Obstacle-induced lateral dispersion and nontrivial trapping of flexible fibers settling in a viscous fluid

TL;DR

This study explores how flexible fibers interact with obstacles in viscous fluids, revealing complex trapping and gliding behaviors that depend on fiber properties, with potential applications in passive particle sorting.

Contribution

It introduces a comprehensive analysis combining simulations, experiments, and theory to understand fiber-obstacle interactions and proposes a passive sorting strategy based on these dynamics.

Findings

Identification of trapping and gliding events

Nontrivial trapping conformations on sharp obstacles

Lateral displacement depends on fiber properties

Abstract

The motion of flexible fibers through structured fluidic environments is ubiquitous in nature and industrial applications. Most often, their dynamics results 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 and characterize two types of events: trapping and gliding, for which we detail the mechanisms at play. We observe nontrivial trapping conformations on sharp obstacles that result from a subtle balance between elasticity, gravity and friction. In the gliding case, a flexible fiber reorients and drifts sideways after sliding along the obstacle. The subsequent lateral displacement is large compared to the fiber length and strongly depends on its mechanical and geometrical properties. We show how these effects can be leveraged to propose a new strategy to sort particles based on their size and/or elasticity. This approach has the major advantage of being simple to implement and fully passive, since no energy is needed.