Negative drag force on beating flagellar-shaped bodies in active fluids

TL;DR

This study experimentally and theoretically explores how active particles exert both positive and negative drag forces on a flagellum-shaped object, revealing complex interactions influenced by motion and geometry in active fluids.

Contribution

It introduces the first experimental observation of negative drag forces on a beating flagellum in active fluids, supported by simulations and an analytical model.

Findings

Negative drag forces occur depending on beating frequency.

Active particle redistribution influences drag direction.

Results applicable to microrobotics and slender body dynamics.

Abstract

We experimentally investigate the drag force exerted by a suspension of light-induced active particles (APs) on a translating and beating idealized flagellum-shaped object realized through negative phototactic interactions with the APs. We observe both positive and negative drag forces, depending on the beating frequency and translational velocity, driven by the dynamic redistribution of APs in response to the object's motion. These findings are supported by numerical simulations and an analytical model, extendable to a range of slender geometries. Our results illustrate the complex interplay between geometric body changes and the density distribution in active baths, which may also be relevant for microrobotic applications.