The rheology of suspensions of submicron particles depends on the particle size, shape, and concentration and the nature and magnitude of the interparticle potentials. Indeed the rheological response differs qualitatively between rigid rods and hard spheres, between submicron and macroscopic particles, between dilute and concentrated suspensions, and between stable and flocculated colloidal suspensions. Hence a useful first step in understanding the subject is to classify systems according to the size and shape of the particles and the dominant interaction potential. Our work focusses on submicron, i.e. colloidal, particles which are spherical and strives to assess the effects of concentration and interparticle potential.
For colloidal systems the rheology is closely linked to the stability or phase behavior, which also reflects the nature and magnitude of the interparticle potentials. Dramatic rheological behavior is generally related to thermodynamic non-idealities, e.g. the solid-like rest state and plastic flow associated with flocculation due to van der Waals forces or phase separation induced by dissolved polymer, the elasticity of colloidal crystals generated by long range electrostatic repulsions, and the shear thinning and shear thickening phenomena associated with disorder-order transitions in stable dispersions near close packing. Thus an initial classification of colloidal systems might take the form.