Colloids near phase transition lines under shear

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

Flow dichroism in critical colloidal fluids

Due to long-range correlations and slow dynamics of concentration fluctuations in the vicinity of the gas-liquid critical point, shear flow is very effective in distorting the microstructure of near-critical fluids. The anisotropic nature of the shear-field renders the microstructure highly anisotropic, leading to dichroism. Experiments on the dichroic behavior can thus be used to test theoretical predictions on microstructural order under shear flow conditions. We performed both static and dynamic dichroism and turbidity measurements on a colloid-polymer mixture, existing of silica spheres (radius 51 nm) and polydimethylsiloxane polymer (molar weight 204 kg/mol). Sufficiently far away from the critical point, in the mean-field region, the experimental data are in good agreement with theory. Very close to the critical point, beyond mean field, for which no theory exists yet, an unexpected decrease of dichroism on approach of the critical point is observed. Moreover, we do not observe critical slowing down of shear-induced dichroism, right up to the critical point, in contrast to the turbidity

Dispersions and mixtures of particles with complex architectures in shear flow

We review the effect of shear flow on the phase behavior and structure of colloidal dispersions with increasing degree of complexity. We discuss dispersions of colloidal rods, stiff living polymers like wormlike micelles, and colloidal platelets. In addition, a review is presented on sheared binary dispersions. For all cases we discuss the interplay between thermodynamic instabilities and hydrodynamic instabilities