Shape and Displacement Fluctuations in Soft Vesicles Filled by Active Particles

TL;DR

This study numerically explores how active particles inside flexible vesicles influence shape and displacement, revealing concentration-dependent shape transitions, boundary accumulation effects, and enhanced vesicle mobility.

Contribution

It introduces a detailed numerical model of active particles within vesicles, highlighting shape-shifting, boundary accumulation, and migration phenomena not previously characterized.

Findings

Low concentration leads to boundary accumulation and pinched high-density regions.

High concentration results in homogeneous pressure and nearly circular shape.

Elongated particles significantly increase vesicle diffusivity.

Abstract

We investigate numerically the dynamics of shape and displacement fluctuations of two-dimensional flexible vesicles filled with active particles. At low concentration most of the active particles accumulate at the boundary of the vesicle where positive particle number fluctuations are amplified by trapping, leading to the formation of pinched spots of high density, curvature and pressure. At high concentration the active particles cover the vesicle boundary almost uniformly, resulting in fairly homogeneous pressure and curvature, and nearly circular vesicle shape. The change between polarized and spherical shapes is driven by the number of active particles. The center-of-mass of the vesicle performs a persistent random walk with a long time diffusivity that is strongly enhanced for elongated active particles due to orientational correlations in their direction of propulsive motion. In our model shape-shifting induces directional sensing and the cell spontaneously migrate along the polarization direction.