Thermosensitive Colloidal Atoms and Molecules
Prof. Dr. Matthias Ballauff (Helmholtz-Zentrum, Berlin)

Suspensions of colloidal particles and in particular spheres with diameters of 100 – 200nm suspended in water may be viewed as colloidal “atoms” which provide a model for the fluid-to-solid transition and in particular the glass transition. Due to the low number density of “atoms” the modulus of the resulting colloidal crystals is lowered by ca. 9 order of magnitude (“soft matter”) as compared to atomic systems. Moreover, motions of these colloidal atoms are slowed down by ca. 10 orders of magnitude which allows us to study dynamic phenomena in a detailed fashion. We have introduced thermosensitive particles as model systems. These particles consist of a solid polystyrene core onto which a thermosensitive network of crosslinked poly(N-isopropylacryl amide) (PNIPAM) is grafted. In cold water this thermosensitive shell will swell while a marked shrinking occurs when the temperature is raised above the volume transition of the network at 32°C. In this way the volume fraction, that is, the decisive control parameter can be adjusted and finely tuned in these systems.
In my lecture, I’ll present our recent work on these systems. First, suspensions of spherical particles will be discussed. These “colloidal atoms” provide ideal model systems for the fluid-to-glass transition and I’ll discuss our recent rheological studies of colloidal glasses. Special emphasis is laid on non-stationary experiments. In a second part I’ll discuss work on thermosensitive dumbbell-shaped colloids that serve as a model system of simple diatomic molecules. We observe a phase transition of the microgels to ordered phase. Comparison with previous computer simulations strongly suggests that the thermosensitive dumbbells form a plastic crystal. Rheological studies will be presented that reveal the subtle relation between ordering and the glass transition in these systems.