Screening of a local force dipole by a nonlinear elastic network through self-organized buckling

Many fundamental cellular processes require exquisitely orchestrated large-scale reorganization of structural filaments. One mechanism of reorganization is via internal forces generated by motor proteins, which are transmitted by a highly non-linear network of fiber-like filaments. The mechanics of such active networks are highly tunable. Experiments in composites of passive actin filaments and microtubule-motor assemblies show activity-driven flow that can transition from being extensile to contractile (Berezney et .al., PNAS (2022)). In this talk, we will present results from a minimal model of a polymer network driven by active-force dipoles. Such networks are known to exhibit amplification and rectification of active stresses (Ronceray et al PNAS 2016).

In this work, we show that the rectification can be framed as an “elastic screening” of the force dipole. The screening emerges from self-organized buckling patterns, resulting in the redistribution of stresses. We trace the microscopic origin of the organized response to soft modes of an underlying Kagome lattice. We demonstrate that this organizing principle survives disorder introduced by diluting the lattice. The far-field stress response is captured well by a non-linear dielectric screening mechanism, where the nonlinearity can be represented by an emergent Poisson ratio that depends sensitively on the magnitude of the force dipole.