Phase-separation during sedimentation of dilute bacterial suspensions

TL;DR

This study investigates how swimming bacteria influence the sedimentation of passive particles, revealing complex front dynamics, speed decay behaviors, and a unifying timescale, supported by a phenomenological model.

Contribution

It introduces a phenomenological model capturing sedimentation dynamics of passive particles in active bacterial suspensions, highlighting the role of bacteria concentration and front behavior.

Findings

Bacteria front behaves diffusive at short times and linearly decays at long times.

Sedimentation speed decreases with bacteria concentration and becomes independent at high concentrations.

All experiments collapse onto a single master line using the bacteria front timescale.

Abstract

Numerous natural systems depend on the sedimentation of passive particles in presence of swimming microorganisms. Here, we investigate the dynamics of the sedimentation of spherical colloids at various E. coli concentration within the dilute regime. Results show the appearance of two sedimentation fronts, a spherical particle front and the bacteria front. We find that the bacteria front behave diffusive at short times, whereas at long times decays linearly. The sedimentation speed of passive particles decays at a constant speed and decreases as bacteria concentration ($\phi_b$) is increased. As $\phi_b$ is increased further, the sedimentation speed becomes independent of $\phi_b$. The timescales of the bacteria front is associated with the particle settling speeds. Remarkably, all experiments collapse onto a single master line by using the bacteria front timescale. A phenomenological model is proposed that captures the sedimentation of passive particles in active fluids.