Effective interactions between inclusions in an active bath

Abstract

We study effective two- and three-body interactions between non-active colloidal inclusions in an active bath of chiral or non-chiral particles, using Brownian dynamics simulations within a standard, two-dimensional model of disk-shaped inclusions and active particles. In a non-chiral active bath, we first corroborate previous findings on effective two-body repulsion mediated between the inclusions by elucidating the detailed non-monotonic features of the two-body force profiles, including a primary maximum and a secondary hump at larger separations that was not previously reported. We then show that these features arise directly from the formation, and sequential overlaps, of circular layers (or "rings") of active particles around the inclusions, as the latter are brought to small surface separations. These rings extend to radial distances of a few active-particle radii from the surface of inclusions, giving the hard-core inclusions relatively thick, soft, repulsive "shoulders," whose multiple overlaps then enable significant (non-pairwise) three-body forces in both non-chiral and chiral active baths. The resulting three-body forces can even exceed the two-body forces in magnitude and display distinct repulsive and attractive regimes at intermediate to large self-propulsion strengths. In a chiral active bath, we show that, while active particles still tend to accumulate at the immediate vicinity of the inclusions, they exhibit strong depletion from the intervening region between the inclusions and partial depletion from relatively thick, circular zones further away from the inclusions. In this case, the effective, predominantly repulsive interactions between the inclusions turn to active, chirality-induced, depletion-type attractions, acting over an extended range of separations.

Active fluids at circular boundaries: swim pressure and anomalous droplet ripening

“…To disentangle the effects of the swim pressure on curved boundaries from other possible effects such as collective alignment [2,5,7], hydrodynamic interactions [22][23][24][25][26][27][28][29], and steric particle layering [34,93,94], we focus on the so-called minimal model of ideal (non-interacting) active Brownian particles in their commonly used twodimensional formulation [4]. The active particles are considered in contact with a circular interface of radius R, which represents the bounding surface of a fixed, diskshaped inclusion immersed in the active bath, or that of a cavity enclosing the active particles (see Fig.…”

Effective interactions mediated between two permeable disks in an active fluid

“…In the particular example of hard disklike inclusions that will be of interest here, the effective interaction between two such inclusions fixed within a two-dimensional active bath was shown to be predominantly repulsive 11 , 13 , 16 , 24 , featuring nonmonotonic distance-dependent behaviors due to the mentioned ABP layering, or ring formation around the disks 18 , 20 , 23 , 24 . Such active bath-mediated interactions have been investigated in different cases, elucidating their dependence on the geometric shape and relative size of the inclusions 11 – 16 as well as motility strength, concentration 13 – 16 , 19 , 24 and chirality of ABPs 18 . In certain cases, attractive interactions 12 – 15 , 18 , 20 , 23 and noncentral forces 20 have been reported.…”

“…This leads to near-wall layering of ABPs 11 , 13 – 15 , 18 , 20 , 23 due to the counterbalancing steric repulsions between them in the highly populated near-surface regions, which in turn causes qualitative changes in the effective interactions mediated between inclusions in the active bath. In the particular example of hard disklike inclusions that will be of interest here, the effective interaction between two such inclusions fixed within a two-dimensional active bath was shown to be predominantly repulsive 11 , 13 , 16 , 24 , featuring nonmonotonic distance-dependent behaviors due to the mentioned ABP layering, or ring formation around the disks 18 , 20 , 23 , 24 . Such active bath-mediated interactions have been investigated in different cases, elucidating their dependence on the geometric shape and relative size of the inclusions 11 – 16 as well as motility strength, concentration 13 – 16 , 19 , 24 and chirality of ABPs 18 .…”

“…Such active bath-mediated interactions have been investigated in different cases, elucidating their dependence on the geometric shape and relative size of the inclusions 11 – 16 as well as motility strength, concentration 13 – 16 , 19 , 24 and chirality of ABPs 18 . In certain cases, attractive interactions 12 – 15 , 18 , 20 , 23 and noncentral forces 20 have been reported. While the focus has mainly been on two-body interactions between fixed inclusions, other cases with mobile inclusions 10 , 11 , 17 , 19 , 22 , 24 , 26 and many-body mixtures 26 have also been studied, and importance of inclusion mobility 24 and many-body interactions 18 have been noted.…”

“…In certain cases, attractive interactions 12 – 15 , 18 , 20 , 23 and noncentral forces 20 have been reported. While the focus has mainly been on two-body interactions between fixed inclusions, other cases with mobile inclusions 10 , 11 , 17 , 19 , 22 , 24 , 26 and many-body mixtures 26 have also been studied, and importance of inclusion mobility 24 and many-body interactions 18 have been noted.…”