Slender-ribbon theory

TL;DR

This paper develops an asymptotic theory for the hydrodynamic behavior of slender ribbons in Stokes flows, providing a fundamental framework for understanding their motion in viscous fluids, with applications in biology and engineering.

Contribution

It introduces a novel integral equation approach for the hydrodynamics of slender ribbons, extending slender-body theory to ribbon geometries with three length scales.

Findings

Good agreement with known hydrodynamics of flat ellipsoids

Successfully models swimming behavior of microscopic swimmers

Analyzes hydrodynamics of bent and twisted ribbon shapes

Abstract

Ribbons are long narrow strips possessing three distinct material length scales (thickness, width, and length) which allow them to produce unique shapes unobtainable by wires or filaments. For example when a ribbon has half a twist and is bent into a circle it produces a M\"obius strip. Significant effort has gone into determining the structural shapes of ribbons but less is know about their behavior in viscous fluids. In this paper we determine, asymptotically, the leading-order hydrodynamic behavior of a slender ribbon in Stokes flows. The derivation, reminiscent of slender-body theory for filaments, assumes that the length of the ribbon is much larger than its width, which itself is much larger than its thickness. The final result is an integral equation for the force density on a mathematical ruled surface, termed the ribbon plane, located inside the ribbon. A numerical implementation of our derivation shows good agreement with the known hydrodynamics of long flat ellipsoids, and successfully captures the swimming behavior of artificial microscopic swimmers recently explored experimentally. We also study the asymptotic behavior of a ribbon bent into a helix, that of a twisted ellipsoid, and we investigate how accurately the hydrodynamics of a ribbon can be effectively captured by that of a slender filament. Our asymptotic results provide the fundamental framework necessary to predict the behavior of slender ribbons at low Reynolds numbers in a variety of biological and engineering problems.