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[AL43] A. Lindner
Flow of complex suspensions
Phys. Fluids 26 101307 (2014)

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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.

[AL42] C. Duprat, H. Berthet, J. S. Wexler, O. du Roure and A. Lindner
Microfluidic in situ mechanical testing of photopolymerized gels
Lab Chip, 2014, DOI: 10.1039/C4LC01034E.

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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.

[AL41] Chelsea S. Davis, Florian Lemoine, Thierry Darnige, David Martina, Costantino Creton, and Anke Lindner
Debonding Mechanisms of Soft Materials at Short Contact Times
Langmuir 2014, 30, 10626?10636, dx.doi.org/10.1021/la5023592

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A carefully controlled, custom-built adhesion testing device was developed which allows a precise, short dwell time on the order of milliseconds to be applied during a contact adhesion experiment. The dwell time dependence of the adhesive strength of crosslinked polydimethyl siloxane (PDMS) in contact with glass and uncrosslinked styrene butadiene rubber (SBR) in contact with glass and with itself was tested with a spherical probe in a confined Johnson-Kendall-Roberts (JKR) geometry. Analysis of the contact images revealed several unique separation mechanisms which are dependent on dwell time and interfacial properties. PDMS-Glass interfaces show essentially no dependence of adhesion on the dwell time while the adhesive strength and separation mechanisms of SBR interfaces are shown to vary drastically for dwell times ranging from 40 ms to 10,000 ms. This influence of dwell time is particularly pronounced for polymer-polymer (SBR-SBR) interfaces. Observations of cavitation due to trapped air pockets in the center of the contact at very short contact times illustrate a transition between a defect-controlled debonding and an interface-controlled debonding which has not been previously reported.

[AL40] Jeremie Gachelin, Annie Rousselet, Anke Lindner, Eric Clement
Collective motion in an active suspension of Escherichia coli bacteria
New J. Phys. 16 (2014) 02

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We investigate experimentally the emergence of collective motion in the bulk of an active suspension of E. coli bacteria. When increasing the concentration from a dilute to a semi-dilute regime, we observe a continuous cross-over from a dynamical cluster regime to a regime of `bio-turbulence' convection patterns. We measure a length scale characterizing the collective motion as a function of the bacteria concentration. For bacteria fullly supplied with oxygen, the increase of the correlation length is almost linear with concentration and at the largest concentrations tested, the correlation length could be as large as 24 bacterial body sizes (or 7-8 when including the agella bundle). In contrast, under conditions of oxygen shortage the correlation length saturates at a value around 7 body length.

[AL39] François Tanguy, Matteo Nicol, Anke Lindner, and Costantino Creton
Quantitative analysis of the debonding structure of soft adhesives
Eur. Phys. J. E (2014) 37: 3

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We experimentally investigate the growth dynamics of cavities nucleating during the first stages of debonding of three different model adhesives. The material properties of these adhesives range from a more liquid-like material to a soft viscoelastic solid and are carefully characterized by small strain oscillatory shear rheology as well as large strain uniaxial extension. The debonding experiments are performed on a probe tack set-up. Using high contrast images of the debonding process and precise image analysis tools we quantify the total projected area of the cavities, the average cavity shape and growth rate and link these observations to the material properties. These measurements are then used to access corrected effective stress and strain curves that can be directly compared to the results from the uniaxial extension.

[AL38] J. Zilz, R.J. Poole, M. Alves, C. Wagner, C. Schäfer and A. Lindner
Serpentine channels: micro – rheometers for fluid relaxation times
Lab Chip, 2014, 14 (2), 351 – 358

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We propose a novel device capable of measuring relaxation times of viscoelastic fluids as small as 1 ms. In contrast to most rheometers, which by their very nature are concerned with producing viscometric or nearly-viscometric flows, here we make use of an elastic instability that occurs in the flow of viscoelastic fluids with curved streamlines. To calibrate the rheometer we combine simple scaling arguments with relaxation times obtained from first normal-stress difference data measured in a classical shear rheometer. As an additional check we also compare these relaxation times to those obtained from Zimm theory and good agreement is observed. Once calibrated, we show how the serpentine rheometer can be used to access smaller polymer concentrations and lower solvent viscosities where classical measurements become difficult or impossible to use due to inertial and/or resolution limitations. In the absence of calibration, the serpentine channel can still be a very useful comparative or index device.

[AL37] H. Berthet, M. Fermigier and A. Lindner
Single fiber transport in a confined channel: microfluidic experiments and numerical study.
Phys. Fluids 25, 103601 (2013)

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We present an experimental and numerical study on the transport of a single fiber confined in a microfluidic Hele-Shaw geometry. The fiber has a square cross-section and a typical aspect ratio of ten. We address the question of the fiber velocity as it is freely transported by the flow, and study in particular its dependence on the fiber orientation and confinement in the channel, defined as the ratio of the fiber height with the channel height. Both experiments and simulations are set so that the fiber suspended in the middle of the channel height does not interact with the lateral flow boundaries. At low confinements, the fiber velocity is independent of the fiber orientation with the flow direction and tends to the maximal velocity of the fluid when the confinement tends to zero. The fiber slows down as the confinement increases.We find that as the confinement reaches approximately 0.5, the orientation affects the fiber velocity: a fiber perpendicular to the flow direction moves faster than a parallel one. Consequently, a confined fiber transported in a microchannel at an angle different from 0° or 90° with the flow direction will drift towards a lateral wall, in the opposite direction found in sedimenting fibers. We also characterize the perturbation caused by the presence of the fiber on the flow field, and find that it drops very quickly as the fiber confinement decreases.

[A36] R.J. Poole, A. Lindner, M.A. Alves
Viscoelastic secondary flows in serpentine channels
Journal of Non-Newtonian Fluid Mechanics 201 (2013) 10–16

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We report the results of a detailed numerical investigation of inertialess viscoelastic fluid flow through three-dimensional serpentine (or wavy) channels of varying radius of curvature and aspect ratio using the Oldroyd-B model. The results reveal the existence of a secondary flow which is absent for the equivalent Newtonian fluid flow. The secondary flow arises due to the curvature of the geometry and the streamwise first normal–stress differences generated in the flowing fluid and can be thought of as the viscoelastic equivalent of Dean vortices. The effects of radius of curvature, aspect ratio and solvent-tototal viscosity ratio on the strength of the secondary flow are investigated. The secondary flow strength is shown to be a function of a modified Deborah number over a wide parameter range.

[A35] Julia Nase, Osvanny Ramos, Costantino Creton, and Anke Lindner
Debonding energy of PDMS: A new analysis of a classic adhesion scenario
Eur. Phys. J. E (2013) 36: 103, DOI 10.1140/epje/i2013-13103-3

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We investigated the debonding energy between confined layers of a soft elastic solid (PDMS) and a circular steel indenter in a flat punch geometry. PDMS is extensively used in applications, but also a widespread model system for fundamental research. Varying systematically the pulling speed and the viscoelastic properties, notably the modulus, we determined scaling laws for the debonding energy. We showed that the debonding energy is independent of the sample thickness. Applying a new approach and separating the crack initiation and the propagation part of the force curves, we analyzed the thickness dependence more precisely and we demonstrated that the energy to propagate the crack at given average speed does not only depend on the modulus, but also on the sample thickness.

[A34] Satyam Bhuyan, François Tanguy, David Martina, Costantino Creton, Anke Lindner and Matteo Ciccotti
Propagating cracks at the interface between viscoelastic Pressure Sensitive Adhesives and model surfaces
Soft Matter, (2013), 9, 6515, DOI: 10.1039/c3sm27919g

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The motion of the triple line during the debonding of a soft viscoelastic adhesive from a rigid polymer surface has been investigated quantitatively. In order to make the debonding geometry simpler than the probe tack test, a new technique, called the sticky wedge test, has been developed where the probe is constituted by a horizontal cylinder instead of a flat punch. The crack propagates in the elongated contact area between the cylinder and the flat surface, thereby, allowing us to measure optically in situ the crack-tip velocity and the receding contact angle of the debonding adhesive. Two model pressure sensitive adhesives (PSA) based on poly(n-butyl acrylate-co-acrylic acid) with different molecular weights and branching levels and four polymer substrates (rubbery or glassy at room temperature) were used. Due to the soft and incompressible nature of the adhesive, the strain energy release rate for this test geometry has been estimated by the equations for the pure shear test geometry. The results show three main new insights: first, despite significant approximations, this novel approach holds promise to characterize more quantitatively the interfacial crack propagation between soft viscoelastic solids and hard surfaces and the relationship between the applied energy release rate G and the crack-tip velocity v has been reliably established for eight combinations of the model viscoelastic adhesives and surfaces. Second, at equivalent values of the thermodynamic work of adhesion, the adhesion energy of both the adhesives against the rubbery surface was much lower than that against the glassy polymers. Third, surprisingly, the measured receding contact angle of debonding was close to 90_ for all adhesive/ surface combinations used.

[A33] J. Gachelin, G. Miño, H. Berthet, A. Lindner, A. Rousselet and E. Clément
Non-Newtonian viscosity of E-coli suspensions
PRL 110, 268103 (2013), 10.1103/PhysRevLett.110.268103

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The viscosity of an active suspension of E. coli bacteria is determined experimentally as a function of the shear rate using a Y-shaped microfluidic channel. From the relative suspension viscosity, we identify rheological thickening and thinning regimes as well as situations at low shear rate where the viscosity of the bacteria suspension can be lower than the viscosity of the suspending fluid. In addition, bacteria concentration and velocity profiles in the bulk are directly measured in the microchannel.

[A32] Dadhichi Paretkar, Marleen Kamperman, David Martina, Jiahua Zhao, Costantino Creton, Anke Lindner, Anand Jagota, Robert McMeeking and Eduard Arzt
Preload responsive adhesion: effects of aspect ratio, tip shape, and alignment
J. R. Soc. Interface 2013 10, 20130171, (2013)

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We tested the adhesive response of polymer surfaces structured with arrays of cylindrical fibrils having diameters of 10–20 mm and aspect ratios 1–2.4. Fibrils had two different tip shapes of end-flaps and round edges. A preloadinduced mechanical buckling instability of the fibrils was used to switch between the states of adhesion and non-adhesion. Non-adhesion in fibrils with round edges was reached at preloads that caused fibril buckling, whereas fibrils with end-flaps showed adhesion loss only at very high preloads. The round edge acted as a circumferential flaw prohibiting smooth tip contact recovery leading to an adhesion loss. In situ observations showed that, after reversal of buckling, the end-flaps unfold and re-form contact under prevailing compressive stress, retaining adhesion in spite of buckling. At very high preloads, however, end-flaps are unable to re-form contact resulting in adhesion loss. Additionally, the end-flaps showed varying contact adaptability as a function of the fibril–probe alignment, which further affects the preload for adhesion loss. The combined influence of preload, tip shape and alignment on adhesion can be used to switch adhesion in bioinspired fibrillar arrays.

[A31] M. van Deen, T. Bertrand, N. Vu, D. Quéré, E. Clément and A. Lindner
Particles accelerate the detachment of viscous liquids
Rheologica Acta (2013), DOI 10.1007/s00397-013-0691-9

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During detachment of a viscous fluid extruded from a nozzle a filament linking the droplet to the latter is formed. Under the effect of surface tension the filament thins until pinch off and final detachment of the droplet. In this paper we study the effect of the presence of individual particles trapped in the filament on the detachment dynamics using granular suspensions of small volume fractions (? < 6%). We show that even a single particle strongly modifies the detachment dynamics. The particle perturbs the thinning of the thread and a large droplet of fluid around the particle is formed. This perturbation leads to an acceleration of the detachment of the droplet compared to the detachment observed for a pure fluid. We quantify this acceleration for single particles of different sizes and link it to similar observations for suspensions of small volume fractions. Our study also gives more insight into particulate effects on detachment of more dense suspensions and allows to explain the accelerated detachment close to final pinch off observed previously (Bonnoit et al 2012).

[A30] J. Wexler, P. Trinh, H. Berthet, N. Quennouz, O. du Roure, H. Huppert, A. Lindner and H. Stone
Bending of elastic fibres in viscous flows: the influence of confinement
J. Fluid Mech. (2013), vol. 720, pp. 517-544, doi:10.1017/jfm.2013.49

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We present a mathematical model and corresponding series of micrfluidic 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 side walls 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.

[A29] E. Altshuler, G. Miño, C. Pérez-Penichet, L. del R?o, A. Lindner, A. Rousselet, and E. Clément
Flow-controlled densification and anomalous dispersion of E. coli through a constriction
SoftMatter, 2012, DOI:10.1039/C2SM26460A

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Dispersion and migration of bacteria under flow in tortuous and confined structures such as porous or fractured materials is related to a large spectrum of practical interest, but is still poorly understood. Here, we address the question of transport and dispersion of an E. coli suspension flowing through a micro-fluidic channel with a funnel-like constriction in its center. We show a counter-intuitive symmetry breaking of the bacterial concentration, which increases significantly past the funnel. This concentration enhancement persists over large distances from the funnel and disappears at large flow rate values. We map our results onto a one dimensional convection–diffusion equation predicting quantitatively the experimental results, without free parameters, when a conservative non-local source term is introduced. This last term, measured experimentally, represents a long range memory effect due to the unbalance of wall adsorption and desorption processes past the constriction. Our model experiment points out the generic importance of considering such constriction effects in the description of transport properties of active matter in porous media. It also opens the possibility to control the concentration of bacterial suspensions in micro-fluidic channels by simply tuning the flow intensity or direction.

[A28] C. Davis, D. Martina, C. Creton, A. Lindner and A. Crosby
Enhanced Adhesion of Elastic Materials to Small-Scale Wrinkles
Langmuir (2012) DOI: 10.1021/la302314z

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The adhesive properties of a material can be greatly affected simply by wrinkling its surface. We show the importance of selecting the wrinkle feature sizes (amplitude, b; and wavelength, ?) that complement the material-defined length scale related to the adhesion energy and modulus. A rigid circular cylindrical punch patterned with aligned wrinkles ranging in amplitude from 0.5 to 5.0 ?m with a fixed aspect ratio of 0.1 is used to characterize the adhesion of elastic films of smooth poly(dimethyl siloxane) (PDMS). The cross-linker concentration used to form the PDMS layers is varied to determine the impact of material properties on wrinkled surface adhesion. The elastic films have an average thickness of 240 ?m and the average probe radius is 1 mm, leading to a confined contact scenario. The separation stress and work of debonding are presented for each cross-linker concentration with testing rates ranging over 3 orders of magnitude. For stiffer films (10 wt % cross-linker, E' ? 3.00 MPa), small wrinkles (b ? 0.5 ?m) increase the separation stress by nearly 200% relative to a smooth interface whereas large wrinkles (b ?5.0 ?m) are shown to reduce adhesion significantly. A substantial increase in the debonding energy is also observed for these small-amplitude wrinkles contacting stiff materials. No discernible impact of wrinkled surface topography on the adhesion of softer (2 and 4 wt % cross-linker, 0.05 MPa < E' < 0.30 MPa) films is measured.

[A27] J. Zilz, R. J. Poole, M. A. Alves, D. Bartolo, B. Levache and A. Lindner
Geometric scaling of purely-elastic flow instabilities
J. Fluid Mech.  712 (2012) 203-218

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A combined experimental, numerical and theoretical investigation of the geometric scaling of the onset of a purely elastic flow instability in serpentine channels is presented. Good qualitative agreement is obtained between experiments, using dilute solutions of flexible polymers in microfluidic devices, and three-dimensional numerical simulations using the upper-convected Maxwell model. The results are confirmed by a simple theoretical analysis, based on the dimensionless criterion proposed by Pakdel & McKinley (Phys. Rev. Lett., vol. 77, 1996, pp. 2459–2462) for onset of a purely elastic flow instability. Three-dimensional simulations show that the instability is primarily driven by the curvature of the streamlines induced by the flow geometry and not due to the weak secondary flow in the azimuthal direction. In addition, the simulations also reveal that the instability is time-dependent and that the flow oscillates with a well-defined period and amplitude close to the onset of the supercritical instability

[A26] J. Boujlel, M. Maillard, A. Lindner, G. Ovarlez, X. Chateau, P. Coussot
Boundary layers in pastes - Displacement of a long object through a yield stress fluid
J. Rheol. 56(5), 1083-1108 (2012)

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When it moves through a yield stress fluid, a solid object continuously reaches and liquefies new solid regions, so that both flow in liquid regions and deformations in solid regions occur. In the present work, we focus on the displacement of a plate through simple yield stress fluids (nonthixotropic). Through force vs velocity and particle imaging velocimetry measurements with a detailed analysis of the deformation history, we are able to identify the solid and liquid regions and their respective role in the flow characteristics. It is shown that the displacement of a long object through a yield stress fluid gives rise to the formation of a liquid boundary layer (BL) of uniform thickness at short distance from the leading edge, while the rest of the material remains solid. The original result is that the thickness of this boundary layer, which is of the order of 10mm, only slightly increases with velocity and does not tend to zero when the velocity tends to zero, in contrast with usual flows of yield stress fluids along solid surfaces. Moreover, it does not change for significant variations of the rheological characteristics of the fluid in its liquid regime. We show that these specific characteristics of the liquid layer are mainly governed by the progressive transition from an elastic solid to a liquid, starting slightly ahead of the leading edge of the plate.

[A25] C. Bonnoit, T. Bertrand, E. Clement and A. Lindner
Accelerated drop detachment in dense granular suspensions
Phys. Fluids 24, 043304 (2012)

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We experimentally study the detachment of drops of granular suspensions using a density matched model suspension with varying grain volume fraction (? = 15% to 55%) and grain diameter (d = 20?m to 140?m). We show that at the beginning of the detachment process, the suspensions behave as an effective fluid. The detachment dynamics in this regime can be entirely described by the shear viscosity of the suspension [R. J. Furbank and J. F. Morris, Int. J. Multiphase Flow 33(4), 448–468 (2007)]. At later stages of the detachment, the dynamics become independent of the volume fraction and are found to be identical to the dynamics of the interstitial fluid. Surprisingly, visual observation reveals that at this stage, particles are still present in the neck. We suspect rearrangements of particles to locally free the neck of grains, causing the observed dynamics. Close to the final pinch off, the detachment of the suspensions is further accelerated, compared to the dynamics of pure interstitial fluid. This acceleration might be due to the fact that the neck diameter gets of the order of magnitude of the size of the grains and a continuous thinning of the liquid thread is not possible any more. The crossover between the different detachment regimes is a function of the grain size and the initial volume fraction. We characterize the overall acceleration as a function of the grain size and volume fraction.

[A24] D. Martina, C. Creton, P. Damman, M. Jeusette and A. Lindner
Adhesion of soft viscoelastic adhesives on periodic rough surfaces
Soft Matter, 2012, 8 (19), 5350 - 5357

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We experimentally study adhesion of soft viscoelastic materials on surfaces with periodic roughness. The materials tested are partially crosslinked PDMS layers with a low shear elastic modulus (below 0.1 MPa) and variable viscoelasticity. We use an elastic surface instability to produce wrinkled PDMS surfaces. The hard periodic surfaces are then molded in Norland resin from the master surfaces. The periodic surface roughness is typically of the order of 100 microns. For identical surface roughness, we can use two surface topologies: peaks and holes. Using our model material and the model surfaces, we study in detail the interplay between the elastic penalties needed to conform the adhesive to the rough surface and the enhanced viscous dissipation during debonding. In this way we show, that adhesion on these surfaces can either be enhanced or decreased as a function of the surface topology and the viscoelastic properties of the material.

[A23] T. Bertrand, C. Bonnoit, E. Clement and A. Lindner
Dynamics of drop formation in granular suspensions
Granular Matter (2012) 1–6:169–174

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The presence of grains strongly modifies the detachment of drops of a viscous liquid.We have shown previously that the detachment of drops of granular suspensions takes place via different regimes (Bonnoit et al. submitted, 2011). Here we study the influence of the volume fraction of particles on the formation and shape of the droplets by means of visual observations. We measure the minimal neck diameter as well as the height of the detachment as a function of time to quantify the evolution of the drop shape. We also address the question of the thinning dynamics of the neck in the different regimes. Linking the dynamics to the properties of the effective fluid or to rearrangements of individual grains in the thread gives insights in the origin of the different regimes.

[A22] J. Nase, D. Derks and A. Lindner
Dynamic evolution of fingering patterns in a lifted Hele--Shaw cell
Physics of Fluids, (2011) 23, 123101

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We present a study on pattern formation in a Newtonian liquid during lifting of a circular Hele–Shaw cell. When a confined layer of oil is subject to such a stretch flow, air penetrates into the liquid from the sides and a fingering instability, a variant of the classical Saffman–Taylor instability, evolves. This setting has the particularity that the finger growth takes place in a conserved volume of liquid and that the dimensionless surface tension, the control parameter which governs the Saffman–Taylor instability, is changing with time. This leads to a constantly evolving pattern, which we investigate with regard to number of fingers and finger amplitude. We distinguish in the pattern at each instant growing fingers and stagnant fingers. Systematically varying the properties of the viscous oil and the geometry of the Hele–Shaw cell, we show that the number of growing fingers is at each moment well described by a simple approach based on linear stability analysis and depends only on the dimensionless surface tension. In contrast, the finger amplitude and consequently the total number of fingers (growing and stagnant fingers) depend also on the cell confinement. We demonstrate that the finger amplitude has a distinct influence on the debonding force. Higher finger amplitude and number of fingers lead to lower forces.

[A21] C. Bonnoit, J. Lanuza, A. Lindner and E. Clement
A diverging mesoscopic length scale controls the rheology of dense suspensions
Phys. Rev . Lett. (2010) 105, 108302

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From the flow properties of dense granular suspensions on an inclined plane, we identify a mesoscopic length scale strongly increasing with volume fraction. When the flowing layer height is larger than this length scale, a diverging Newtonian viscosity is determined. However, when the flowing layer height drops below this scale, we evidence a nonlocal effective viscosity, decreasing as a power law of the flow height. We establish a scaling relation between this mesoscopic length scale and the suspension viscosity. These results support recent theoretical and numerical results implying collective and clustered granular motion when the jamming point is approached from below.

[A20] E.Wandersman , N. Quennouz , M. Fermigier, A. Lindner, and O. du Roure
Buckled in translation
Soft Matter, 2010, DOI: 10.1039/C0SM00132E

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We report experiments on the deformation andtransport of an elastic fiber in a viscous cellular flow, namely alattice of counterrotative vortices. We show that the fiber can bucklewhen approaching a stagnation point. By tuning either the flow or fiberproperties, we measure the onset of this buckling instability. Thebuckling threshold is determined by the relative intensity of viscousand elastic forces, the elasto-viscous number Sp. Moreover we show thatflexible fibers escape faster from a vortex (formed by closedstreamlines) compared to rigid fibers. As a consequence, thedeformation of the fiber changes its transport properties in thecellular flow.

[A19] J. Nase, O.Ramos, C. Creton, L. Sonneberg, T. Yamaguchi and A. Lindner
A Measurement of the receeding contact angle at the interface between aviscoelastic material and a rigid surface
Soft Matter, 6 (2010) 2685-2691

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We present the first quantitative measurement ofthe receding contact angle between a soft vis-coelastic material and a solid surface during thedebonding of the material. We developed a newmethod of observation, based on the technique pi-oneered by Yamaguchi et al.. In a probe-tack geometry, we obtained a 3D-like view ofthe debonding pattern as seen through the ad-hesive/probe interface, providing thus an excel-lent view of the details of the contact geometrynear the interface. Combining visual and force-displacement information in a probe tack test, weidentified three different categories of mechanismsand contact geometries as a function of the mate-rial parameters. For very weakly crosslinked vis-coelastic materials around the gel point, air fingersform in the bulk of the layer. For higher degrees ofcrosslinking, the air fingers form and propagate atthe interface, with a large deformation of the bulkof the layer. A well defined and reproducible re-ceding contact angle ? < 90? depending on theviscoelastic properties of the layer was observed atthe leading edge of the moving finger. Finally forwell crosslinked samples the failure occurs by in-terfacial crack propagation with little deformationof the layer; the contact angle at the leading edgethen was close to 90° as expected from elastic frac-ture mechanics.

[A18] C. Bonnoit, T.Darnige, E. Clement, and A. Lindner
Inclined plane rheometry of a dense granular suspension
J. Rheol. 54 (2010) 65-79

We present a new method to measure the viscosityof a dense model suspension using an inclinedplane rheometer. The suspension is made of mono-disperse, spherical,non-Brownian polystyrenebeads immersed in a density matched silicon oil. We show that with thissimple set-up, theviscosity can be directly measured up to volume fractions of ??=61% andthat particle migrationcan be neglected. The results are in excellent agreement with localviscosity measurementsobtained by magnetic resonance imaging techniques by Ovarlez et al. [J.Rheol. 50(3), 259–292,2006]. In the high density regime, we show that the viscosity is withinthe tested range ofparameters, independent of the shear rate and the confinement pressure.Finally, we discussdeviations from the viscous behavior of the suspensions

[A17] A. Lindner andC. Wagner
Viscoelastic surface instabilities
C.R. Physique (2009) 712–727

We review three different types of viscoelasticsurface instabilities: The Rayleigh–Plateau, the Saffman–Taylor and theFaradayinstabilities. These instabilities are classical examples ofhydrodynamic surface instabilities. The addition of a small amount ofpolymer to pure water can alter its flow behavior drastically and thetype of instability may change not only quantitatively but alsoqualitatively. We will show that some of the observed new phenomena canbe explained by the use of simple rheological modelsthat contain most of the underlying physical mechanisms leading to theinstability. A quantitative description however is often onlypossible close to the onset of the instability or for weak deviationsfrom Newtonian behavior. A complete theoretical description isstill lacking when the system is driven far from equilibrium or forfluids with strong non-Newtonian behavior.

[A16] Ø. Johnsen, C.Chevalier, A. Lindner, R. Toussaint, E. Clément, K.J. Måløy, E. Flekkøyand J. Schmittbuhl
Decompaction and fluidization of a saturated and confined granularmedium by injection of a viscous liquid or a gas
Phys. Rev. E 78, 051302 (2008)

We compare quantitatively two experimentalsituations concerning injection of a miscible fluid into an initiallyjammed granular medium saturated with the same fluid, confined in aHele–Shaw cell. Both experiments are identical, apart from theinterstitial and injected fluid, which is in one case air injected intoa dry granular packing, and in the other case silicone oil injectedinto a dense suspension. In spite of the strong differences regardingthe nature of the two fluids, strikingly similar dynamical andgeometrical features are identified as function of the controlparameters: cell thickness and applied fluid injection pressure. Inboth cases an initial hydrodynamically driven decompaction process iscontrolling the unjamming and is preparing the final displacementprocess characterized by finger–like patterns empty of grains. Thepattern shapes are comparable. In addition, the mobilities of thecoupled fluid/grain flow, rescaled by the interstitial fluid viscosityand grain diameter squared, are of the same range and behavecomparably. The mobility proves to depend on the initial solid fractionof the medium. Subtle differences are observed in geometrical aspectslike the finger width with respect to the control parameters.

[A15] J. Nase, A.Lindner and C. Creton
Pattern formation during deformation of a confined viscoelasticlayer:From a viscous liquid to a soft elastic solid
Phys. Rev. Lett. 101, (2008), 074503

We study pattern formation during tensiledeformation of confined viscoelastic layers. The use of a model system[poly(dimethylsiloxane) with different degrees of cross-linking] allowsus to go continuously from a viscous liquid to an elastic solid. Weobserve two distinct regimes of fingering instabilities: a regimecalled ‘‘elastic’’ with interfacial crack propagation, where thefingering wavelength scales only with the film thickness, and a bulkregime called ‘‘viscoelastic,’’ where the fingering instability shows aSaffman-Taylor-like behavior. We find good quantitative agreement withtheory in both cases and present a reduced parameter describing thetransition between the two regimes and allowing us to predict theobserved patterns over the whole range of viscoelastic properties.

[A14] C. Chevalier,A. Lindner et E. Clément
Morphodynamics during air injection into a confined granular suspension
J. Non-Newtonian Fluid Mech. 158 (2009) 63–72 

We study the injection of air into a granularsuspension in a confined geometry. The grains and the suspending fluidare density matched allowing to vary the grain fraction ? over thewhole range accessible: from ? = 0 up to close to the maximum packingfraction ? ~ 60%. In this situation, a large variety of patterns isobserved, that we link directly to the rheology of the suspension inthe confined geometry. For low grain fractions ? < 50%, we showthat taking into account an effective viscosity for the suspensionflow, resulting from particle migration, allows to link quantitativelythe observed structures to the classical Saffman–Taylor selectionmechanism. For higher grain fractions ? > 55% the granularmatrix is jammed and we deal with a weak porous medium. In this case welink the observed patterns to the reorganization occurring in theporous medium as a consequence of the injection process.

[A13] C. Chevalier,A. Lindner et E. Clément
Destabilization of a Saffman-Taylor Fingerlike Pattern in a GranularSuspension
Phys. Rev. Lett, 99 (2007) 174501

We study the Saffman-Taylor instability in agranular suspension formed by micrometric beads immersed in a viscousliquid. When using an effective viscosity for the flow of thesuspension in the Hele-Shaw cell to define the control parameter of thesystem, the results for the finger width of stable fingers are found tobe close to the classical results of Saffman-Taylor. One observes,however, an early destabilization of the fingers that can be attributedto the discrete nature of the individual grains. Classically, thethreshold of destabilization is linked to the noise in the cell and isthus difficult to quantify. We show that the grains represent a‘‘controlled noise’’ and produce an initial perturbation of theinterface with an amplitude proportional to the grain size. The finiteamplitude instability mechanism proposed by Bensimon et al. allows usto link this perturbation to the value of the threshold observed.

[A12] A. Lindner, B.Lestriez, S. Mariot, T. Maevis, R. Brummer, B. Luehmann et C. Creton
Adhesive and rheological properties of lightly crosslinked modelacrylic networks
Journal of Adhesion, 82 (2006) 267-310

The viscoelastic and adhesive properties of aseries of model, lightly crosslinked acrylic polymer networks have beeninvestigated. The model networks were statistical copolymers of2-ethyl-hexyl acrylate and acrylic acid or terpolymers of 2-ethyl-hexylacrylate, acrylic acid, and stearyl acrylate synthesized in solution.All were lightly crosslinked after the polymerization was completed toobtain typical properties of pressure-sensitive adhesives. The bulkrheological properties of the networks were characterized by dynamicalmechanical spectroscopy and in uniaxial extension. Their adhesiveproperties were tested with an instrumented probe tester fitted with acylindrical steel probe. The presence of acrylic acid in the copolymercaused an increase in both elastic modulus and resistance tointerfacial crack propagation characterized by the criticalenergy-release rate Gc and the incorporation of stearyl acrylate causeda decrease in both modulus and Gc. In both cases, however, themodification of Gc controlled the overall behavior. The analysis of thenonlinear elastic properties of the adhesives with the Mooney–Rivlinmodel provided new insights on the role played by the ratio betweenentanglements and crosslink points in controlling the formation andextension of the bridging fibrils observed upon debonding.

[A11] C. Chevalier,M. Ben Amar, D. Bonn et A. Lindner
Inertial effects on Saffman –Taylor viscous fingering
J. Fluid Mech., 552 (2006) 83-97

For the Saffman–Taylor instability, the inertia ofthe fluid may become important for high finger speeds. We investigatethe effects of inertia on the width of the viscous fingersexperimentally. We find that, due to inertia, the finger width canincrease with increasing speed, contrary to what happens at smallReynolds number Re. We find that inertial effects need to be consideredabove a critical Weber number We.In this case it can be shown that the finger width is governed by abalance between viscous forces and inertia. This allows us to define amodified control parameter 1/B’, which takes the corrections due toinertia into account; on rescaling the experimental data with 1/B’,they all collapse onto the universal curve for the classicalSaffman–Taylor instability. Subsequently, we try to rationalize ourobservations. Numerical simulations, taking into account a modificationof Darcy’s law to include inertia, are found to only qualitativelyreproduce the experimental findings, pointing to the importance ofthree-dimensional effects.

[A10] C. Caballero,E. Kolb, A. Lindner, J. Lanuza et E. Clément
Experimental investigation of granular dynamics close to the jammingtransition
J. Phys.: Cond. Matter 17 (2005) S2503

We present different experiments on dense granularassemblies with the aim to clarify the notion of ”jamming transition”for these assemblies of non-Brownian particles. The experimentalset-ups differ in the way external perturbations are applied in orderto unjamm the systems. The first experiment monitors the response to alocally applied deformation of a model packing at rest. The two otherexperiments study local and collective dynamics in a granular assemblyweakly excited by vibration.

[A9] A. Lindner, D.Derks et M. Shelley
Stretch-flow of thin layers of Newtonian liquids : Fingering patternsand lifting forces
Phys. Fluids, 17, 072107, (2005)

We study the stretch flow of a thin layer ofNewtonian liquid constrained between two circular plates. The evolutionof the interface of the originally circular bubble is studied whenlifting one of the plates at a constant velocity and the observedpattern is related to the measured lifting force. By comparingexperimental results to numerical simulations using a Darcy’s law modelwe can account for the fully nonlinear evolution of the observedfingering pattern. One observes an initial destabilization of theinterface by growth of air fingers due to a Saffman–Taylor-likeinstability and then a coarsening of the pattern toward a circularinterface until complete debonding of the two plates occurs. Numericalsimulations reveal that when relating the observed patterns to thelifting force not only the number of fingers but also the amplitude ofthe fingering growth has to be taken into account. This is consistentwith the experimental observations.

[A8] N.Maleki-Jirsaraei, A. Lindner, S. Rouhani et D. Bonn
Saffman – Taylor instability in yield stress fluids
J. Phys.: Condens. Matter, J. Phys.: Cond. Matter 17 (2005) S1219

Pushing a fluid with a less viscous one gives riseto the well known Saffman–Taylor instability. This instability isimportant in a wide variety of applications involving stronglynon-Newtonian fluids that often exhibit a yield stress.Here we investigate the Saffmann–Taylor instability in this type offluid, in longitudinal flows in Hele–Shaw cells. In particular, westudy Darcy’s law for yield stress fluids. The dispersion equation forthe flow is similar to the equations obtained for ordinary viscousfluids but the viscous terms in the dimensionless numbers conditioningthe instability now contain the yield stress.This also has repercussions on the wavelength of the instability as itfollows from a linear stability analysis. As a consequence of thepresence of yield stress, the wavelength of maximum growth is finiteeven at vanishing velocities. We study Darcy’s law and the fingeringpatterns experimentally for a yield stress fluid in a linear Hele–Shawcell. The results are in rather good agreement with the theoreticalpredictions. In addition we observe different regimes that lead todifferent morphologies of the fingering patterns, in both rectangularand circular Hele–Shaw cells.

[A7] A. Lindner, T.Maevis, R. Brummer, B. Lühmann et C. Creton
Sub-critical failure of soft acrylic adhesives under tensile stress
Langmuir 20 (2004) 9156

The mechanisms of failure over time of a series ofmodel acrylic pressure-sensitive adhesives under a moderate level ofstress has been investigated with a probe method. Two competingmechanisms of failure have been observed: a progressive nucleation ofcavities under stress and the propagation of existing cavities at theinterface between the probe and the adhesive layer. Homogeneous creepof the adhesive was never observed as the only failure mechanism. Insituations where the resistance to crack propagation was good relativeto the resistance to cavitation, extensive nucleation of cavities wasobserved until a material-dependent and stable value of stress wasachieved. On the other hand in situations were the resistance to crackpropagation was weak, propagation led invariably to a complete failureof the adhesive bond. In addition to the stress relaxation, the energydissipation was studied allowing to distinguish the different adhesiveseven further. This allowed determination of the optimal amount of acomonomer (acrylic acid) that had to be added to improve the long-termresistance of the adhesives under study. Further more we investigatethe compliance of the confined adhesive layers and compare the obtainedresults to predictions from theoretical models.

[A6] D. Derks, A.Lindner C. Creton et D. Bonn
Cohesive failure of thin layers of soft model adhesives under tension
J. Appl. Phys. 93 (2003) 1557

The cohesive failure of soft adhesives is studiedusing a yield stress fluid as a model adhesive, which allow to relatethe viscoelastic properties of the fluid directly to its adhesiveperformance. We derive a theoretical expression for the force—distancecurve as a function of the yield stress, which describes ourexperimental results very well. The theoretical prediction is obtainedby assuming a circular air—adhesive interface; surprisingly, goodagreement between theory and experiment is also obtained when strongfingering instabilities are observed in the experiment. This result isconfirmed by the fact that we do not find a significant reduction inthe work of adhesion when fingering instabilities are present. Inaddition, we discuss the morphology of the fingering instabilities.

[A5] A. Lindner, J.Vermant et D. Bonn
Elongational viscosity of dilute polymer solutions
Physica A 319 (2003) 125

Measurement of the resistance to a stretchingmotion, the so-called elongational viscosity he,has proven to bedifficult for dilute polymer solutions, although it is important for alarge number of applications. We attempt to deduce heof dilutesolutions of flexible polymers in two different ways. First, hecan beinferred from measurements of the shear viscosity h and the firstnormal stress difference N1, invoking an appropriate constitutiveequation. Second, it is measured in the opposing jet elongationalviscometer. Although the results of the latter method have been muchdebated they agree reasonably well with those from the first method.The combination of the two methods thus allows to obtain the correctorder of magnitude of he

[A4] A. Lindner, D.Bonn, E . Corvera Poiré, M.Ben Amar et J. Meunier
Viscous fingering in non-Newtonian fluids
J.Fluid Mech. 469 (2002) 237

We study the viscous fingering or Saffman-Taylorinstability in two different dilute or semi-dilute polymer solutions.The different solutions exhibit only one non-Newtonian property, in thesense that other non-Newtonian effects can be neglected. The viscosityof solutions of stiff polymers has a strong shear rate dependence.Relative to Newtonian fluids, narrower fingers are found for rigidpolymers. For solutions of flexible polymers, elastic effects such asnormal stresses are dominant, whereas the shear viscosity is almostconstant. Wider fingers are found in this case. We characterize thenon-Newtonian flow properties of these polymer solutions completely,allowing for separate and quantitative investigation of the influenceof the two most common non-Newtonian properties on the Saffman-Taylorinstability. The effects of the non-Newtonian flow properties on theinstability can in all cases be understood quantitatively by redefiningthe control parameter of the instability.

[A3] A. Lindner, P.Coussot et D. Bonn
Viscous fingering in a yield stress fluid
Phys. Rev. Lett. 85 (2000) 314

We study the Saffman-Taylor or viscous fingeringinstability in yield stress fluids. The theory for yield stress fluidsshows that the dispersion equation of the instability is similar tothat for Newtonian fluids; however, the capillary number governing theinstability now contains the yield stress. Experiments using gels andfoams reveal very branched fingers in the gel. The results are inexcellent agreement with theory for the gel, with, in addition, acrossover from yield stress dominated to viscous behaviour. The resultsfor foams are very different due to the existence of wall slip.

[A2] A. Lindner, D.Bonn et J. Meunier
Viscousfingering in complexfluids
Phys.: Condensed Matter 12 (2000), A477

Viscous fingers form when a less viscous fluidpushes a more viscous fluid in a linear channel. The instability of theinterface results from a competition between viscous and capillaryforces. We show here that by using complex fluids such as polymer orsurfactant solutions one can act on the viscosity or the surfacetension and modify the instability drastically. Two different polymersolutions, that exhibit either shear thinning or normal stress effects,are used. For the first fluid the viscous forces are altered leading tofinger narrowing, whereas for the second fluid the viscous forces aresupplemented by normal stresses, which leads to finger widening. Forthe surfactant solutions the modification of the capillary forces leadsto finger widening.

[A1] A. Lindner, D.Bonn et J. Meunier
Viscousfingering in a shear thinning fluid
Physics of Fluids, 12 (2000) 256

We study the Saffman–Taylor instability in arectangular Hele-Shaw cell. The driven fluid is a dilute or semidilutepolymer solution, with a viscosity that exhibits shear thinning. Othernon-Newtonian properties such as elastic effects are negligible underthe present experimental conditions; the system thus allows forseparate investigation of the influence of shear thinning on theinstability. The experiments show that, for weak shear-thinning, theresults for the width of the fingers as a function of the capillarynumber collapse onto the universal curve for Newtonian fluids, providedthe shear-thinning viscosity is used to calculate the capillary number.For stronger shear thinning, narrower fingers are found. The experimentallows also for a study of the applicability ofDarcy’s law to shear thinning fluids. For Newtonian fluids, this lawgives the finger velocity as a function of the pressure gradient. Forweakly shear-thinning fluids, we find that an effective Darcy’s law, inwhich the constant viscosity is replaced by the shear-thinningviscosity, gives good agreement with the experiments. For strongershear thinning, the predictions from the effective Darcy’s lawdeteriorate. Satisfactory agreement with experimental data can beobtained when using a ‘‘shear-thinning’’ Darcy’s law, which can bederived using a power law model for the shear rate dependence of theviscosity.