Issue 31, 2019

Stability of the interface of an isotropic active fluid

Abstract

We study the linear stability of an isotropic active fluid in three different geometries: a film of active fluid on a rigid substrate, a cylindrical thread of fluid, and a spherical fluid droplet. The active fluid is modeled by the hydrodynamic theory of an active nematic liquid crystal in the isotropic phase. In each geometry, we calculate the growth rate of sinusoidal modes of deformation of the interface. There are two distinct branches of growth rates; at long wavelength, one corresponds to the deformation of the interface, and one corresponds to the evolution of the liquid crystalline degrees of freedom. The passive cases of the film and the spherical droplet are always stable. For these geometries, a sufficiently large activity leads to instability. Activity also leads to propagating damped or growing modes. The passive cylindrical thread is unstable for perturbations with wavelength longer than the circumference. A sufficiently large activity can make any wavelength unstable, and again leads to propagating damped or growing modes. Our calculations are carried out for the case of zero Frank elasticity. While Frank elasticity is a stabilizing mechanism as it penalizes distortions of the order parameter tensor, we show that it has a small effect on the instabilities considered here.

Graphical abstract: Stability of the interface of an isotropic active fluid

Article information

Article type
Paper
Submitted
18 Jun 2019
Accepted
11 Jul 2019
First published
19 Jul 2019

Soft Matter, 2019,15, 6318-6330

Author version available

Stability of the interface of an isotropic active fluid

H. Soni, W. Luo, R. A. Pelcovits and T. R. Powers, Soft Matter, 2019, 15, 6318 DOI: 10.1039/C9SM01216H

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