Hydrodynamic diffusion in apolar active suspensions

TL;DR

This study uses advanced simulations to analyze hydrodynamic diffusion in non-self-propelling active suspensions, revealing how particle activity influences collective motion and transport properties.

Contribution

It provides the first detailed numerical analysis of hydrodynamic diffusion in apolar active suspensions of squirmers, highlighting differences from self-propelling systems.

Findings

Peak diffusivity at 10-20% volume fraction

Little difference between puller and pusher dynamics

Rotational dynamics increase with volume fraction

Abstract

Active suspensions encompass a wide range of complex fluids containing microscale energy-injecting particles, such as cells, bacteria or artificially powered active colloids. Because they are intrinsically non-equilibrium, active suspensions can display a number of fascinating phenomena, including turbulent-like large-scale coherent motion and enhanced diffusion. Here, using a recently developed active Fast Stokesian Dynamics method, we present a detailed numerical study on the hydrodynamic diffusion in apolar active suspensions. Specifically, we simulate suspensions of active but non-self-propelling spherical squirmers, of either puller- or pusher-type, at volume fractions from 0.5% to 55%. Our results show little difference between pullers and pushers in their instantaneous and long-time dynamics, where the translational dynamics vary non-monotonically with the volume fraction, with a peak diffusivity at around 10% to 20%, in stark contrast to suspensions of self-propelling particles. On the other hand, the rotational dynamics tend to increase with the volume fraction as is the case for self-propelling particles. To explain these dynamics, we provide detailed scaling and statistical analyses based on the activity-induced hydrodynamic interactions and the observed microstructural correlations, which display a weak local order. Overall, these results elucidate and highlight the different effects of particle activity on the collective dynamics and transport phenomena in active fluids.