| 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. |
- Honkonen, J., et al.
(author)
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Critical behavior of directed percolation process in the presence of compressible velocity field
- 2017
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In: CHAOS 2017 - Proceedings. - : ISAST: International Society for the Advancement of Science and Technology. ; , s. 383-402
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Conference paper (peer-reviewed)abstract
- Various systems exhibit universal behavior at the critical point. A typical example of the non-equilibrium critical behavior is the directed bond percolation that exhibits an active-to-absorbing state phase transition in the vicinity of critical percolation probability. An interesting question is how the turbulent mixing influences its critical behavior. In this work we assume that the turbulent mixing is generated by the compressible Navier-Stokes equation where the compressibility is described by an additional field related to the density. Using field-theoretic models and renormalization group methods we investigate large scale and long time behavior.
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| 3. |
- Honkonen, J., et al.
(author)
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Influence of turbulent mixing on critical behavior of directed percolation process : Effect of compressibility
- 2018
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In: Physical review. E. - 2470-0045 .- 2470-0053. ; 97:2
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Journal article (peer-reviewed)abstract
- Universal behavior is a typical emergent feature of critical systems. A paramount model of the nonequilibrium critical behavior is the directed bond percolation process that exhibits an active-to-absorbing state phase transition in the vicinity of a percolation threshold. Fluctuations of the ambient environment might affect or destroy the universality properties completely. In this work, we assume that the random environment can be described by means of compressible velocity fluctuations. Using field-theoretic models and renormalization group methods, we investigate large-scale and long-time behavior. Altogether, 11 universality classes are found, out of which 4 are stable in the infrared limit and thus macroscopically accessible. In contrast to the model without velocity fluctuations, a possible candidate for a realistic three-dimensional case, a regime with relevant short-range noise, is identified. Depending on the dimensionality of space and the structure of the turbulent flow, we calculate critical exponents of the directed percolation process. In the limit of the purely transversal random force, critical exponents comply with the incompressible results obtained by previous authors. We have found intriguing nonuniversal behavior related to the mutual effect of compressibility and advection.
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| 4. |
- 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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| 5. |
- Škultéty, Viktor, et al.
(author)
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Swimming Suppresses Correlations in Dilute Suspensions of Pusher Microorganisms
- 2020
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In: Physical Review X. - 2160-3308. ; 10:3
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Journal article (peer-reviewed)abstract
- Active matter exhibits various forms of nonequilibrium states in the absence of external forcing, including macroscopic steady-state currents. Such states are often too complex to be modeled from first principles, and our understanding of their physics relies heavily on minimal models. These are mostly studied in the case of "dry"active matter, where particle dynamics are dominated by friction with their surroundings. Significantly less is known about systems with long-range hydrodynamic interactions that belong to "wet"active matter. Dilute suspensions of motile bacteria, modeled as self-propelled dipolar particles interacting solely through long-ranged hydrodynamic fields, are arguably the most studied example from this class of active systems. Their phenomenology is well established: At a sufficiently high density of bacteria, there appear large-scale vortices and jets comprising many individual organisms, forming a chaotic state commonly known as bacterial turbulence. As revealed by computer simulations, below the onset of collective motion, the suspension exhibits very strong correlations between individual microswimmers stemming from the long-ranged nature of dipolar fields. Here, we demonstrate that this phenomenology is captured by the minimal model of microswimmers. We develop a kinetic theory that goes beyond the commonly used mean-field assumption and explicitly takes into account such correlations. Notably, these can be computed exactly within our theory. We calculate the fluid velocity variance, spatial and temporal correlation functions, the fluid velocity spectrum, and the enhanced diffusivity of tracer particles. We find that correlations are suppressed by particle self-propulsion, although the mean-field behavior is not restored even in the limit of very fast swimming. Our theory is not perturbative and is valid for any value of the microswimmer density below the onset of collective motion. This work constitutes a significant methodological advance and allows us to make qualitative and quantitative predictions that can be directly compared to experiments and computer simulations of microswimmer suspensions.
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