  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.
