Séminaire Silke Henkes (Leiden University)

Activity and spatiotemporal correlations in model cell sheets

In living materials such as epithelial cell sheets, the origin of active driving lies in the net forces that cells exchange with their substrate, and in the stresses acting between cells. Thus their unjamming transitions are modified both by their complex mechanical interactions, captured e.g. by the vertex model potential, and active driving. Here I will speak about two different incarnations of this process.  First I consider simple uncorrelated active driving forces for cells or particles coupled to a substrate. This leads to an emergent mesoscopic correlation length due to the interactions between driving and the overdamped elasticity of the sheet. I will show that this provides a good null model for the fingering ‘instability’ at the sheet edge, which instead represents long-lived fluctuations. Second, for developing embryos and other systems without a substrate, activity necessarily takes the form of balanced forces, or equivalently, active stresses. I will first again consider the simplest system, purely fluctuating active stresses, and show how they generate long range order and hyperuniformity in active crystals and solids, while being incompatible with collective motion. I will end by showing that including coupling between active stresses can lead to collective motion and exotic convergence-extension states with negative viscosity.