The dynamics of flexible and Brownian filaments in viscous flows

The morphological dynamics, instabilities and transitions of elastic filaments in viscous flows underlie a wealth of biophysical processes, and are also key to deciphering the rheological behavior of many complex fluids and soft materials. Here, we combine experiments with actin filaments as an experimental model system and computational modeling with non-local slender body theory to elucidate the dynamical regimes and morphological transitions of elastic Brownian filaments in a simple shear flow. The filament dynamics and transitions are primarily governed by a dimensionless elasto-viscous number comparing viscous drag forces to elastic bending forces, with thermal fluctuations only playing a secondary role. The transitions are characterized by a buckling instability and the appearance of localized high-curvature bends that propagate along the filaments in apparent “snaking” motions. We present a complete characterization of filament morphologies and transitions as a function of elasto-viscous number and scaled persistence length and demonstrate excellent agreement between theory, experiments and simulations.