Colloids near phase transition lines under shear

Abstract

The aim of this thesis is to investigate the structure formation and deformation in colloidal systems due to an externally applied shear flow. The focus is on two different kind of colloidal systems: suspensions of attractive spherical colloidal particles in the neighbourhood of a gas-liquid critical point and suspensions of rod-like colloidal particles. Chapter 2 deals with the shear induced deformation of long ranged, critical microstructure of a colloid-polymer mixture close to it's gas-liquid critical point. Close to the critical point, the spatial extent of effective interactions between the colloidal particles is very large and the dynamics of concentration fluctuations is very slow. These two properties of near-critical systems causes their microstructure to be sensitive to an externally applied shear flow. In Chapter 3, the shear-rate dependent location of paranematic-nematic spinodals for suspensions of fd-virus particles is investigated. Fd-virus is a rod-like plant virus with a length of 880 nm, a thickness of 6 nm and has a persistence length of 2000 nm. The tendency of the shear flow to align the rods is the most important feature that causes the shear rate induced shift of phase transition lines. In the last chapter of this thesis, we report preliminary results on a flow instability that is observed in suspensions of stiff, rod-like colloids. Under shear flow, two kind of flow instabilities can in principle occur: the classic Taylor instability and the shear-banding instability. In both case on observes a banded structure along the vorticity direction in a couette cell.