Effective Interactions between Colloidal Particles Suspended in a Bath of Swimming Cells

Abstract

The dynamics of passive colloidal tracers in a bath of self-propelled particles is receiving a lot of attention in the context of nonequilibrium statistical mechanics. Here we demonstrate that active baths are also capable of mediating effective interactions between suspended bodies. In particular we observe that a bath of swimming bacteria gives rise to a short range attraction similar to depletion forces in equilibrium colloidal suspensions. Using numerical simulations and experiments we show how the features of this interaction arise from the combination of nonequilibrium dynamics (peculiar of bacterial baths) and excluded volume effects.

Effective equilibrium states in mixtures of active particles driven by colored noise

“…(7) that the UCNA results for the off-diagonal friction matrix elements in Eq. (17) and for the correlators in Eq. (16) are different from those found in the present treatment, based on the Fox approximation, even in the small τ (α) limit, given by Eq.…”

“…On the other hand, active dopants with a short persistence length were reported to aggregate in cages [14]. On immersing large colloids into a bath * rene.wittmann@unifr.ch of smaller active colloids, the former effectively are rendered active, which becomes manifest through activity-enhanced diffusivities [15,16] and depletion forces [8,[17][18][19]. Employing shape-anisotropic colloids [19][20][21] or manufacturing activity gradients [22], these effects can be used to generate directed motion of the colloids.…”

Depletion forces on circular and elliptical obstacles induced by active matter

“…Hence, the total entropy increases, as shown in the pioneering paper of Asakura and Oosawa [19] almost fifty years ago. However, the excluded volume effect observed in equilibrium systems can not be the sole cause for the effective interaction between passive colloids in a bath of active particles [20]. Instead, this effect, added to the peculiar non-equilibrium features of the dynamics of the self-propelled particles, generates an effective attractive or repulsive interaction which depends, e.g.…”

Motility fractionation of bacteria by centrifugation

“…Flagellar motility also plays an important role in medicine, being a major contributing factor to pathogenicity and colonization in bacteria like Vibrio cholerae [2][3][4] . More recently, the possibility of exploiting swimming micro-organisms as actuators for microstructures has extended the interest for flagellar motility to the physical domain of micro-engineering applications [5][6][7][8][9][10][11] . Recognizing the primary role of motility has led to the development of new tools that are capable of a precise and quick characterization of the dynamical properties of cells.…”