# Generalized Archimedes' principle in active fluids

## TL;DR

This paper demonstrates how gradients in motility properties of active particles generate pressure gradients that can move large objects, extending Archimedes' principle to active matter systems with potential experimental applications.

## Contribution

It introduces a generalized Archimedes' principle for active fluids with position-dependent motility, providing theoretical calculations and measurable predictions.

## Key findings

- Pressure gradients can be induced by motility gradients in active particles.
- A modified Archimedes' principle applies to active matter systems.
- The model aligns with recent experimental observations in biological and artificial systems.

## Abstract

We show how a gradient in the motility properties of non-interacting point-like active particles can cause a pressure gradient that pushes a large inert object. We calculate the force on an object inside a system of active particles with position dependent motion parameters, in one and two dimensions, and show that a modified Archimedes' principle is satisfied. We characterize the system, both in terms of the model parameters and in terms of experimentally measurable quantities: the spatial profiles of the density, velocity and pressure. This theoretical analysis is motivated by recent experiments, which showed that the nucleus of a mouse oocyte (immature egg cell) moves from the cortex to the center due to a gradient of activity of vesicles propelled by molecular motors; it more generally applies to artificial systems of controlled localized activity.