Individual behavior and pairwise interactions between microswimmers in anisotropic liquid

Andrey Sokolov, Shuang Zhou, Oleg D. Lavrentovich, and Igor S. Aranson

Phys. Rev. E 91, 013009 – Published 15 January 2015

Abstract

A motile bacterium swims by generating flow in its surrounding liquid. Anisotropy of the suspending liquid significantly modifies the swimming dynamics and corresponding flow signatures of an individual bacterium and impacts collective behavior. We study the interactions between swimming bacteria in an anisotropic environment exemplified by lyotropic chromonic liquid crystal. Our analysis reveals a significant localization of the bacteria-induced flow along a line coaxial with the bacterial body, which is due to strong viscosity anisotropy of the liquid crystal. Despite the fact that the average viscosity of the liquid crystal is two to three orders of magnitude higher than the viscosity of pure water, the speed of bacteria in the liquid crystal is of the same order of magnitude as in water. We show that bacteria can transport a cargo (a fluorescent particle) along a predetermined trajectory defined by the direction of molecular orientation of the liquid crystal. We demonstrate that while the hydrodynamic interaction between flagella of two close-by bacteria is negligible, the observed convergence of the swimming speeds as well as flagella waves' phase velocities may occur due to viscoelastic interaction between the bacterial bodies.

Received 27 June 2014
Revised 11 November 2014

DOI:https://doi.org/10.1103/PhysRevE.91.013009

Authors & Affiliations

Andrey Sokolov¹, Shuang Zhou², Oleg D. Lavrentovich², and Igor S. Aranson^1,*

¹Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
²Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, USA

^*aronson@anl.gov

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Vol. 91, Iss. 1 — January 2015

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Images

Figure 1
Probability distributions of bacteria swimming speeds in water (red circles), in 12% solution of DSCG in water (isotropic phase, green diamonds) and in 14% solution of DSCG in water (nematic phase, blue squares). Dashed lines are Gaussian fits. Inset: Time autocorrelations of swimming speed in water (red circles) and LC (blue squares). Dashed lines show exponential fits.
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Figure 2
Relationship between the swimming speed and flagella frequency. The black line represents the dependence of a bacterium swimming speed on flagella frequency in LC. The dashed brown (light gray) line shows a linear fit. Phase speed of the flagella wave as a function of the flagella frequency is shown by blue (dark gray) line. Inset: Example of time fluctuations of swimming speed and flagella frequency for an individual bacterium.
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Figure 3
A bacterium (marked by a yellow dashed ellipse) pushes a fluorescent tracer (green dashed circle) located on a bacterial path. Particles located slightly off the bacterial path (marked by solid blue circles) are not advected.
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Figure 4
Velocities of a bacterium (red circles) and a tracer (green triangles) and bacterium-tracer distance (blue squares) as a function of time. Bacteria length $L = 5 μ m$ . Inset: Decay of the flow magnitude in the direction of swimming normalized by bacteria swimming speed, compared with a typical decay of flow in isotropic liquid (dashed line). Distance is measured from the center of a bacterium.
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Figure 5
Reconstruction of the flow velocity field. (a) The magnitude of the flow field created by an isolated bacterium in LC. Arrows indicate the direction of flow. (b) Flow profiles; $r = 0$ corresponds to the center of the bacterium. Blue lines (circles) corresponds to the horizontal component (parallel to the bacterial body) of the flow field along the line coaxial with the bacterial body (see inset). The red line (squares) corresponds to the horizontal component along the line shifted by $2 μ m$ in the perpendicular direction. The green line (diamonds) represents the vertical component of flow along the line perpendicular to the bacterial body.
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Figure 6
Interaction of nearby bacteria. (A) Swimming speeds (dashed lines) and flagella phase velocities (solid lines) vs time for two bacteria swimming next to each other. Convergence of the flagella phase speeds occurs between 1.5 and 2 s. (B) and (C) Snapshots of bacteria positions. Bacteria are swimming from right to left. Bacteria are marked by numbers corresponding to plot (A). See Movie 3 in [26].
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