Séminaire exceptionnel – 18 novembre 2021 – 11h salle 357

Active mechanics of living fluids: mucus and tissues

Simon Gsell – IBDM, Marseille

In our body, many physiological processes rely on active soft material flows. To ensure their biological function, these “living fluids” are kept out of mechanical equilibrium through active processes at play within the fluid volume. How active biological processes couple to fluid dynamics to maintain functional spatio-temporal organization remains largely unclear for many of these systems. A first example of living fluid is the mucus protecting bronchial tissue in the lung. This complex fluid is transported along the bronchial tree through active beating of microscopic cilia attached to the tissue. Experiments performed by E. Loiseau and A. Viallat at CINAM (Marseille) on reconstituted in-vitro tissues have shown that directional organization of cilia is coupled to ciliary-driven mucus flow. A minimal hydrodynamic model shows that this self-organization process is controlled by a hydrodynamic length closely connected to the mucus viscosity. The model predicts crucial organizational phase transitions and directional correlation length. Soft cellular tissues can also exhibit fluid-like behavior. During embryonic development, tissue “fluidity” is known to be crucial in several morphogenetic processes. However, the way embryonic tissue flows couple to bio-chemical processes controlling local active cell-cell interactions is generally unknown. This question is currently addressed experimentally in the lab of P.-F. Lenne at IBDM (Marseille) on the basis of embryonic organoids (embryoids). I will present some physical approaches employed to analyze the self-organization dynamics of in-vitro embryoids, as a well as a continuum theoretical/numerical framework that I develop to explore the coupling between advection and cellular organization in such heterogeneous cell aggregates. Finally, I will conclude my talk by presenting potential future works on complex active fluids.