Active binary mixtures of fast and slow hard spheres

Abstract

We computationally studied the phase behavior and dynamics of binary mixtures of active particles, where each 'species' had distinct activities leading to distinct velocities, fast and slow. We obtained phase diagrams demonstrating motility-induced phase separation (MIPS) upon varying the activity and concentration of each species, and extended current kinetic theory of active/passive mixtures to active/active mixtures. We discovered two regimes of behavior quantified through the participation of each species in the dense phase compared to their monodisperse counterparts. In regime I (active/passive and active/weakly-active), we found that the dense phase was segregated by particle type into domains of fast and slow particles. Moreover, fast particles were suppressed from entering the dense phase while slow particles were enhanced entering the dense phase, compared to monodisperse systems of all-fast or all-slow particles. These effects decayed asymptotically as the activity of the slow species increased, approaching the activity of the fast species until they were negligible (regime II). In regime II, the dense phase was homogeneously mixed and each species participated in the dense phase as if it were it a monodisperse system; each species behaved as if it weren't mixed at all. Finally, we collapsed our data defining two quantities that measure the total activity of the system. We thus found expressions that predict, a priori, the percentage of each particle type that participates in the dense phase through MIPS.

Fast and slow self-propelled particles interacting with asymmetric obstacles: Wetting, segregation, rectification, and vorticity

“…In fact, the passive particles continue to contribute positively to the total v x . Such behavior where the motion of passive particles is "enhanced" by active ones has been previously reported in the context of motilityinduced phase separation: the presence of active particles in fact induces clustering for the passive ones [28,30,56]. Here, what we find is that the active particles induce a finite degree of rectification for the passive ones, which increases with φ.…”

“…However, in natural colonies of bacteria and other microorganisms, a broad dispersion of motility parameters exists due to different ages, reproduction stages, shapes, sizes, and running modes [20][21][22][23]. For either passive or active fluids, it is known that "diversity" of some particle attribute generates several new phase behavior phenomena, including changing the nature and loci of phase diagram boundaries and introducing particle-type spatial segregation [4,[24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41]. Still, it remains unclear what are the effects of particle diversity on active rectification by convex asymmetric obstacles.…”

Scalar Active Mixtures: The Nonreciprocal Cahn-Hilliard Model

“…Formation of spatial structure has also been reported in multicomponent mixtures of self-propelled active particles with different average speeds [27]. Significant departure from equilibrium behavior is observed even with isotropic mutual interactions.…”

Active Matter Shepherding and Clustering in Inhomogeneous Environments

“…There have been several studies examining bidisperse mixtures of passive and active particles which have shown that both motility induced phase separation and species separation can occur [61][62][63][64]. It has also been demonstrated that active particles can move passive particles and organize them into particular states even when there are only a small number of active particles present, which is known as active doping [65][66][67][68].…”