Bias- and bath-mediated pairing of particles driven through a quiescent medium

Abstract

A particle driven by an external force in a molecular crowding environment—a quiescent bath of other
particles, makes their spatial distribution inhomogeneous: the bath particles accumulate in front of the
biased particle (BP) and are depleted behind. In fact, a BP travels together with the inhomogeneity it
creates. A natural question is what will happen with two BPs when they appear sufficiently close to each
other such that the inhomogeneities around each of them start to interfere? In quest for the answer we
examine here, via Monte Carlo simulations, the dynamics of two BPs in a lattice gas of bath particles.
We observe that for a sufficiently dense medium, surprisingly, both BPs spend most of the time together
which signifies that the interference of the microstructural inhomogeneities results in effectively
attractive interactions between them. Such statistical pairing of BPs minimizes the size of the
inhomogeneity and hence reduces the frictional drag force exerted on the BPs by the medium. As
a result, in some configurations the center-of-mass of a pair of BPs propagates faster than a single
isolated BP. These jamming-induced forces are very different from fundamental physical interactions,
exist only in presence of an external force, and require the presence of a quiescent bath to mediate the
interactions between the driven particles.