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Unexpected trapping of active particles and efficient sorting of flexible fibers with obstacles
The motion of active or elastic particles through structured environments is ubiquitous in nature and industrial applications. Their dynamics result from the complex interplay between internal elastic stresses, activity, contact forces, thermal fluctuations and hydrodynamic interactions with the obstacles.
In this talk I will present two very different systems : 1) colloidal microrollers moving across a structured landscape and 2) flexible fibers settling against obstacles in a viscous fluid. In both systems the motion of the particles is induced by an external forcing : magnetic torques for the rigid microrollers, and gravity for the deformable fibers. Depending on their initial condition, both of these particles can either get trapped or pass by the obstacles. While the fibers get stuck upstream as they hit the obstacle, the microrollers get instead trapped in the wake of the obstacle.
Combining experiments, numerical simulations and analytical models, we detail the mechanisms that lead to these trapped state and characterize the particle trajectories when they pass-by the obstacles.. Two remarkable features arise from our investigation :
1) trapping of the microrollers by obstacles is purely hydrodynamic and can only occur with Brownian motion. While noise is usually needed to escape traps in dynamical systems, here it is the only means to reach the hydrodynamic attractor behind the obstacle.
2) after hitting an obstacle, a flexible fiber reorients and drifts sideways. The subsequent lateral displacement is large compared to the fiber length and strongly depends on its mechanical and geometrical properties. Sedimentation against obstacles could therefore be a promising strategy to sort particles based on their size and/or deformability.