Soft Matter Seminar: How rods give structure to fluids and how structure is distorted by flow

Pavlik Lettinga, Forschungszentrum Jülich GmbH, Institute of Complex Systems

The most effective way to structure a fluid is by immersing slender rods, as rods have a huge excluded volume. In equilibrium, this leads to an entropy-driven cascade of phase transitions with increasing concentration of rods. When subjecting isotropic, randomly oriented, 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.

Both subjects will be addressed in this talk, using biological engineered rod-like viruses, with different well-defined length and stiffness. First, I will discuss video microscopy experiments on highly ordered rods, revealing very unusual dynamics that challenge fundamental notions on diffusion [1,2]. Second, I will show how  the geometry and stiffness of rods play a key role in the rheological response, using a combination of Rheology and Small Angle Neutron Scattering (Rheo-SANS), resolving the orientational ordering of rodlike viruses in all relevant directions.  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 [3]. Finally, I will show how long semi-flexible F-actin filaments order in shear flow, using in situ confocal microscopy. Here, theoretical input is still missing to explain this extreme non-linear response [4].

1. M. P. Lettinga and E. Grelet. Self-Diffusion of Rodlike Viruses through Smectic Layers. Phy. Rev. Lett. (2007) Vol. 99, 197802.
2. L. Alvarez, M. P. Lettinga, E.Grelet, Fast Diffusion of Long Guest Rods in a Lamellar Phase of Short Host Particles. PRL 118 (2017) 17800
3.  C. Lang, L. Porcar, J. Kohlbrecher, M.P. Lettinga, The Connection between Biaxial Orientation and Shear Thinning for Quasi-Ideal Rods. Polymers, 8 (2016) 291.
4. Kirchenbuechler, I., D. Guu, N. A. Kurniawan, G. H. Koenderink, M. P. Lettinga. Direct visualization of flow-induced conformational transitions of single actin filaments in entangled solutions. Nature Communications, (2014) Vol. 5, 5060.