Depletion forces on circular and elliptical obstacles induced by active matter

Abstract

Depletion forces exerted by self-propelled particles on circular and elliptical passive objects are studied using numerical simulations. We show that a bath of active particles can induce repulsive and attractive forces which are sensitive to the shape and orientation of the passive objects (either horizontal or vertical ellipses). The resultant force on the passive objects due to the active particles is studied as a function of the shape and orientation of the passive objects, magnitude of the angular noise, distance between the passive objects. By increasing the distance between obstacles the magnitude of the repulsive depletion force increases, as long as such a distance is less than one active particle diameter. For longer distances, the magnitude of the force always decrease with increasing distance. We also found that attractive forces may arise for vertical ellipses at high enough area fraction.

Universal reshaping of arrested colloidal gels via active doping

“…This behavior is in good agreement with several studies that have looked to characterize the accumulation of active particles on flat and curved surfaces [79][80][81][82][83][84][85][86][87]. Surface accumulation of active Brownian particles is characterized by a density profile that peaks at the surface and decays exponentially as one moves away from it.…”

“…Once the clusters have achieved this sufficiently large size, there is a sta-tistical force, similar in spirit to depletion in equilibrium system, which is extremely repulsive at short interaction distances and weakly attractive at longer distances. This effect in active matter systems has been studied in some detail elsewhere [80,81,84,91]. The active stabilization of these clusters is quite remarkable and cannot be described by mapping the active forces into an effective temperature of the solution.…”

Effective interactions mediated between two permeable disks in an active fluid

“…Our model assumes that the inclusions are placed at fixed positions within the active bath, an assumption that has frequently been used in the literature 8 , 9 , 11 – 16 , 18 , 20 , 21 , 23 – 25 , as it facilitates direct analogies with textbook examples of equilibrium depletion forces between colloidal inclusions fixed within a bath of nonactive depletants 1 , 2 , where the bath-mediated interactions represent the corresponding potentials of mean force between the inclusions. Several studies involving mobile (hard) inclusions in an active bath have also been published 10 , 11 , 19 , 22 , 24 , 26 .…”

“…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 .…”

“…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.…”