Rotational dynamics of a soft filament: wrapping transition and propulsive forces

TL;DR

This study investigates how a long elastic filament rotating in a viscous fluid transitions from straight to helical shape, generating propulsive forces, with a model explaining the observed nonlinear dynamics and force-torque behavior.

Contribution

It provides experimental evidence and a linearized model for the shape transition and propulsion in rotating elastic filaments in viscous fluids.

Findings

Identified a sharp transition from straight to helical shape.

Discovered the generation of propulsive forces during the transition.

Proposed a linearized fluid-structure interaction model.

Abstract

We analyze experimentally the shape of a long elastic filament rotating in a viscous liquid. We identify a continuous but sharp transition from a straight to an helical shape, resulting from the competition between viscous stresses and elastic forces. This induced helicity generates a propulsive force along the axis of rotation. In addition, we show that the shape transition is associated with an unstable branch in the force-torque relation. A linearized model of the fluid-structure interaction is proposed to account for all the features of the non-linear filament dynamics.