Physics-Biology interface seminar: Ulrich Schwarz


11:00 - 12:00

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Physics of active contractile matter

Ulrich S. Schwarz (Heidelberg University)

Biological systems such as cells and tissue use non-equilibrium processes to actively generate mechanical stress, movement and growth. Some of these processes can actually be reconstituted in biomimetic experiments with active soft matter. In this talk, we will first discuss why and how contractile forces are generated by biological systems and how they can be measured with soft elastic substrates (« traction force microscopy »). Because kilo-Pascal stresses are typically transmitted through micrometer-sized contacts, the relevant force scale is nano-Newton. We then introduce different theoretical approaches to understand and model the contractility of biological matter. Because closed systems have to conserve momentum, the most central concept here is the one of a « force dipole », similarly as for the theoretical description of microswimmers, but now coupled to a mechanical rather than to a hydrodynamic environment. We present a stochastic theory for the biologically most relevant example of a contractile force dipole, namely the « myosin II minifilament ». We then explain why on the large length scale of cells and tissue, the mechanical properties of these systems are dominated by tensions rather than by their elastic modulus, with dramatic consequences for their shape and force transmission to the environment.

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