Core–shell microgels as model colloids for rheological studies

We review recent work done on the rheology of thermosensitive suspensions. These systems consist of aqueous suspensions of core–shell particles having a solid polystyrene core and a shell of thermosensitive crosslinked poly(N-isopropylacrylamide) (PNIPA). In cold water the thermosensitive PNIPA-network is swollen leading to a high effective volume fraction of the particles in suspension. Approaching the volume transition at 32 °C the network shrinks by expelling water. Hence, the effective volume fraction can be adjusted by the temperature. We demonstrate that these suspensions are a well-characterized model system for the study of the flow behavior of concentrated suspensions. In particular, experimental work done on this system can be compared to the predictions of the mode-coupling theory (MCT) of the fluid-to-glass transition. Excellent agreement is found demonstrating that MCT captures the essential features of the dynamics of flowing suspensions. In particular, MCT predicts a melting of the glass by shear which is fully corroborated by the experimental data.