There are a number of applications in which particles are added to rheologically ”complex” fluids. The latter include polymer melts and solutions, surfactant systems, associative poly- mers and gels. In order to process these materials in a rational manner, controlling the the rheological behavior is a prerequisite. In addition, the end-use properties may also require certain rheological features or a specific suspension microstructure, such as e.g. particle orientation.
When particles are dispersed in visco-elastic media, there will be changes in both the thermodynamic and the hydrodynamic interactions as compared to particles dispersed in low-molecular weight Newtonian fluids. These changes are reflected in the rheological behavior, for which some general trends emerge from the available literature on polymeric solutions and melts.
• The flow curves can be reduced using an internal shear rate concept. This has, however, limited predictive value as the limits of applicability have not yet been established.
• Adding particles will weaken the impact of elasticity, which has a pronounced effect on processing operations (e.g. reduction of the die swell, reduction of the strain hardening, reduction of flow instabilities).
• The addition of fibers or non-spherical (nano-)particles allows control of the elonga- tional viscosities, whithout increasing the elasticity of the medium (inkjet printing, coatings, sprays.)
A vast amount of data can be found in literature. However, few truly systematic data sets, in which there is a systematic variation of the medium rheology, particle size and interactions have been reported. Therefore no quantitative, predictive scalings, theories or simulations exist.
The changed hydrodynamic interactions also lead to very specific microstructural fea- tures. The formation of chains of spherical particles, as well as segregation and migration effects are qualitative understood as being associated with normal stress effects. Less at- tention has been paid as to how these specific microstructural features can be exploited technologically. Also rational control over structure development in complex, processing flows or mixing operations is lacking. The effects of medium elasticity on flow induced ori- entation of non-spherical particles, such as fibers or some nanofillers, are also non-trivial. Overall, for the rational formulation or intelligent processing of dispersions in viscoelastic media, the key features seem to be understanding how particles interact (hydrodynam- ically) and how the structural changes that occur during flow relate to the rheology of the medium. This calls for combined experiments on the rheology, flow-induced structure and dynamics, of suspensions containing particles with known interparticle interactions, dispersed in fluids with a well defined rheological behaviour. This should enable one to establish predictive scaling laws and provide a solid base for theoretical work.