Undulatory motion is utilised by crawlers and swimmers, such as snakes and spermatozoa, at length scales spanning more than seven orders of magnitude. The understanding of this highly efficient form of locomotion requires experimental characterisation of the material properties of the crawling or swimming organism, as well as of its active force output on the surrounding medium. Here we present a novel experimental technique used to study the properties of the millmeter-sized nematode and microswimmer Caenorhabditis elegans. By using the deflection of a very flexible, force-calibrated micropipette, the viscoelastic material properties of the model organism were

directly probed and characterised. The worm was shown to have a self-similar elastic structure as well as a, surprising, shear-thinning viscous component. The complex fluid properties were directly determined through dynamic relaxation measurements and successfully described by a power-law fluid model. The excellent force (pN) and time (ms) resolution provided by the micropipette deflection technique also enabled measurements of the active forces experienced by C. elegans swimming through a liquid. Our experimental study of this tiny nematode has revealed fascinating aspects of the design of an ultimate, undulatory microswimmer.

Complex viscoelastic fluid properties of a tiny worm

Matilda Backholm1*, William S. Ryu2 and Kari Dalnoki-Veress1,2

1 McMaster University, Hamilton, ON, Canada 2 University of Toronto, Toronto, ON, Canada

* Present address: Aalto University, Helsinki, Finland

Figure 1. The material properties and swimming dynamics of the C. elegans nematode was directly measured using the micropipette deflection technique.

Simultaneous In-situ Analysis of Instabilities and First Normal StressDifference during Polymer Melt Extrusion Flows

Roland Kádár1,2, Ingo F. C. Naue2 and Manfred Wilhelm2

1 Chalmers University of Technology, 41258 Gothenburg, Sweden2 Karlsruhe Institute of Technology - KIT, 76128 Karlsruhe, Germany

ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 24, 2016

ABSTRACTA high sensitivity system for capillaryrheometry capable of simultaneously de-tecting the onset and propagation of insta-bilities and the first normal stress differ-ence during polymer melt extrusion flowsis here presented. The main goals of thestudy are to analyse the nonlinear dynam-ics of extrusion instabilities and to deter-mine the first normal stress difference inthe presence of an induced streamline cur-vature via the so-called ’hole effect’. Anoverview of the system, general analysisprinciples, preliminary results and overallframework are herein discussed.

INTRODUCTIONCapillary rheometry is the preferredrheological characterisation method forpressure-driven processing applications,e.g. extrusion, injection moulding. Themain reason is that capillary rheometry isthe only method of probing material rheo-logical properties in processing-like condi-tions, i.e. high shear rate, nonlinear vis-coelastic regime, albeit in a controlledenvironment and using a comparativelysmall amount of material.1 Thus, it isof paramount importance to develop newtechniques to enhance capillary rheome-ters for a more comprehensive probing ofmaterial properties. Extrusion alone ac-counts for the processing of approximately35% of the worldwide production of plas-tics, currently 280⇥ 106 tons (Plastics Eu-rope, 2014). This makes it the most im-portant single polymer processing opera-

tion for the industry and can be found ina variety of forms in many manufacturingoperations. Extrusion throughput is lim-ited by the onset of instabilities, i.e. prod-uct defects. Comprehensive reviews on thesubject of polymer melt extrusion insta-bilities can be found elsewhere.4,6 A re-cent method proposed for the detectionand analysis of these instabilities is that ofa high sensitivity in-situ mechanical pres-sure instability detection system for cap-illary rheometry.8,10 The system consistsof high sensitivity piezoelectric transducersplaced along the extrusion slit die. In thisway all instability types detectable, thusopening new means of scientific inquiry. Asa result, new insights into the nonlinear dy-namics of the flow have been provided.9,14

Moreover, the possibility of investigatingthe reconstructed nonlinear dynamics wasconsidered, whereby a reconstructed phasespace is an embedding of the original phasespace.2,14 It was shown that a positive Lya-punov exponent was detected for the pri-mary and secondary instabilities in lin-ear and linear low density polyethylenes,LDPE and LLDPE,.14 Furthermore, it wasdetermined that Lyapunov exponents aresensitive to the changes in flow regime andbehave qualitatively different for the iden-tified transition sequences.14 It was alsoshown that it is possible to transfer thehigh sensitivity instability detection sys-tem to lab-sized extruders for inline ad-vanced processing control and quality con-trol systems.13

A very recent possibility considered

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REFERENCES 1. Backholm, M., Ryu, W.S., Dalnoki-Veress, K. (2013), "Viscoelastic properties of the nematode Caenorhabditis elegans, a self-similar, shear-thinning worm", Proc. Natl. Acad. Sci. USA (PNAS), 110, 4529-4533. 2. Backholm, M., Ryu, W.S., Dalnoki-Veress, K. (2015), "Copolymer of star poly(epsilon-caprolactone) and polyglycidols as potential carriers for hydrophobic drugs", Eur. Phys. J. E, 38, 118. 3. Schulman, R. D., Backholm, M., Ryu, W. S., Dalnoki-Veress, K. (2014), "Dynamic force patterns of an undulatory microswimmer", Phys. Rev. E, 89, 050701.

M. Backholm et al.

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