Self-organisation and convection of confined magnetotactic bacteria

Abstract

Collective motion is found at all scales in biological and artificial systems, and extensive research is devoted to describing the interplay between interactions and external cues in collective dynamics. Magnetotactic bacteria constitute a remarkable example of living organisms for which motion can be easily controlled remotely. Here, we report a new type of collective motion where a uniform distribution of magnetotactic bacteria is rendered unstable by a magnetic field. A new state of "bacterial magneto-convection" results, wherein bacterial plumes emerge spontaneously perpendicular to an interface and develop into self-sustained flow convection cells. While there are similarities to gravity driven bioconvection and the Rayleigh-Bénard instability, these rely on a density mismatch between layers of the fluids. Remarkably, here no external forces are applied on the fluid and the magnetic field only exerts an external torque aligning magnetotactic bacteria with the field. Using a theoretical model based on hydrodynamic singularities, we capture quantitatively the instability and the observed long-time growth. Bacterial magneto-convection represents a new class of collective behaviour resulting only from the balance between hydrodynamic interactions and external alignment.

Controlling confined collective organisation with taxis

“…1d and 3a, extensile force dipoles ( s ) create a lateral attractive flow [37]. For aligned pusher swimmers parallel to one another, in particular for bacteria swimming perpendicular to a plane wall, this lateral flows results in a well-known attraction [38] and clustering [11, 14]. As a result, for sufficiently strong fields the swimmers from a population in the drop align, attract, and form clusters that move cohesively on the boundary.…”

“…For example, magnetotactic bacteria (MTB) exhibit magnetic moments and align to external fields [8]. In suspensions, this leads to pearling instabilities [9,10], boundary-mediated clustering [11][12][13] and plume formation [14]. A consistent feature of biased collective dynamics is the prominence of confinement-mediated interactions; oriented swimmers tend to accumulate at boundaries and create dense regions where self-organisation occurs [14][15][16].…”

“…In suspensions, this leads to pearling instabilities [9,10], boundary-mediated clustering [11][12][13] and plume formation [14]. A consistent feature of biased collective dynamics is the prominence of confinement-mediated interactions; oriented swimmers tend to accumulate at boundaries and create dense regions where self-organisation occurs [14][15][16]. Given its significant potential for biological and biomedical control, a predictive framework for such self-organisation is needed.…”

The narrow escape problem of a chiral active particle (CAP): an optimal scheme

“…Henceforth, the study of active systems confined within enclosed spaces has garnered substantial attention from various groups since the past few decades. [10][11][12][13][14][15][16][17][18][19][20] Furthermore, various research groups [21][22][23] have explored geometrical confinement as a means to segregate a mixture of active particles with different activities.…”

Gyrotactic cluster formation of bottom-heavy squirmers

“…Investigating how active particles react on external fields reveals novel aspects of these non-equilibrium systems [14,15]. This includes sedimentation of active particles [16,17,18,19,20,21], microswimmers in magnetic fields [22,23,24], their pearling transition and plume formation in microchannels [25,26], as well as the question of optimal steering of active entities under flow and in a potential landscape [27,28,29].…”