Buckling of a beam extruded into highly viscous fluid

TL;DR

This study investigates the buckling behavior of a slender beam extruded into a viscous fluid through experiments and modeling, revealing a critical length for stability and implications for microscopic propulsion mechanisms.

Contribution

The paper introduces a combined experimental and numerical approach to analyze buckling of beams in viscous fluids, providing a predictive model for critical buckling length and force behavior.

Findings

Force increases linearly then plateaus at large deformation

Critical buckling length depends on extrusion speed, rigidity, and viscosity

Predicted optimal rigidity for biological trichocysts to maximize thrust without buckling

Abstract

Inspired by microscopic paramecies which use trichocyst extrusion to propel themselves away from thermal aggressions, we propose a macroscopic experiment to study the stability of a slender beam extruded in a highly viscous fluid. Piano wires were extruded axially at constant speed in a tank filled with corn syrup. The force necessary to extrude the wire was measured to increase linearly at first until the compressive viscous force causes the wire to buckle. A numerical model, coupling a lengthening elastica formulation with resistive force theory, predicts a similar behaviour. The model is used to study the dynamics at large time when the beam is highly deformed. It is found that at large time, a large deformation regime exists in which the force necessary to extrude the beam at constant speed becomes constant and length-independent. With a proper dimensional analysis, the beam can be shown to buckle at a critical length based on the extrusion speed, the bending rigidity and the dynamic viscosity of the fluid. Hypothesising that the trichocysts of paramercies must be sized to maximise their thrust per unit volume as well as avoid buckling instabilities, we predict that their bending rigidity must be about $3\times 10^{-9}~\mathrm{N\cdot \mu m^2}$. The verification of this prediction is left for future work.