Nonequilibrium dynamics of semiflexible filaments in an active fluid

Active fluids exhibit a variety of complex dynamical phenomena that are not observed in their passive counterparts, largely due to the breaking of detailed balance at the particle level. This breaking of detailed balance can also be manifested in the dynamics of passive objects immersed in an active fluid. For instance, Nikola et al. have demonstrated numerically that a semiflexible filament exhibits spontaneous buckling and directed motion when immersed in an active bath. Despite considerable progress on the theoretical and numerical front, experimental investigations of the dynamics of passive objects in active media were so far restricted to the simplest cases. Here, we have developed an experimental system to systematically analyze the nonequilibrium dynamics of semiflexible filaments in an active fluid. We perform optical video microscopy on DNA-linked magnetic colloidal chains immersed in a quasi-2D bacterial bath, and quantify the diffusivity and the amplitudes of the bending mode fluctuations of the chains as a function of chain length, stiffness and bacterial density. Using fluorescent bacteria, we compute the active flow fields to uncover the underlying mechanism of the nonequilibrium dynamical phenomena.