Dynamical Clustering and Phase Separation in Suspensions of Self-Propelled Colloidal Particles

Abstract

We study experimentally and numerically a (quasi) two dimensional colloidal suspension of selfpropelled spherical particles. The particles are carbon-coated Janus particles, which are propelled due to diffusiophoresis in a near-critical water-lutidine mixture. At low densities, we find that the driving stabilizes small clusters. At higher densities, the suspension undergoes a phase separation into large clusters and a dilute gas phase. The same qualitative behavior is observed in simulations of a minimal model for repulsive self-propelled particles lacking any alignment interactions. The observed behavior is rationalized in terms of a dynamical instability due to the self-trapping of self-propelled particles.PACS numbers: 82.70. Dd,64.60.Cn Following our physical intuition, "agitating" a system by, e.g., increasing the temperature also increases disorder. The most simple and paradigmatic example is the Ising model of interacting spins on a lattice, which, in two or more dimensions, displays a second-order phase transition from an ordered state to a disordered state as we increase the temperature [1]. Non-equilibrium driven systems, however, may defy our intuition and show the opposite behavior: increasing the noise strength leads to the emergence of an ordered state [2,3], for example the "freezing by heating" transition of oppositely driven particles in a narrow channel [4].One class of non-equilibrium systems that currently receives considerable attention are self-propelled, or "active", particles [5][6][7][8][9][10][11][12][13]. These are model systems for "living active matter" ranging from microtubules [14] to dense bacterial solutions [15][16][17] to flocks of birds [18]. A common feature of many of these models is that the particle orientations align, which leads to a multitude of collective phenomena such as swarming [19] and even micro-bacterial turbulence [20]. This alignment interaction can be either explicit (Vicsek-type models [21]) or indirect. For example, in dense granular systems of rods [22] and disks [23], the combination of hardcore repulsion and propulsion implies an effective alignment. Somewhat surprisingly, recently it has been found that also self-propelled suspensions lacking any alignment mechanism are able to show collective behavior. Specifically, simulations of a minimal model for a suspension of repulsive disks below the freezing transition [24] show phase separation into a dense large cluster and a dilute gas phase [25,26]. Phase separation due to a densitydependent mobility has been discussed theoretically in the context of run-and-tumble bacteria [27], and a link has been made recently to self-propelled Brownian particles [28].Experimentally, active clustering of spherical colloidal particles has been observed for sedimenting, platinumcoated gold particles [10] and colloidal particles with an embedded hematite cube [13], where platinum and hematite act as catalysts for the decomposition of water peroxide. In both studies, aggregation is attributed to attractive forces. In thi...

Hydrodynamic simulations of self-phoretic microswimmers

“…Self-phoretic effects have shown to be an effective and promising strategy to design such artificial microswimmers [3-5, 7, 10-12], where the microswimmers are driven by gradient fields locally produced by swimmers themselves in the surrounding solvent. In particular, the collective behavior of a suspension of selfdiffusiophoretic swimmers has recently been studied in experiments [13][14][15][16].…”

The force on a boundary in active matter

“…Owing to the particles' self-motion, active matter can spontaneously phase-separate into dense and dilute regions (Cates et al 2010;Fily & Marchetti 2012;Buttinoni et al 2013;Palacci et al 2013;Stenhammar et al 2013Stenhammar et al , 2014Takatori, Yan & Brady 2014;Wysocki, Winkler & Gompper 2014;Yang, Manning & Marchetti 2014;Takatori & Brady 2015) and can move collectively under an orienting field .…”

First-order phase transition in a model of self-propelled particles with variable angular range of interaction

“…Biological systems such as schools of fish [1], flocks of birds [2,3], bacterial colonies [4][5][6], artificial systems such as 'Kobots' (robots specially developed for the study of flocking) [7], platinum-silica particles in hydrogen peroxide solution [8], carbon coated Janus particles in water-lutidine mixture [9], and vibrating rods [10][11][12] are some examples. One remarkable characteristic of these systems is that under certain conditions they are capable of displaying extraordinary collective dynamics, such as highly cooperative collective motion and complex moving patterns [13][14][15][16][17][18][19].…”