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 coreshell particIes having asolid 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 stlldy ofthe flow behavior ofconcentrated suspensions. Tn particular, experimental work done on this system can be compared to the predictions of the modecoupling theory (MCT) 01' the fluid-to-glass transition. Excellent agreement is found demonstrating that MCT captures the essential features of the dyn amics of flowing suspensions. In particular, MeT predicts a melting of the glass by shear which is fulJy corroborated by the experimental data.