Shape and Displacement Fluctuations in Soft Vesicles Filled by Active Particles.

Matteo Paoluzzi,M Cristina Marchetti,Roberto Di Leonardo,Luca Angelani

doi:10.1038/srep34146

Matteo Paoluzzi, M Cristina Marchetti + Show 2 more

Open Access

https://doi.org/10.1038/srep34146

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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.

Highlights

Active systems are collections of agents that convert the energy of the environment in systematic movement[1,2,3]
The corresponding equilibrium system would be a vesicle filled with a suspension and bounded by a flexible membrane that is permeable to the solvent but not to the solute molecules
We examine the center of mass dynamics of the whole vesicle and show that it performs a persistent random walk with a long time diffusivity that is larger for elongated swimmers due to orientational correlations

Summary

Introduction

Active systems are collections of agents that convert the energy of the environment in systematic movement[1,2,3]. When the swimmers packing fraction falls below a characteristic value, depending on particles shape, the vesicle acquires an asymmetric shape characterized by a bimodal distribution of the local curvatures, with a high curvature peak and a near zero curvature component This effect is driven by a feedback mechanism coupling swimmers density and membrane curvature through local pressure. Since active particles tend to accumulate at concave boundaries, this local curvature increase drives further accumulation of swimmers, which in turn raises the local pressure. The presence of this feedback mechanism is confirmed by a strong correlation between the local swimmers density (or local pressure on the membrane) and the local curvature of the membrane. Interesting, the resulting migratory behavior shares some similarities with Eukaryotic directed cell migration[44,45]

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Conclusion