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

TL;DR

This work presents 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 and demonstrates that filament dynamics are governed by its length relative to the characteristic size of the bath.

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.

Figures (4)

• Figure 1: Transport dynamics of semi-flexible filaments in collective bacterial baths. Typical trajectories of the filament CM under (a) low and (b) high bacterial activity. Triangle and square mark the starting and ending location of the filament during 10s of motion. Green arrows represent the 2D flow field measured from PIV. (c) MSD of the filament CM and (d) of the end-to-end vector for short $\left(l_c\sim20\mathrm{\mu m}\right)$ and long $(l_c\sim200\mathrm{\mu m})$ filaments under low$(|\mathbf{v}|\sim13\mathrm{\mu m/s})$ and high $(|\mathbf{v}|\sim 55\mathrm{\mu m/s})$ activities. Green scale bar: $100\mathrm{\mu m/s}$; black scale bar:$\mathrm{100\mu m}$
• Figure 2: Conformations sampled by filaments of different contour lengths in bacterial baths. (a) Short filaments remain straight in both low and high activity baths, and occupy the corner of the $A^2-\tilde{R}_g$ configuration space. (b) Intermediate filaments are bent into C-shapes at high bath activity, and show an extended coverage in the configuration space. (c)Long filaments show stronger deformations and a richer variety of conformation states, represented by the expansion of $A^2$ values at low $\tilde{R}_g$ values. Color bar: probability of conformation states normalized by the maximum probability for each case. Scale bar: $100\mathrm{\mu m}$
• Figure 3: Characterization of the mesoscale flow in bacterial baths. (a) Direct observation of jet-like structures impinging on and deforming filaments. Representative image of the passive filament in the bath with color and arrows corresponding to the local velocity field measured from PIV. (b) Zoomed-in view showing the elongated shape of a typical jet marked by the white boundary (SI). Fitting the boundary with an ellipse results in the length $l^{\prime}$ and width $w^{\prime}$ of the jet.(c) A typical anisotropic velocity correlation used to estimate average $l$ and $w$ values for a given active bath. (d) Jet widths $w$ for all active baths studied, which shows no clear dependence on the activity. Scale bar: $150\mathrm{\mu m}$.
• Figure 4: Competition between viscous drag and filament rigidity governs filament configurations. (a) Average curvature $\langle \kappa \rangle$ of the filaments at different activity levels of the bath show distinct behaviors according to its length. Short filaments remain straight regardless of bath activity level, while long filaments show increased curvature as activity increases. (b) Average curvature shows a clear threshold behavior on the dimensionless elastoviscous number $\tilde{\mu}$, with a transition close to unity.