Competing active and passive interactions drive amoebalike crystallites and ordered bands in active colloids

Abstract

Swimmers and self-propelled particles are physical models for the collective behavior and motility of a wide variety of living systems, such as bacteria colonies, bird flocks, and fish schools. Such artificial active materials are amenable to physical models which reveal the microscopic mechanisms underlying the collective behavior. Here we study colloids in a dc electric field. Our quasi-two-dimensional system of electrically driven particles exhibits a rich and exotic phase behavior exhibiting passive crystallites, motile crystallites, an active gas, and banding. Amongst these are two mesophases, reminiscent of systems with competing interactions. At low field strengths activity suppresses demixing, leading to motile crystallites. Meanwhile, at high field strengths, activity drives partial demixing to traveling bands. We parametrize a particulate simulation model which reproduces the experimentally observed phases.

Dynamics and Interactions of Quincke Roller Clusters: from Orbits and Flips to Excited States

“…We emphasize that the collision processes here are different to the phoretic and hydrodynamic interactions of Janus particles [1] and Quincke rollers [11]. The formation of tetramers and hexamers is due to dynamical selftrapping, akin to systems displaying phase separation [6].…”

“…In recent years, much effort has been devoted to investigating the motility of microorganisms, driven, e.g., by flagellae and to the realisation of synthetic active matter by means of diffusophoretic [1], thermophoretic [2], field driven [3] and vibrated particles [4]. These active particles exhibit fascinating collective phenomena not found in passive systems, such as flock formation [3,5], dynamical clustering and phase separation [6][7][8], anomalous density fluctuations [9], vortices [10] activitydependent phase behaviour [11,12] and novel dynamical transitions [13]. While many experiments have focused on self-propelled spherical colloids, e.g.…”

“…In active colloidal systems attention has often focussed on assembly of large numbers of particles, i.e. the emergence of macroscopic states or active "phase behavior" [3,11,27,28]. By contrast, assembly of fixed numbers of passive colloids, often through careful control of interactions [29] has led to supracolloidal chemistry with reaction pathways at the colloidal rather than molecular, level [30][31][32][33][34][35][36][37][38][39][40][41][42][43] which exhibit some aspects of moleculae interactions [35,44].…”

Crystallisation and Polymorph Selection in Active Brownian Particles

“…This is due in no small part to the difficulties in stabilising active colloids at high density against aggregation. However, recently this has begun to change and excluded volume interactions have driven ordering in a few experiments in two dimensions [45][46][47][48]. The study of 3d active colloids is in its infancy, however one system that has emerged of active multi-polar colloids [49] does exhibit crystallisation to a variety of polymorphs also exhibited by related passive dipolar colloids [50,51].…”

Collective Motion of Quincke Rollers with Fully Resolved Hydrodynamics

“…A variety of distinct regimes have been observed in experimental studies, including gas, polar liquid, active solid and cluster states [1,[3][4][5][6][7][8], as well as an orderdisorder transition between the polar liquid and gas states [1], mobility enhancement due to paired-up states [3], and solidification at high density [5]. Numerical studies have also been conducted, mostly using twodimensional particle-based models.…”