear effect tends to promote aggregation due to the formation of a network in the suspension, and hence, the viscosity increases. The macroscopic rheological property variation due to the microstructural aspects of the suspensions associated with the shear-induced effect are addressed.     

Negative thixotropy of solutions of partially hydrolyzed polyacrylamide

We found that the character of negative thixotropy of partially hydrolyzed polyacrylamide in aqueous glycerol strongly depends on polymer concentration, glycerol content and shear rate applied. At low polymer and glycerol concentrations, shear stress and viscosity slowly increased during shearing to a limiting value. In addition to this behavior, a steep increase in shear stress as well as normal stress followed by their pronounced oscillations occurred at higher concentrations of both components and at higher shear rates. Similarly to the negative thixotropic effect in solutions of other polymers in organic solvents, the hydrodynamic conditions in which the effects set in seem to be controlled by the shear stress acting in the flowing solution; initial kinetics of the effect depends on solvent viscosity and shear stress applied. To explain the influence of the glycerol content and degree of ionization of the polymer on the minimum shear stress at which the effect sets in, a decisive role of intermolecular electrostatic repulsions in association of the polymer molecules in shear field is assumed.     

Stabilization of emulsions by heterocoagulation of clay minerals and layered double hydroxides

 Paraffin/water emulsions were stabilized by colloidal particles without surface active agents. Mixtures of two types of particles with opposite signs of charge were used: a layered double hydroxide (the hydroxide layers carry positive charges) and the clay mineral montmorillonite (the silicate layers carry negative charges). The emulsions were very stable and did not separate a coherent oil phase. The stability of the emulsion (no oil coalescence after centrifugation) was independent of the mixing ratio of both the compounds when the total solid content was >0.5%. Solid contents up to 2.0% were optimal. In contrast to stability, flow behavior of the emulsion was dependent on the mass fraction χ of the hydroxide. The maxima of viscosity and yield value were observed at χ≈0.2–0.3; the emulsion showed pronounced antithixotropic behavior. Weak thixotropic properties, smaller viscosities and yield values were observed at χ≤0.2 and ≥0.5. In the absence of the clay mineral, the double hydroxide particles stabilize by forming envelopes around the oil droplets. Addition of bentonites creates a three-dimensional network of particles with high elasticity which impedes coalescence of the oil droplets. Received: 9 March 1998 Accepted: 6 April 1998     

The influence of salt concentration on negative thixotropy in solutions of partially hydrolyzed polyacrylamide

The influence of NaCl concentration on negative thixotropy in aqueous glycerol solutions of partially hydrolyzed polyacrylamide have been investigated. It was found that negative thixotropy type I (a slow increase in viscosity with time of shearing) sets in at higher critical shear rates when the salt is present. On the other hand, critical shear rates for negative thixotropy type II (a rapid increase in viscosity followed by viscosity oscillation) did not depend on the salt addition. Using the critical shear stress as a hydrodynamic criterion for the occurrence of negative thixotropy, a possible explanation of the behavior is proposed.