Sedimenting flexible fibers against obstacles in a viscous fluid*

The motion of flexible fibers often happens in complex media that are structured by obstacles. Examples range from the transport of biofilm streamers through porous media to the design of sorting devices for DNA molecules. For large number of such problems, the dynamic of the fibers result from the complex interplay between internal elastic stresses, contact forces and hydrodynamic interactions with the walls and obstacles. By means of numerical simulations, experiments and analytical predictions, we investigate the dynamics of flexible fibers settling in a viscous fluid embedded with obstacles of arbitrary shapes. We identify various regimes on the dynamics of a single fiber, such as prolonged periods of trapping around the obstacle and lateral dispersion. The magnitude of this lateral displacement, in the same order as its length, depends on the fiber characteristics, an effect we leverage on to propose a sensitive sorting solution based on the fiber length and/or mechanical properties. Furthermore, we examine the conditions under which a fiber may remain trapped on the obstacle, and identify non-trivial wrapping configurations, providing design clues for optimal sorting/filtration.