Emergent vortices in populations of colloidal rollers

Abstract

Coherent vortical motion has been reported in a wide variety of populations including living organisms (bacteria, fishes, human crowds) and synthetic active matter (shaken grains, mixtures of biopolymers), yet a unified description of the formation and structure of this pattern remains lacking. Here we report the self-organization of motile colloids into a macroscopic steadily rotating vortex. Combining physical experiments and numerical simulations, we elucidate this collective behaviour. We demonstrate that the emergent-vortex structure lives on the verge of a phase separation, and single out the very constituents responsible for this state of polar active matter. Building on this observation, we establish a continuum theory and lay out a strong foundation for the description of vortical collective motion in a broad class of motile populations constrained by geometrical boundaries.

Density Shock Waves in Confined Microswimmers

“…Our results are consistent with experimental observations on driven droplets and self-propelled colloids, [3,15,18,19], but go beyond these observations to report an unobserved transition from 'subsonic' to 'supersonic' compression shocks as the intensity of the background flow increases. The physical mechanisms underlying this transition are elucidated via discrete simulations and a continuum model that properly captures the effects of HI and geometric confinement.…”

“…The effects of confinement to narrow flow channels are less well understood; see, e.g., [10][11][12][13][14] and references therein. Recent experiments on driven droplets [3,[15][16][17] and self-propelled colloids [18,19] in quasi two-dimensional (Hele-Shaw type) channels show the emergence of traveling density waves, including density shocks at the wave front. Similar observations are reported in simulations of confined self-propelled particles [20], albeit with density shocks forming at the back of the wave.…”

Dynamics of inert spheres in active suspensions of micro-rotors

“…We anticipate growing interest in the topic of microrotor given the increase of practical realizations of selfrotating particles in synthetic systems, e.g., applying a torque to a particle by magnetic [24-26, 43, 44], electrical [27,28] or optical fields [45], or biological systems, e.g., T. majus bacteria [46], Volvox algae [47] or aggregates of swimming bacteria [48]. We hope our numerical work will provide useful insights to understand the behavior of existing experimental systems and design new ones.…”

“…Unlike self-propelled particles such as bacteria and colloids, spinning particles ("rotors") and their collective dynamics are far less explored [23] mostly because there are fewer experimental realizations, e.g., magneticallydriven colloids [24][25][26] and electrically-driven "Quincke" colloids [27,28]. Self-organization in rotor suspensions have been analyzed mostly computationally [29][30][31][32][33], though not all studies consider the effects of the immersing liquid on the rotor motions.…”

Self-Driven Droplet Powered By Active Nematics

“…Alternatively, we can take advantage of the particle interactions, either individually or collectively, with obstacles and geometric boundaries to manipulate the system more directly. By trapping active suspensions (such as Pusher swimmers or Quincke rollers) within the straight and curved boundaries, stable flow patterns, such as unidirectional circulations, traveling waves, density shocks, and rotating vortices, have already been constructed [17][18][19][20][21][22][23]. More interestingly, active polar gels under soft confinement by surface tension are able to generate internal flows to break symmetry and drive the whole-body movement [24][25][26].…”