Flowing fibers in the presence of obstacles: toward a sorting device

Flowing suspensions of rigid and flexible particles in structured media are encountered in many biological and industrial systems. The motion of the particles results from the complex interplay between the surrounding flow, internal elastic forces, as well as hydrodynamic and steric interactions with obstacles. In this work, we study numerically and experimentally the dynamics of flexible and rigid fibers interacting with triangular obstacles in a microchannel. We identify various types of fiber trajectories around a single obstacle depending on their mechanical and geometrical properties. Long and rigid fibers are found to be more laterally shifted in the presence of obstacles while short and flexible fibers tend to remain on the same streamline with no visible deviation. In the rigid case, the trajectories are highly sensitive to the initial orientation of the fibers. We also show that the channel height and width strongly affect the flow field around the obstacle, and therefore the fiber motion. We finally suggest how these findings could be used to optimize a microfluidic device to sort fibers by length and/or deformability.