Computer simulations of dynamics, structure and rheology of packed soft colloids

'Soft' colloids are typically micron or sub-micron scale structured objects such as polymer microgels, which consist of chemically cross-linked polymer networks that are compressible and deformable. Experiments suggest that at packing ratios where the structural dynamics of hard colloids are arrested, a soft colloid system may still be able to flow as a consequence of cage-breaking due to particle deformation. However, the link between the detailed elastic properties of soft colloids and the resulting dynamics are presently not well understood. Soft packed colloids show rich and complex rheological and flow behaviour and it is important to derive the links between the single particle elastic properties and the resulting suspension's behaviour.

The simulations described in this thesis utilise a recently developed computational algorithm, Fluctuating Finite Element Analysis, for simulating viscoelastic objects undergoing thermal excitation. This approach captures the detailed shape deformations of the colloidal particles allowing the structure of the objects as well as the effect of anisotropic deformation to be considered.

I apply Fluctuating Finite Element Analysis to soft colloidal systems, investigating the effects of varying effective volume fraction and material parameters on the dynamics, structure and rheology of both thermally diffusing and linearly sheared soft colloidal systems. Additionally, I present results of an experimental rheology investigation of ultrasoft polymer microgels, and compare to sheared simulation results.

I find evidence of a diffusive regime between cages in all quiescent simulations, and frustration of long rage ordering. I find the structural modulus of systems depends on the volume fraction, while mean squared displacement does not. Applying shear, I find a relationship between the diffusion timescale of the system and the timescale at which the system yields and layers. I find that shear response is similar to less expensive simulation techniques, and does not reproduce ultrasoft behaviour.