Multiphase coexistence and non-linear rheology of colloidal. Dispersions as observed in a model capillary viscosimeter

Investigations of the flow properties of colloidal substances by viscometry and rheometry are a valuable tool in understanding many transport processes of importance in biology, medicine and industrial treatment of materials. The streaming of cytoplasm, blood, micellar solutions or crude oil emulsions are but some obvious examples. One of the most intriguing properties of colloidal systems is their ability of thinning or thickening under shear. To characterise this non-Newtonian flow behaviour different visco- and rheometric experiments have been devised, the capillary viscometer being one of the classical instruments. The underlying physical mechanisms of non-linear rheometry are the shear-induced formation and destruction of long range positional and orientational order. Since only in rare cases comprehensive structure and velocity information is accessible from inside a viscosimeter, generally, homogeneous samples are assumed. However, there are indications of a geometry dependent evolution of inhomogeneous phase and flow behaviour from recent experiments on colloidal model systems, in particular for denser systems of strongly interacting particles. We here present investigations performed on a well characterised suspension of spherical particles interacting via a screened electrostatic potential. We give a detailed study of the local structures and shear rates in an optical model capillary viscosimeter. As a function of the overall Bur several different flow scenarios are observed within the viscosimeter and the most striking feature is the simultaneous existence of up to four concentrically arranged phases under conditions of stationary flow.