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Theses

Dynamics of flexible and Brownian filaments in viscous flow

Abstract : The dynamics of individual flexible filament in a viscous flow is the key to deciphering the rheolog- ical behavior of many complex fluids and soft materials. It also underlies a wealth of biophysical processes from flagellar propulsion to intracellular streaming. This thesis presents systematic exper- iments to investigate the dynamics of flexible and Brownian filaments in viscous flows. Biopolymer actin has been chosen to be our experimental model filament: its typical length can be varied from 1 to 100 μm, it is flexible at these dimensions with a persistence length in the order of 20μm, it is Brow- nian due to its small diameter with bending fluctuations and it can be labelled by fluorescent dye. Microfluidic channels and flow control systems are combined to optical microscope with automated stage to carry out well-controlled experiments on the diverse dynamics of actin filaments in shear flow and pure straining flow. In shear flow, simulations matching the experimental conditions have also been performed using inextensible Euler-Bernoulli beam theory and non-local slender body hy- drodynamics in the presence of thermal fluctuations and agree quantitatively with the experimental results. We demonstrate that filament dynamics in this flow geometry is primarily governed by a dimension- less elasto-viscous number comparing viscous forces to elastic forces with thermal fluctuations only playing a secondary role. We present a complete characterization of the different modes of defor- mation undergone by the filament while rotating as well as of the transitions between these different modes. In pure straining flow, we opt to use an optimized hyperbolic channel to allow long resi- dence time at constant strain rate to be applied. We directly observe the suppression of transverse fluctuations in the extensional part of the hyperbolic channel while we observe, in the compressive part of the flow, the formation of three dimensional helical structures subsequent to the initial buck- ling of the filament. Finally, this thesis manuscript also reports on experimental developments to fabricate suspensions of actin filament with a narrow distribution of lengths and on preliminary re- sults on shear-thinning effects. All together the results presented here pave the way of future studies towards the understanding of filament dynamics in more complex flows, as Poiseuille flows or oscil- latory flows, as well as towards establishing the link between filament deformations and rheological response in dilute suspensions of flexible Brownian filaments, which remains nearly unexplored from an experimental point of view.
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Yanan Liu. Dynamics of flexible and Brownian filaments in viscous flow. Physics [physics]. Université Sorbonne Paris Cité, 2018. English. ⟨NNT : 2018USPCC196⟩. ⟨tel-02451935⟩

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