| 1. |
- Bárdfalvy, Dóra, et al.
(author)
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Collective motion in a sheet of microswimmers
- 2024
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In: Communications Physics. - 2399-3650. ; 7:1
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Journal article (peer-reviewed)abstract
- Self-propelled particles such as bacteria or algae swimming through a fluid are non-equilibrium systems where particle motility breaks microscopic detailed balance, often resulting in large-scale collective motion. Previous theoretical work has identified long-ranged hydrodynamic interactions as the driver of collective motion in unbounded suspensions of rear-actuated (“pusher”) microswimmers. In contrast, most experimental studies of collective motion in microswimmer suspensions have been carried out in restricted geometries where both the swimmers’ motion and their long-range flow fields become altered due to the proximity of a boundary. Here, we study numerically a minimal model of microswimmers in such a restricted geometry, where the particles move in the midplane between two no-slip walls. For pushers, we demonstrate collective motion with short-ranged order, in contrast with the long-ranged flows observed in unbounded systems. For front-actuated (“puller”) microswimmers, we discover a long-wavelength density instability resulting in the formation of dense microswimmer clusters. Both types of collective motion are fundamentally different from their previously studied counterparts in unbounded domains. Our results show that this difference is dictated by the geometrical restriction of the swimmers’ motion, while hydrodynamic screening due to the presence of a wall is subdominant in determining the suspension’s collective state.
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| 2. |
- Bárdfalvy, Dóra, et al.
(author)
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Particle-resolved lattice Boltzmann simulations of 3-dimensional active turbulence
- 2019
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In: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-6848 .- 1744-683X. ; 15:39, s. 7747-7756
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Journal article (peer-reviewed)abstract
- Collective behaviour in suspensions of microswimmers is often dominated by the impact of long-ranged hydrodynamic interactions. These phenomena include active turbulence, where suspensions of pusher bacteria at sufficient densities exhibit large-scale, chaotic flows. To study this collective phenomenon, we use large-scale (up to N = 3 × 106) particle-resolved lattice Boltzmann simulations of model microswimmers described by extended stresslets. Such system sizes enable us to obtain quantitative information about both the transition to active turbulence and characteristic features of the turbulent state itself. In the dilute limit, we test analytical predictions for a number of static and dynamic properties against our simulation results. For higher swimmer densities, where swimmer-swimmer interactions become significant, we numerically show that the length- and timescales of the turbulent flows increase steeply near the predicted finite-system transition density.
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| 3. |
- Bardfalvy, Dora
(author)
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Simulation of collective phenomena in microswimmer suspensions
- 2020
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Doctoral thesis (other academic/artistic)abstract
- Collective motion is ubiquitous in biological and synthetic systems across manylength- and timescales. On the macroscopic scale, examples include schools of fish, herds of sheep and flocks of birds. On the microscopic scale, bacteria, algae and synthetic self-propelled particles exhibit a range of collective phenomena. In suspensions of swimming bacteria, collective motion is often caused by hydrodynamic interactions between the swimmers, and is manifested as long-ranged chaotic flows, dubbed active turbulence. In this work, we study collective motion in simplified models of bacterial and algal suspensions with particle-resolved lattice Boltzmann simulations. Using anextended force dipole as a minimal model for a microswimmer, we have been able to study large systems, containing up to 3 × 10^6 particles, and to capture information about large-scale collective behaviours. We have studied four separate aspects of collective motion in microswimmer suspensions. First, we performed unprecedentedly large simulations of 3-dimensional active suspensions to test predictions from kinetic theory about the transition to active turbulence and characterize the ensuing turbulent state. The focus was then turned to the effects of swimming velocity on the transition to active turbulence of pusher suspensions. In nature, front- and rearactuated microswimmers (so called pushers and pullers, respectively) coexist, which motivated us to study how the presence of pullers in the suspension changes the collective behaviour of pushers. Finally, motivated by the fact that most experiments are performed in 2-dimensional geometries, we also investigated and characterized the collective phenomena in a quasi-2-dimensional system, finding important qualitative differences compared to unbounded suspensions.
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| 4. |
- Bárdfalvy, Dóra, et al.
(author)
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Symmetric Mixtures of Pusher and Puller Microswimmers Behave as Noninteracting Suspensions
- 2020
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In: Physical Review Letters. - 0031-9007. ; 125:1
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Journal article (peer-reviewed)abstract
- Suspensions of rear- and front-actuated microswimmers immersed in a fluid, known respectively as "pushers"and "pullers,"display qualitatively different collective behaviors: beyond a characteristic density, pusher suspensions exhibit a hydrodynamic instability leading to collective motion known as active turbulence, a phenomenon which is absent for pullers. In this Letter, we describe the collective dynamics of a binary pusher-puller mixture using kinetic theory and large-scale particle-resolved simulations. We derive and verify an instability criterion, showing that the critical density for active turbulence moves to higher values as the fraction χ of pullers is increased and disappears for χ≥0.5. We then show analytically and numerically that the two-point hydrodynamic correlations of the 1:1 mixture are equal to those of a suspension of noninteracting swimmers. Strikingly, our numerical analysis furthermore shows that the full probability distribution of the fluid velocity fluctuations collapses onto the one of a noninteracting system at the same density, where swimmer-swimmer correlations are strictly absent. Our results thus indicate that the fluid velocity fluctuations in 1:1 pusher-puller mixtures are exactly equal to those of the corresponding noninteracting suspension at any density, a surprising cancellation with no counterpart in equilibrium long-range interacting systems.
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| 5. |
- Škultéty, Viktor, et al.
(author)
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Hydrodynamic instabilities in a two-dimensional sheet of microswimmers embedded in a three-dimensional fluid
- 2024
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In: Journal of Fluid Mechanics. - 0022-1120. ; 980
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Journal article (peer-reviewed)abstract
- A collection of microswimmers immersed in an incompressible fluid is characterised by strong interactions due to the long-range nature of the hydrodynamic fields generated by individual organisms. As a result, suspensions of rear-actuated 'pusher' swimmers such as bacteria exhibit a collective motion state often referred to as 'bacterial turbulence', characterised by large-scale chaotic flows. The onset of collective motion in pusher suspensions is classically understood within the framework of mean-field kinetic theories for dipolar swimmers. In bulk two and three dimensions, the theory predicts that the instability leading to bacterial turbulence is due to mutual swimmer reorientation and sets in at the largest length scale available to the suspension. Here, we construct a similar kinetic theory for the case of a dipolar microswimmer suspension restricted to a two-dimensional plane embedded in a three-dimensional incompressible fluid. This setting qualitatively mimics the effect of swimming close to a two-dimensional interface. We show that the in-plane flow fields are effectively compressible in spite of the incompressibility of the three-dimensional bulk fluid, and that microswimmers on average act as sources (pushers) or sinks (pullers). We analyse the stability of the homogeneous and isotropic state, and find two types of instability that are qualitatively different from the bulk, three-dimensional case: first, we show that the analogue of the orientational pusher instability leading to bacterial turbulence in bulk systems instead occurs at the smallest length scale available to the system. Second, an instability associated with density variations arises in puller suspensions as a generic consequence of the effective in-plane compressibility. Given these qualitative differences with respect to the standard bulk setting, we conclude that confinement can have a crucial role in determining the collective behaviour of microswimmer suspensions.
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