Confinement controlled bend instability of three-dimensional active fluids

TL;DR

This study explores how biaxial confinement influences bend instabilities in 3D active fluids, revealing that confinement controls the wavelength and growth rate of deformations through a balance of active forces and elastic stresses.

Contribution

It demonstrates how confinement geometry affects instability characteristics in active fluids, supported by experimental and hydrodynamic modeling insights.

Findings

Confinement increases instability wavelength and growth rate.

Instability wavelength is determined by active and elastic stress balance.

Experimental results align with hydrodynamic model predictions.

Abstract

Spontaneous growth of long-wavelength deformations is a defining feature of active fluids with orientational order. We investigate the effect of biaxial rectangular confinement on the instability of initially shear-aligned 3D isotropic active fluids composed of extensile microtubule bundles and kinesin molecular motors. Under confinement, such fluids exhibit finite-wavelength self-amplifying bend deformations which grow in the plane orthogonal to the direction of the strongest confinement. Both the instability wavelength and the growth rate increase with weakening confinement. These findings are consistent with a minimal hydrodynamic model, which predicts that the fastest growing deformation is set by a balance of active driving and elastic relaxation. Experiments in the highly confined regime confirm that the instability wavelength is set by the balance of active and elastic stresses, which are independently controlled by the concentration of motors and non-motile crosslinkers.