Symmetric Mixtures of Pusher and Puller Microswimmers Behave as Noninteracting Suspensions

Bárdfalvy, Dóra; Anjum, Shan; Nardini, Cesare; Morozov, Alexander; Stenhammar, Joakim

doi:10.1103/physrevlett.125.018003

Cited by 22 publications

(17 citation statements)

References 49 publications

(90 reference statements)

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“…28 × 10 −8 kg/s is the calculated drag coefficient of bacterial body orientated along its major axis, which is slightly smaller than the measured value shown above. The linear stability analysis of the original kinetic theories can be easily extended for the mixture of mobile and immobile bacteria (14,44), which results in f as the prefactor of v in Eq. 2.…”

Section: D Phase Diagrammentioning

confidence: 99%

Imaging the emergence of bacterial turbulence: Phase diagram and transition kinetics

2021

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We experimentally study the emergence of collective bacterial swimming, a phenomenon often referred to as bacterial turbulence. A phase diagram of the flow of 3D Escherichia coli suspensions spanned by bacterial concentration, the swimming speed of bacteria, and the number fraction of active swimmers is systematically mapped, which shows quantitative agreement with kinetic theories and demonstrates the dominant role of hydrodynamic interactions in bacterial collective swimming. We trigger bacterial turbulence by suddenly increasing the swimming speed of light-powered bacteria and image the transition to the turbulence in real time. Our experiments identify two unusual kinetic pathways, i.e., the one-step transition with long incubation periods near the phase boundary and the two-step transition driven by long-wavelength instabilities deep inside the turbulent phase. Our study provides not only a quantitative verification of existing theories but also insights into interparticle interactions and transition kinetics of bacterial turbulence.

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Section: D Phase Diagrammentioning

confidence: 99%

Imaging the emergence of bacterial turbulence: Phase diagram and transition kinetics

2021

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“…There has been considerable work examining the interactions of other kinds of microswimmers, e.g., phoretic particles [46], active droplets [26], active particles [10] and mixed suspensions [47]. However, in addition to hydrodynamic interactions, chemical trails can be far-reaching, and therefore affect the far-field dynamics.…”

Section: Discussionmentioning

confidence: 99%

Pairwise scattering and bound states of spherical microorganisms

C.¹,

Ishikawa²,

Pedley³

et al. 2021

Preprint

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The dynamic interactions between pairs of swimming microorganisms underpin the collective behaviour of larger suspensions, but accurately calculating pairwise collisions has typically required the use of numerical simulations in which hydrodynamic interactions are fully resolved. In this paper, we utilise analytical expressions for forces and torques acting on two closely separated spherical squirmers -accurate to second order in the ratio of cell-cell spacing to squirmer radius -in order to calculate their scattering dynamics. Attention is limited to squirmers whose orientation vectors lie in the same plane. We characterise the outgoing angles of pairs of bottom-heavy squirmers in terms of their incoming angles, the squirmer parameter β, and the strength of the external gravitational field, discovering transient scattering, stationary bound states, pairwise swimming motion, and circular orbits. These results compare well with full numerical solutions obtained using boundary element methods, highlighting the utility of lubrication theory. We expect these results will be useful for the foundations of mesoscale continuum models for suspensions of spherical microorganisms.

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“…These can in turn be related to physical units by rescaling with the corresponding dipole lengths and swimming speeds of the experimental system in question: for E. coli, l ≈ 2 µm, v s ≈ 20 µm/s, and F ≈ 0.4 pN [7]. The model introduced in the previous Section has been extensively used to study the onset of collective motion in dilute suspensions of pusher-like microswimmers [35,39,49], and its phenomenology is wellunderstood. Below the threshold value n c of the number density n = N/V of microswimmers, the suspension remains homogeneous and isotropic while exhibiting significant correlations between microswimmers.…”

Section: Model and Methodsmentioning

confidence: 99%

Anisotropic diffusion of ellipsoidal tracers in microswimmer suspensions

Nordanger,

Morozov,

Stenhammar

2021

Preprint

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Tracer particles immersed in suspensions of biological microswimmers such as E. coli or Chlamydomonas display phenomena unseen in conventional equilibrium systems, including strongly enhanced diffusivity relative to the Brownian value and non-Gaussian displacement statistics. In dilute, 3-dimensional suspensions, these phenomena have typically been explained by the hydrodynamic advection of point tracers by isolated microswimmers, while, at higher concentrations, correlations between pusher microswimmers such as E. coli can increase the effective diffusivity even further. Anisotropic tracers in active suspensions can be expected to exhibit even more complex behaviour than spherical ones, due to the presence of a nontrivial translation-rotation coupling. Using largescale lattice Boltzmann simulations of model microswimmers described by extended force dipoles, we study the motion of ellipsoidal point tracers immersed in 3-dimensional microswimmer suspensions. We find that the rotational diffusivity of tracers is much less affected by swimmer-swimmer correlations than the translational diffusivity. We furthermore study the anisotropic translational diffusion in the particle frame and find that, in pusher suspensions, the diffusivity along the ellipsoid major axis is higher than in the direction perpendicular to it, albeit with a smaller ratio than for Brownian diffusion. Thus, we find that far field hydrodynamics cannot account for the anomalous coupling between translation and rotation observed in experiments, as was recently proposed. Finally, we study the probability distributions (PDFs) of translational and rotational displacements. In accordance with experimental observations, for short observation times we observe strongly non-Gaussian PDFs that collapse when rescaled with their variance, which we attribute to the ballistic nature of tracer motion at short times.

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Symmetric Mixtures of Pusher and Puller Microswimmers Behave as Noninteracting Suspensions

Cited by 22 publications

References 49 publications

Imaging the emergence of bacterial turbulence: Phase diagram and transition kinetics

Imaging the emergence of bacterial turbulence: Phase diagram and transition kinetics

Pairwise scattering and bound states of spherical microorganisms

Anisotropic diffusion of ellipsoidal tracers in microswimmer suspensions

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