Assoc. Prof. Dr. Pavlik Lettinga (KU Leuven, FS Juelich): How rods give structure to fluids and how structure is distorted by flow (and confinement)

The most effective way to structure a fluid is by immersing slender colloidal rods, as rods have a large excluded volume. In equilibrium, this leads to an entropy-driven cascade of phase transitions where ordering increases with increasing concentration of rods. When subjecting randomly oriented rods but entangled rods to flow, this leads to a strong shear thinning as volume becomes available by flow aligning the rods. While the first phenomenon is a hall mark for self-assembly, the second phenomenon is of special interest to understand industrial and biological processes industrial applications. I will first elucidate the underlying dynamics in equilibrium, using video microscopy of fluorescently labeled rod-like viruses in a host dispersion of unlabeled rods. For each phase (transition) this approach uncovers surprising dynamics that influence the way to think about the origin of the self-assembly. Moreover, I will (shortly) discuss the effect of confinement on the structure. Second, I will show how the geometry and stiffness of rods play a key role in the rheological response, using a combination of Rheology, Small Angle Neutron Scattering (Rheo-SANS), and biological engineered rod-like viruses with different well-defined length and stiffness. Interpretation of this data for the different model rods on the basis of microscopic theory gives a full understanding of the non-linear flow behavior.
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