Archives de l'auteur: oduroure

Bending of elastic fibres in viscous flows: the influence of confinement.

Wexler, J. S., Trinh, P. H., Berthet, H., Quennouz, N., du Roure, O., Huppert, H. E., Lindner, A. & Stone, H. A.

 Journal of Fluid Mechanics, (2013)  720, 517–544. doi:10.1017/jfm.2013.49

 

Photopolymerized fiber confined in a microfluidic channel and bent by a flow.

Photopolymerized fiber confined in a microfluidic channel and bent by a flow.

We present a mathematical model and corresponding series of microfluidic experiments examining the flow of a viscous fluid past an elastic fibre in a three-dimensional channel. The fibre’s axis lies perpendicular to the direction of flow and its base is clamped to one wall of the channel; the sidewalls of the channel are close to the fibre, confining the flow. Experiments show that there is a linear relationship between deflection and flow rate for highly confined fibres at low flow rates, which inspires an asymptotic treatment of the problem in this regime. The three-dimensional problem is reduced to a two-dimensional model, consisting of Hele-Shaw flow past a barrier, with boundary conditions at the barrier that allow for the effects of flexibility and three-dimensional leakage. The analysis yields insight into the competing effects of flexion and leakage, and an analytical solution is derived for the leading-order pressure field corresponding to a slit that partially blocks a two-dimensional channel. The predictions of our model show favourable agreement with experimental results, allowing measurement of the fibre’s elasticity and the flow rate in the channel.

The fabrication and directed self-assembly of micron-sized superparamagnetic non-spherical particles

 Tavacoli JW, Bauër P, Fermigier M, Bartolo D, Heuvingh J & du Roure O

Soft Matter, 2013,9, 9103-9110

 

We outline a simple techniq2013_Tavacoli_SoftMatterue to engineer monodisperse, superparamagnetic, micron-sized prisms of arbitrary cross-section and large magnetic susceptibility. The fabrication process allows pre-positioning of the particles that introduces another lever to guide self-assembly. In this method, a dispersion of magnetic colloids in a UV-curable monomer is molded in PDMS wells and subsequently reticulated. High homogeneous magnetic content is achieved by preventing colloidal aggregation through careful choice of the colloid and monomer. Additionally, on their removal from the PDMS molds, the relative position of the magnetic particles is conserved: they are extracted as arrays whose patterns are set by the PDMS mold. This novel method therefore offers unique control of the self-assembly of specific ‘higher order’ structures mediated by dipolar interactions and directed by the geometry and initial positioning of the particles. This is also a promising approach to develop devices with complex responses to external fields.

Human adipocyte function is impacted by mechanical cues.

Pellegrinelli V, Heuvingh J, du Roure O, Rouault C, Devulder A, Klein C, Lacasa M, Clément E, Lacasa D, Clément K.

J Pathol. 2014 Jun;233(2):183-95. doi: 10.1002/path.4347.

Fibrosis is a hallmark of human white adipose tissue (WAT) during obesity-induced chronic inflammation. The functional impact of increased interstitial fibrosis (peri-adipocyte fibrosis) on adjacent adipocytes remains unknown. Here we developed a novel in vitro 3D culture system in which human adipocytes and decellularized material of adipose tissue (dMAT) from obese subjects are embedded in a peptide hydrogel. When cultured with dMAT, adipocytes showed decreased lipolysis and adipokine secretion and increased expression/production of cytokines (IL-6, G-CSF) and fibrotic mediators (LOXL2 and the matricellular proteins THSB2 and CTGF). Moreover, some alterations including lipolytic activity and fibro-inflammation also occurred when the adipocyte/hydrogel culture was mechanically compressed. Notably, CTGF expression levels correlated with the amount of peri-adipocyte fibrosis in WAT from obese individuals. Moreover, dMAT-dependent CTGF promoter activity, which depends on β1-integrin/cytoskeleton pathways, was enhanced in the presence of YAP, a mechanosensitive co-activator of TEAD transcription factors. Mutation of TEAD binding sites abolished the dMAT-induced promoter activity. In conclusion, fibrosis may negatively affect human adipocyte function via mechanosensitive molecules, in part stimulated by cell deformation.

Microfluidic in situ mechanical testing of photopolymerized gels

Duprat, C., Berthet, H., Wexler, JS., du Roure, O., & Lindner, A.

Lab on a Chip (2015)  15 244

Photopolymerized fiber deformed by increasing flow rates.

Photopolymerized fiber deformed by increasing flow rates.

Gels are a functional template for micro-particle fabrication and microbiology experiments. The control and knowledge of their mechanical properties is critical in a number of applications, but no simple in situ method exists to determine these properties. We propose a novel microfluidic based method that directly measures the mechanical properties of the gel upon its fabrication. We measure the deformation of a gel beam under a controlled flow forcing, which gives us a direct access to the Young’s modulus of the material itself. We then use this method to determine the mechanical properties of poly(ethylene glycol) diacrylate (PEGDA) under various experimental conditions. The mechanical properties of the gel can be highly tuned, yielding two order of magnitude in the Young’s modulus. The method can be easily implemented to allow for an in situ direct measurement and control of Young’s moduli under various experimental conditions.

Deformation and shape of flexible, microscale helices in viscous flow

2015_Pham_PREPham, J.T., Morozov, A., Crosby, A.J., Lindner, A. & du Roure, O.

Physical Review E. (2015) 011004(R) 1-5

 

We examine experimentally the deformation of flexible, microscale helical ribbons with nanoscale thickness
subject to viscous flow in a microfluidic channel. Two aspects of flexible microhelices are quantified: the overall shape of the helix and the viscous frictional properties. The frictional coefficients determined by our experiments are consistent with calculated values in the context of resistive-force theory. The deformation of helices by viscous flow is well described by nonlinear finite extensibility. Under distributed loading, the pitch distribution is nonuniform, and from thiswe identify both linear and nonlinear behavior along the contour length of a single helix. Moreover, flexible helices are found to display reversible global to local helical transitions at a high flow rate.

Transport and buckling dynamics of an elastic fiber in a viscous cellular flow

2015_Quennouz_JFMQuennouz N., Shelley M.J., du Roure, O. & Lindner, A.

Journal of Fluid Mechanics (2015) 769 387- 402

We study, using both experiment and theory, the coupling of transport and shape dynamics for elastomeric fibres moving through an inhomogeneous flow. The cellular flow, created electromagnetically in our experiment, comprises many identical cells of counter-rotating vortices, with a global flow geometry characterized by a backbone of stable and unstable manifolds connecting hyperbolic stagnation points. Our mathematical model is based upon slender-body theory for the Stokes equations, with the fibres modelled as inextensible elastica. Above a certain threshold of the control parameter, the elasto-viscous number, transport of fibres is mediated by their episodic buckling by compressive stagnation point flows, lending an effectively chaotic component to their dynamics. We use simulations of the model to construct phase diagrams of the fibre state (buckled or not) near stagnation points in terms of two variables that arise in characterizing the transport dynamics. We show that this reduced statistical description quantitatively captures our experimental observations. By carefully reproducing the experimental protocols and time scales of observation within our numerical simulations, we also quantitatively explain features of the measured buckling probability curve as a function of the effective flow forcing. Finally, we show within both experiment and simulation the existence of short and long time scales in the evolution of fibre conformation.