Mesoscale flows in active baths dictate the dynamics of semi-flexible filaments

TL;DR

This study experimentally investigates how mesoscale active flows in bacterial baths influence the movement and deformation of passive semi-flexible filaments, revealing length-dependent dynamics governed by active viscous drag and elastic resistance.

Contribution

It provides the first experimental analysis of passive filament behavior in mesoscale active flows, highlighting the role of filament length relative to flow structures.

Findings

Short filaments show enhanced translation and rotation with minimal deformation.

Long filaments experience significant deformation but less transport.

Filament dynamics are governed by a competition between active viscous drag and elastic resistance.

Abstract

Semi-flexible filaments in living systems are constantly driven by active forces that often organize into mesoscale coherent flows. Although theory and simulations predict rich filament dynamics, experimental studies of passive filaments in collective active baths remain scarce. Here we present an experimental study on passive colloidal filaments confined to the air-liquid interface beneath a free-standing, quasi-two-dimensional bacterial film featuring jet-like mesoscale flows. By varying filament contour length and bacterial activity, we demonstrate that filament dynamics are governed by its length relative to the characteristic size of the bath. Filaments shorter than the jet width exhibit greatly enhanced translation and rotation with minimal deformation, while long filaments show dramatic deformation but less enhanced transport. We explain our findings through the competition between the active viscous drag of the bath and passive elastic resistance of the filaments, using a modified elastoviscous number that considers the mesoscale flows.