Forces in inhomogeneous open active-particle systems

TL;DR

This paper investigates the forces exerted by non-interacting active particles on objects within inhomogeneous environments, analyzing how activity gradients and particle sources influence net forces through analytical and numerical methods.

Contribution

It introduces a simplified 1D model and numerical simulations to understand forces in active-particle systems with sources and sinks, connecting theory to biological observations.

Findings

Particle flux can smooth density profiles caused by activity gradients.

Net forces depend on the interplay between activity gradients and particle creation/annihilation.

Results qualitatively match biological phenomena like nuclear motion in oocytes.

Abstract

We study the force that non-interacting point-like active particles apply to a symmetric inert object in the presence of a gradient of activity and particle sources and sinks. We consider two simple patterns of sources and sinks that are common in biological systems. We analytically solve a one dimensional model designed to emulate higher dimensional systems, and study a two dimensional model by numerical simulations. We specify when the particle flux due to the creation and annihilation of particles can act to smooth the density profile that is induced by a gradient in the velocity of the active particles, and find the net resultant force due to both the gradient in activity and the particle flux. These results are compared qualitatively to observations of nuclear motion inside the oocyte, that is driven by a gradient in activity of actin-coated vesicles.