Anisotropic diffusion of ellipsoidal tracers in microswimmer suspensions
(2022) In Physical Review Fluids 7(1).- Abstract
Tracer particles immersed in suspensions of biological microswimmers such as E. coli or C. reinhardtii display phenomena unseen in conventional equilibrium systems, including strongly enhanced diffusivity relative to the Brownian value and non-Gaussian displacement statistics. In dilute, three-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 behavior than spherical ones due to the presence of a nontrivial... (More)
Tracer particles immersed in suspensions of biological microswimmers such as E. coli or C. reinhardtii display phenomena unseen in conventional equilibrium systems, including strongly enhanced diffusivity relative to the Brownian value and non-Gaussian displacement statistics. In dilute, three-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 behavior than spherical ones due to the presence of a nontrivial translation-rotation coupling. Using large-scale lattice Boltzmann simulations of model microswimmers described by extended force dipoles, we study the motion of ellipsoidal point tracers immersed in three-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 the 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.
(Less)
- author
- Nordanger, Henrik LU ; Morozov, Alexander and Stenhammar, Joakim LU
- organization
- publishing date
- 2022
- type
- Contribution to journal
- publication status
- published
- subject
- in
- Physical Review Fluids
- volume
- 7
- issue
- 1
- article number
- 013103
- publisher
- American Physical Society
- external identifiers
-
- scopus:85124481203
- ISSN
- 2469-990X
- DOI
- 10.1103/PhysRevFluids.7.013103
- project
- Large-scale Lattice-Boltzmann simulations of tracer diffusion in colloidal suspensions out of equilibrium
- language
- English
- LU publication?
- yes
- id
- eda03ccf-9539-4b53-81ea-107e423675a7
- date added to LUP
- 2022-04-13 12:31:11
- date last changed
- 2023-04-02 23:13:29
@article{eda03ccf-9539-4b53-81ea-107e423675a7,
abstract = {{<p>Tracer particles immersed in suspensions of biological microswimmers such as E. coli or C. reinhardtii display phenomena unseen in conventional equilibrium systems, including strongly enhanced diffusivity relative to the Brownian value and non-Gaussian displacement statistics. In dilute, three-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 behavior than spherical ones due to the presence of a nontrivial translation-rotation coupling. Using large-scale lattice Boltzmann simulations of model microswimmers described by extended force dipoles, we study the motion of ellipsoidal point tracers immersed in three-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 the 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.</p>}},
author = {{Nordanger, Henrik and Morozov, Alexander and Stenhammar, Joakim}},
issn = {{2469-990X}},
language = {{eng}},
number = {{1}},
publisher = {{American Physical Society}},
series = {{Physical Review Fluids}},
title = {{Anisotropic diffusion of ellipsoidal tracers in microswimmer suspensions}},
url = {{http://dx.doi.org/10.1103/PhysRevFluids.7.013103}},
doi = {{10.1103/PhysRevFluids.7.013103}},
volume = {{7}},
year = {{2022}},
}