Self-organization of active rod suspensions on fluid membranes and thin viscous films

TL;DR

This study investigates how active hydrodynamic flows influence the self-organization of rod-like inclusions on fluid interfaces, revealing different behaviors for extensile and contractile stresses and emphasizing the importance of hydrodynamic interactions.

Contribution

The paper introduces a continuum model analyzing the effects of active stresses and self-propulsion on rod suspensions at fluid interfaces, highlighting the role of hydrodynamics in self-organization.

Findings

Extensile (pusher) stresses induce finite wavelength nematic ordering at high activity.

Ordered domain size decreases with increasing rod length to Saffman-Delbrück length ratio.

Contractile (puller) stresses do not lead to ordering, remaining uniform across activities.

Abstract

Many biological processes involve transport and organization of inclusions in thin fluid interfaces. A key aspect of these assemblies is the active dissipative stresses applied from the inclusions to the fluid interface, resulting in long-range active interfacial flows. We study the effect of these active flows on the self-organization of rod-like inclusions in the interface. Specifically, we consider a dilute suspension of Brownian rods of length $L$, embedded in a thin fluid interface of 2D viscosity $\eta_m$ and surrounded on both sides with 3D fluid domains of viscosity $\eta_f$. The momentum transfer from the interfacial flows to the surrounding fluids occurs over length $\ell_0=\eta_m/\eta_f$, known as Saffman-Delbr\"uck length. We use zeroth, first and second moments of Smoluchowski equation to obtain the conservation equations for concentration, polar order and nematic order fields, and use linear stability analysis and continuum simulations to study the dynamic variations of these fields as a function of $L/\ell_0$, the ratio of active to thermal stresses, and the dimensionless self-propulsion velocity of the embedded particles. We find that at sufficiently large activities, the suspensions of active extensile stress (pusher) with no directed motion undergo a finite wavelength nematic ordering, with the length of the ordered domains decreasing with increasing $L/\ell_0$. The ordering transition is hindered with further increases in $L/\ell_0$. In contrast, the suspensions with active contractile stress (puller) remain uniform with variations of activity. We notice that the self-propulsion velocity results in significant concentration fluctuations and changes in the size of the order domains that depend on $L/\ell_0$. Our research highlights the role of hydrodynamic interactions in the self-organization of active inclusions on biological interfaces.