Particle simulations of nonlinear rheology in core-shell systems

In the thesis we report on simulations of the nonlinear rheology of core-shell sytsems. The core-shell system is a model for resins to be used in novel water borne coating systems. Resin particles can be considered as a hard spheres, grafted with soft, in water soluble, polymeric coronas. A new Brownian dynamics model is presented to describe the coarse grain dynamics of these particles with long-lived memory. Instead of solving a set of generalised Langevin equations we introduce a set of variable describing the slowly fluctuating thermodynamic state of the ignored degrees of freedom. These variables give rise to additional transient forces on the simulated particles.
The parameters in the model are tuned to represent experimental shear thinning data of resin systems correctly. Increasing the influence of the transient forces results in shear banding. In a well defined range of imposed shear rates, the system settles in a banded state with two coexisting bands stacked along the gradient direction. Both bands have the same symmetry but differ in shear rate and density. Additional, the influence of other model parameters on the rheology is studied. Continuous planar elongation flow is implsed in the simulation. The measured elongational viscosity as a function of the deformation rate is compared to the shear viscosity. Using superposition we studied the response of a sheared system to a perturbation causing a flow in the vorticity direction with a gradient in the direction of the shear flow. For low shear rates, the viscosity measured from the perturbation agrees with the shear viscosity, at higher shear rates a deviation is observed.
As an application we studied the string formation of colloids dissolved in a strong shear thinning solvent. The solvent is modelled by the newly developed resin model. Starting from a random configuration the simulated colloids are seen to align in string-like structures, in agreement with experimental observations.