Micro-phase separation in two dimensional suspensions of self-propelled spheres and dumbbells

Abstract

We use numerical simulations to study the phase behavior of self-propelled spherical and dumbbellar particles interacting via micro-phase separation inducing potentials. Our results indicate that under the appropriate conditions, it is possible to drive the formation of two new active states; a spinning cluster crystal, i.e. an ordered mesoscopic phase having finite size spinning crystallites as lattice sites, and a fluid of living clusters, i.e. a two dimensional fluid where each "particle" is a finite size living cluster. We discuss the dynamics of these phases and suggest ways of extending their stability under a wide range of active forces. INTRODUCTIONSpontaneous pattern formation is a ubiquitous phenomenon in nature and arises in both equilibrium and out-ofequilibrium systems. Apart from the many biological examples [1,2], important synthetic systems such as colloids and block copolymers have been shown to exhibit complex spatial equilibrium patterns upon self-assembly. Control of patterns at the micro and nanoscale is integral to the development of materials with novel optical, electrical, and rheological properties [3].One route to spontaneous pattern formation in equilibrium systems is achieved via micro-phase separation [4], a phenomenon that typically occurs when geometrical or chemical constraints prevent a system from fully phase separating. Block copolymers, for instance, exhibit a wide variety of patterns upon micro-phase separation that can be controlled by tuning the relative length of the two blocks, from lamellae to cylinders to networks [5][6][7]. An alternative route is achieved with competing interactions. In colloidal supensions, for instance, the interplay between a short range attraction and a long range repulsion has been shown to lead to micro-phase separation into a variety of patterns [8] with symmetry dependent on the relative weight of the two interactions [9][10][11][12][13]. In these cases, the short range attraction is usually induced by depletion, hydrophobic or van der Waals forces, while the long range repulsion may come from dipolar forces or screened electrostatics [14][15][16][17].Recent experiments in two dimensions have shown that dilute suspensions of self-propelled colloidal particles can self-assemble into "dynamic", "living" crystals [18][19][20], where finite-sized aggregates continually join, break apart, dissolve, and reform. In this case, what limits the growth of the crystal to a macroscopic size − the scenario that would be favored due to inter-particle attractive interactions − is the self-propulsion of the active particles. This behavior seems to be specific to spherical particles, as these are able to freely re-orient within the developing crystallites under the influence of thermal forces, thus creating large stresses within the crystal. The formation of living clusters has also been recently observed in computer simulations of a three dimensional diluted suspension of self-propelled attractive spheres [21,22]; however, in three dimensions clusters did not ...

Dimensionality matters in the collective behaviour of active emulsions

“…Non-doped clusters do not rotate. In this respect, our system differs from simulations of dumbbell aggregates [53,54] and studies of bacterial micro-rotors [55], where rotation is caused by the random addition of individual torques and there is no global chirality in the system.…”

Morphology of clusters of attractive dry and wet self-propelled spherical particle suspensions

“…It has been shown that repulsive active rods exhibit a phaseseparated state characterized by the formation of polar clusters [32]. Similarly, dilute suspensions of active dumbbells form rotating clusters when particles interact both via a short-range attraction and long-range repul-sion [28] and an isotropic attraction [33], in the latter case displaying a nematic order with spiral patterns in two dimensions [34].…”

“…Inspired by experiments on dilute suspensions of colloids [35] and bacteria [33], showing cluster formation, and motivated by understanding how hydrodynamics could affect the formation of living clusters [27,28,36], we have carried out a systematic numerical analysis on dilute 2D suspensions of attractive self-propelled spheres both in the presence (wet active system) and absence (dry active system) of hydrodynamic interactions [37]. We identify the conditions at which the system forms clusters, and characterize their morphology considering and neglecting hydrodynamics.…”

Dewetting and spreading transitions for active matter on random pinning substrates

“…One of the simplest models of active matter is monodisperse sterically interacting disks undergoing active Brownian motion or run and tumble dynamics. These can transition from a uniform liquid state to a clump or phase separated state with increasing disk density, increasing persistence length [15][16][17][18][19][20] or increasing run length [21][22][23] . In the phase separated regime, which appears even in the absence of an attractive component in the particle-particle interactions, large clumps of densely packed disks are separated by a low density gas of active particles.…”