Dynamics of freely transported fibers in confined viscous flows : Role of shape and flexibility

We study the dynamics of a model system constituted by a fiber freely transported in pressure-driven flows in a Hele-Shaw cell. Fiber height is comparable to channel height and in this confined geometry the fiber transport shows specific characteristics. Due to viscous friction with top and bottom walls transported particles act as moving obstacles and induce strong flow perturbations. These perturbations are at the origin of anisotropic friction forces leading to lateral drift and oscillatory movement between lateral walls. In this work, we ask how the transport dynamics are perturbed when going from an initially straight and rigid particle to a more complex object. Two approaches have been studied here: we add flexibility to the object and focus our investigations on the deformation of perpendicular and parallel fibers during their transport by an external viscous flow. Fibers perpendicular to the flow will bend while parallel fibers can deform in a wavy shape. We show that the bending of the perpendicular fiber is proportional to an elasto-viscous number and we fully characterize the influence of the confinement on the deformation of the fiber. Experiments on parallel flexible fibers reveal the existence of an instability threshold. Complementary, we also choose to change the shape of the fiber by adding an additional arm and forming an L shaped fiber. This induces fiber rotation until a stable equilibrium orientation is reached. Lateral drift is then observed until interaction with side walls becomes important. Tuning the fiber asymmetry allows for a precise control of particle trajectories, including the approach of side walls.