Morphology of clusters of attractive dry and wet self-propelled spherical particle suspensions

Abstract: In order to asses the effect of hydrodynamics in the assembly of active attractive spheres, we simulate a semi-dilute suspension of attractive self-propelled spherical particles in a quasi two dimensional geometry comparing the case with and without hydrodynamics interactions. To start with, independently on the presence of hydrodynamics, we observe that depending on the ratio between attraction and propulsion, particles either coarsen or aggregate forming finite-size clusters. Focusing on the clustering regime, we characterize two different clusters parameters, i.e. their morphology and orientational order, and compare the case when active particles behave either as pushers or pullers (always in the regime where inter-particles attractions competes with self-propulsion). Studying cluster phases for squirmers with respect to those obtained for active Brownian disks (indicated as ABP), we have shown that hydrodynamics alone can sustain a cluster phase of active swimmers (pullers), while ABP form cluster phases due to the competition between attraction and self propulsion. The structural properties of the cluster phases of squirmers and ABP are similar, although squirmers show sensitivity to active stresses. Active Brownian disks resemble weakly pusher squirmer suspensions in terms of cluster size distribution, structure of the radius of gyration on cluster size and degree of cluster polarity.