Colloidal Suspensions in Shear Flow : a Real Space Study

Abstract

We investigate the effect of shear flow on the microstructure of colloidal suspensions by means of microscopy. Systems of nearly equally sized particles are used, whose interactions and phase behavior are predominantly determined by their size and shape, and can further be tuned by the addition of polymers. Recently, a new type of shear cell was developed to study flowing suspensions in real space. The key property of this setup is the counter-rotating principle of the cone and plate, opening up the possibility to create a stationary layer in the bulk of the cell. In Chapter 2, we elaborate on the details of this setup and its performance. Fluorescence confocal microscopy is used to visualize the sheared suspension, and allows imaging of individual particles in the bulk in the stationary plane for a prolonged time. This way, the particle positions in a layer of, for example, a sheared colloidal crystal can be tracked. The particle dynamics in colloidal crystals in shear flow are the subject of Chapter 3. Here, the particles interact through a (nearly) hard sphere potential. Apart from the alignment of the crystal in the shear field and the collective zigzag motion, which had also been deduced from early scattering experiments, we find that random particle displacements increase with shear rate. Those increased fluctuations result in shear induced melting when their mean square displacement has reached about 13 % of the particle separation. Apart from hard spheres, we investigate mixtures of colloids and polymers in shear flow. The polymers cause an effective attraction between the spheres, leading to phase separation into a colloid rich (polymer poor) and a colloid poor (polymer rich) phase at sufficiently high colloid and polymer concentration. In Chapter 4, we study the demixing process in the (spinodal) two-phase region of the phase diagram. The system is quenched from an initially almost homogeneous state at very high shear rate to a low shear rate. A spinodal decomposition pattern is observed. As the structure coarsens, the domains become highly stretched along the flow direction, and the domain width along the vorticity axis reaches a stationary size, corresponding to a steady state. In the final stage of phase separation the denser colloidal liquid phase settles on the bottom of the cell, while the gas phase floats on top. The interface between these phases is the topic of Chapter 5. We investigate the thermal fluctuations of the colloidal gas-liquid interface subjected to a shear flow parallel to the interface. Strikingly, we find that the shear strongly suppresses capillary waves, making the interface smoother. Finally, we consider the demixing process in systems of attractive rods (Chapter 6). A mixture of rod-like viruses (fd) and polymer (dextran) is quenched from a flow-induced fully nematic state into the region where the nematic and the isotropic phase coexist (at zero shear). Dependent on the concentration of rods we observe either demixing by nucleation-and-growth (high concentration) or spinodal decomposition (low concentration). At intermediate concentrations we see the transition between both types of demixing processes, where we locate the spinodal point.