Structural Transitions in Sheared Electrically Stabilized Colloidal Crystals

A Landau theory is presented for the structural transition of electrically stabilized colloidal crystals under shear. The model suggests that a structural transition from an ordered layered colloidal crystal into a disordered structure occurs at a critical shear stress. The shear induced structural transition is related to a change of the rheological properties caused by the variation of the microstructure which can be verified by scattering experiments. The theory is used to establish the shape of the flow curves. A good qualitative agreement with experimental results can be achieved, while a scaling relation similar to the elastic scaling is established. 1. Introduction The rheology of suspensions containing small solid particles continues to generate great interest not merely because of its relevance to many industrial processes but also because of the theoretical understanding of many-particle systems. Here we want to focus on shear induced transitions in a subclass of suspensions, those with uniform spherical particles carrying an electric charge. In these suspensions the electrostatic interaction is responsible for the creation of long-range periodic crystal structures in equilibrium. They occur as body-centered-cubic (bcc) colloidal lattices for small particles at low ionic strengths or as face-centered-cubic (fcc) lattices for larger particles and higher ionic strengths [1–3]. A large number of rheological investigations have been carried out on model systems of electrically stabilized colloidal suspensions. Experimental techniques allow varying the particle interaction over a wide range by altering the particle size, the volume fraction, the surface charge, and the electrolyte concentration (e.g., [4–9]). The application of numerical simulations (e.g., [10–13]). The simultaneous investigation of the rheological properties and the microstructure by scattering techniques revealed a connection between the microstructure and the flow properties of concentrated colloidal dispersions (e.g., [4, 14–20]). Hoffman [4] first demonstrated that electrically stabilized colloidal suspensions undergo an order-disorder transition accompanied by shear thickening. This microstructural transition can be understood as a disturbance of the balance between stabilizing forces due to the mutual repulsion of the colloidal particles and hydrodynamic forces in a sheared suspension [21–24]. Experimental evidence of order-disorder transitions in low density colloidal crystals induced by shear perturbations were given first by Ackerson et al. [25]. It was found that under