Title: Instabilities in contractile active matter
Abstract: One of the hallmark of living systems is their ability to transform chemical energy into mechanical work. This is achieved in particular by the cytoskeleton, a polymer network that plays an essential role, for example, in cell division and migration. The actomyosin cytoskeletal network consist of actin filaments and myosin molecular motors. The assembly of actin filaments and stress generation by the motor molecules are driven by the hydrolysis of Adenosine-Triphosphate maintaining the network continuously out of thermodynamic equilibrium. From a physical point of view, the actomyosin network thus belongs to the class of active matter. In living cells, the active mechanical cytoskeletal stress is typically contractile in nature. In this talk, I will discuss two examples of dynamic instabilities exhibited by the contracting actin cytoskeleton. First, I will discuss the formation of myosin clusters in contractile actin rings that drive the fission of animal cells and yeast. These clusters appear to be stationary in mammalian cells whereas they travel along the actin ring in fission yeast. I will present a common mechanism underlying cluster formation in both cell types. It crucially depends on the formation of bipolar filament assemblies, which leads to a rich spectrum of possible myosin cluster dynamics and suggests different functions of the clusters in the two cell types. In the end I will briefly discuss buckling instabilities of contracting reconstituted actomyosin sheets.