Rheology of Entangled Active Polymer-Like T. Tubifex Worms

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Rheology of Entangled Active Polymer-Like T. Tubifex Worms

A. Deblais, S. Woutersen, and D. Bonn

Phys. Rev. Lett. 124, 188002 – Published 8 May 2020

See Focus story: Worm Viscosity

Abstract

We experimentally study the rheology of long, slender, and entangled living worms (Tubifex Tubifex). Their level of activity can be controlled by changing the temperature or by adding small amounts of alcohol to make the worms temporarily inactive. Performing classical rheology experiments on this entangled polymer-like system, we find that the rheology is qualitatively similar to that of usual polymers, but, quantitatively, (i) shear thinning is reduced by activity, (ii) the characteristic shear rate for the onset of shear-thinning is given by the time scale of the activity, and (iii) the low shear viscosity as a function of concentration shows a very different scaling from that of regular polymers. Our study paves the way towards a new experimental research field of active “polymer-like worms.”

Received 18 November 2019
Revised 18 March 2020
Accepted 13 April 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.188002

Physics Subject Headings (PhySH)

Fluid-particle interactions Non-Newtonian fluids Polymer entanglement

Living matter & active matter

Rheological properties Rheology Shear thinning

Fluid DynamicsNonlinear Dynamics

Focus

Worm Viscosity

Published 8 May 2020

The activity of sludge worms can alter the viscosity of a fluid, a finding that could help researchers understand active systems in cellular biology.

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Authors & Affiliations

A. Deblais^1,*, S. Woutersen^2,†, and D. Bonn^1,‡

¹Van der Waals-Zeeman Institute, Institute of Physics, University of Amsterdam, 1098XH Amsterdam, The Netherlands
²Van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands

^*A.Deblais@uva.nl
^†S.Woutersen@uva.nl
^‡D.Bonn@uva.nl

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Issue

Vol. 124, Iss. 18 — 8 May 2020

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Images

Figure 1
Rheology experiments of polymer-like worms. (a) Picture of the custom-designed rheology cell (see Supplemental Video 1 [26]) used for the measurements. The cell is mounted on a Peltier cell to control the in situ temperature $T$ and thus the level of activity of the worms. The enlargement emphasizes the entangled state of the worms. (b) Shear-thinning curves (shear viscosity $η_{s}$ as a function of shear rate $\dot{γ}$ ) of the polymer-like solution ( $ϕ = 28 %_{vol}$ ) at different levels of activity, as tuned by adding 5% of alcohol and varying the temperature ( $T = 5$ , 20, $30 ° C$ ). Colored solid lines are fits to the Cross model (1).
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Figure 2
Microscopic dynamics of a single worm. (a) Sequence of images (initially recorded at a rate of 50 frames per second) of a single worm at a high level of activity in water at $T = 30 ° C$ constrained to move in a quasi-two-dimensional thermo-controlled aquarium. Similar analysis as in the higher panel for the same worm at a lower level of activity, achieved by lowering the temperature to $T = 20 ° C$ and exposing the worm to 5% of alcohol. (b) Corresponding plot of the fluctuation of the end-to-end distance $δ r_{e}$ with respect to its averaged value versus time. Scale bar in the sequence of images represents 2 mm. The red shaded area in the plot indicates the six-second time period during which the sequence of images shown in the top panel were recorded. (c) Autocorrelation function of $δ (r_{e})$ measured at different levels of activity (i.e., different temperatures $T = 5$ , 20, $30 ° C$ and in the presence of alcohol). From these graphs, the (microscopic) characteristic time $τ_{worm}$ of a single worm was determined as indicated by the dotted lines. (d) Characteristic time of the active solution as deduced from rheology $τ_{rheo}$ plotted against the (microscopic) characteristic time $τ_{worm}$ of a single worm at four levels of activity. The dashed line is a linear fit of the data.
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Figure 3
Effect of the concentration on the polymer-like worm rheology. (a),(b) Shear viscosity as a function of the shear rate for different concentrations of worms in solution at a low [(a): $T = 0 ° C$ ] and high [(b): $T = 20 ° C$ ] level of activity. Insets: same data as main graph, but normalized by the fitting parameters of the Cross model (continuous lines): $\tilde{η} = η_{s} / η_{s, 0}$ and $\tilde{\dot{γ}} = (\dot{γ} τ_{rheo})^{α}$ . The sequence of images shows top views of the container holding typical Tubifex solutions at different concentrations from low (yellow, left) to high (purple, right). (c) Zero-shear viscosity $η_{s, 0}$ as function of living polymer-like concentration at a high level of activity ( $T = 20 ° C$ , circles) and low level of activity ( $T = 0 ° C$ , triangles). Dashed line has slope 1.5. (d) Effect of activity on the zero-shear viscosity as a function of activity as quantified by the (microscopic) characteristic time of the worms ( $τ_{worm}$ ) at a polymer concentration $C = 28 %_{vol}$ . Dashed line has slope 1.7.
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