Geometric control of active collective motion

Abstract

Recent experimental studies have shown that confinement can profoundly affect selforganization in semi-dilute active suspensions, leading to striking features such as the formation of steady and spontaneous vortices in circular domains and the emergence of unidirectional pumping motions in periodic racetrack geometries. Motivated by these findings, we analyze the two-dimensional dynamics in confined suspensions of active self-propelled swimmers using a mean-field kinetic theory where conservation equations for the particle configurations are coupled to the forced Navier-Stokes equations for the self-generated fluid flow. In circular domains, a systematic exploration of the parameter space casts light on three distinct states: equilibrium with no flow, stable vortex, and chaotic motion, and the transitions between these are explained and predicted quantitatively using a linearized theory. In periodic racetracks, similar transitions from equilibrium to net pumping to traveling waves to chaos are observed in agreement with experimental observations and are also explained theoretically. Our results underscore the subtle effects of geometry on the morphology and dynamics of emerging patterns in active suspensions and pave the way for the control of active collective motion in microfluidic devices.

Self-organized dynamics and the transition to turbulence of confined active nematics

“…To quantify this phenomena, we defined the signed order parameter that indicates the degree of flow circulation as Φ (t) = u ·ê θ / |u| , where u is the velocity field extracted from the PIV algorithm ( Fig.5C). Φ = ±1 for system-scale coherent circular flows, while Φ = 0 indicates lack of net transport along the azimuthal direction [31,37]. Temporal evolution of the flow order parameter confirms that weakly confined active nematics exhibited persistent circular flows that switched handedness on the time scale of tens of minutes ( Fig.5D).…”

“…1). The hard-wall boundaries enforce both the no-slip condition and the parallel anchoring that are readily described by hydrodynamic models of confined active nematics [21][22][23][24][25][26].…”

Magnetotactic bacteria in a droplet self-assemble into a rotary motor

“…Like the many artificial active systems recently proposed to tackle this question [11][12][13][14][15], assemblies of motile bacteria turned out to be a rich and insightful experimental playground [16][17][18][19][20][21][22][23]. Among the rich topics that were investigated, the confinement of bacteria and of active particles has been the focus of many experimental [24][25][26][27] and theoretical studies [27,28], showing that, under strong confinement, vortical collective motions may spontaneously appear.…”

Gravity induced formation of spinners and polar order of spherical microswimmers on a surface

“…On the micrometer length scale, a natural example of active materials is given by bacteria [4,12], while recently artificial active materials, based on colloidal particles, have become an important tool to study collective motion in laboratory [13][14][15]. A large variety of different collectively moving states have been reported [16][17][18][19][20][21][22] and geometrical constraint can strongly affect the dynamics [17,20,21,[23][24][25][26][27]. Examples of this, from bacterial world, include the hydrodynamic stabilisation of rotating Volvox pairs [28] due to a interplay between sedimentation and hydrodynamic effects and the formation of vortex arrays [29] near confining surfaces, while guiding [30,31] and flow-induced phase separation [32] have been observed with artificial swimmers in confinement.…”