laboratoire de physique statistique
laboratoire de physique statistique




One-dimensional collective migration of a proliferating cell monolayer - Recho, Pierre and Ranft, Jonas and Marcq, Philippe
SOFT MATTER 122381-2391 (2016)

Abstract : The importance of collective cellular migration during embryogenesis and tissue repair asks for a sound understanding of underlying principles and mechanisms. Here, we address recent in vitro experiments on cell monolayers, which show that the advancement of the leading edge relies on cell proliferation and protrusive activity at the tissue margin. Within a simple viscoelastic mechanical model amenable to detailed analysis, we identify a key parameter responsible for tissue expansion, and we determine the dependence of the monolayer velocity as a function of measurable rheological parameters. Our results allow us to discuss the effects of pharmacological perturbations on the observed tissue dynamics.
Theoretical analysis of growth or swelling wrinkles on constrained soft slabs - Jia, Fei and Ben Amar, Martine
SOFT MATTER 98216-8226 (2013)

Abstract : Growth or swelling of soft slabs attached to a rigid substrate generates large compressive stresses at their surfaces. When the stresses exceed a critical value, the smooth surface becomes unstable. For an in-plane isotropic material, a nonlinear three dimensional analysis is employed to ascertain the energy in the buckled state for different modes: stripes, squares and hexagons. When increasing the growth control parameter, we show that hexagonal patterns with a dimple at the center minimize the elastic energy and will be the dominant mode if the mode with minimal energy is the most likely to be observed. The growth of an anisotropic material reinforced by fibers is also considered. The results provide a way to understand surface wrinkling patterns induced by equi-biaxial growth or swelling of elastic layers, with possible applications for micro-patterns fabrication through an appropriate fiber arrangement.
Viscous fingering at ultralow interfacial tension - Setu, Siti Aminah and Zacharoudiou, Ioannis and Davies, Gareth J. and Bartolo, Denis and Moulinet, Sebastien and Louis, Ard A. and Yeomans, Julia M. and Aarts, Dirk G. A. L.
SOFT MATTER 910599-10605 (2013)

Abstract : We experimentally study the viscous fingering instability in a fluid-fluid phase separated colloid-polymer mixture by means of laser scanning confocal microscopy and microfluidics. We focus on three aspects of the instability. (i) The interface between the two demixed phases has an ultralow surface tension, such that we can address the role of thermal interface fluctuations. (ii) We image the interface in three dimensions allowing us to study the interplay between interface curvature and flow. (iii) The displacing fluid wets all walls completely, in contrast to traditional viscous fingering experiments, in which the displaced fluid wets the walls. We also perform lattice Boltzmann simulations, which help to interpret the experimental observations.
Peristaltic patterns for swelling and shrinking of soft cylindrical gels - Ciarletta, Pasquale and Ben Amar, Martine
SOFT MATTER 81760-1763 (2012)

Abstract : We propose a variational method for determining the surface patterns of cylindrical gels for both swelling and shrinking. Exact solutions are calculated for the initial stages of such peristaltic instabilities. The morphology and the formation mechanisms depend on a competition between bulk elastic energy and surface tension.
Pattern formation by dewetting and evaporating sedimenting suspensions - Habibi, Mehdi and Moller, Peder and Fall, Abdoulaye and Rafai, Salima and Bonn, Daniel
SOFT MATTER 84682-4686 (2012)

Abstract : Pattern formation from drying droplets containing sedimenting particles and dewetting of thin films of such suspensions was studied. The dewetting causes the formation of finger-like patterns near the contact line which leave behind a deposit of branches. We find that the strikingly low speed of dewetting is due to the high particle concentration in the contact line region, leading to a strongly enhanced viscosity. For pattern formation from drying droplets (containing particles), evaporation also causes dewetting. In both cases, we find a similar relationship between the size of the patterns and the dewetting speed. The coefficient of this relationship gives us the effective viscosity at the contact line. We present a simple model that accounts for this, and that shows that the size of the particles is the relevant length scale in both problems.
Shear thickening of Laponite suspensions with poly(ethylene oxide) - Fall, Abdoulaye and Bonn, Daniel
SOFT MATTER 84645-4651 (2012)

Abstract : We study the effect of addition of polyethylene oxide (PEO) on the rheological behavior of suspensions of Laponite. Experiments were performed on mixtures of PEO and Laponite at different concentrations. These mixtures can exhibit very strong shear thickening behavior: under shear, the suspension can become a yield stress material sufficiently strong to support its own weight. Depending on the polymer concentration, we observe continuous or discontinuous shear thickening mechanisms; it is the discontinuous shear thickening that leads to the formation of yield stress materials.
Dynamical heterogeneity in aging colloidal glasses of Laponite - Jabbari-Farouji, S. and Zargar, R. and Wegdam, G. H. and Bonn, Daniel
SOFT MATTER 85507-5512 (2012)

Abstract : Glasses behave as solids due to their long relaxation time; however the origin of this slow response remains a puzzle. Growing dynamic length scales due to cooperative motion of particles are believed to be central to the understanding of both the slow dynamics and the emergence of rigidity. Here, we provide experimental evidence of a growing dynamical heterogeneity length scale that increases with increasing waiting time in an aging colloidal glass of Laponite. The signature of heterogeneity in the dynamics follows from dynamic light scattering measurements in which we study both the rotational and translational diffusion of the disk-shaped particles of Laponite in suspension. These measurements are accompanied by simultaneous microrheology and macroscopic rheology experiments. We find that rotational diffusion of particles slows down at a faster rate than their translational motion. Such decoupling of translational and orientational degrees of freedom finds its origin in the dynamic heterogeneity since rotation and translation probe different length scales in the sample. The macroscopic rheology experiments show that the low frequency shear viscosity increases at a much faster rate than both rotational and translational diffusive relaxation times.
Micro helical polymeric structures produced by variable voltage direct electrospinning - Shariatpanahi, S. P. and Zad, A. Iraji and Abdollahzadeh, I. and Shirsavar, R. and Bonn, D. and Ejtehadi, R.
SOFT MATTER 710548-10551 (2011)

Abstract : Direct near field electrospinning is used to produce very long helical polystyrene microfibers in water. The pitch length of helices can be controlled by changing the applied voltage, allowing the production of both microsprings and microchannels. Using a novel high frequency variable voltage electrospinning method we determined the helix formation speed and compared the experimental buckling frequency to theoretical expressions for viscous and elastic buckling. Finally we showed that the new method can be used to produce new periodic micro and nano structures.
Capillary wrinkling of elastic membranes - Vella, D. and Adda-Bedia, M. and Cerda, E.
SOFT MATTER 65778-5782 (2010)

Abstract : We present a physically-based model for the deformation of a floating elastic membrane caused by the presence of a liquid drop. Starting from the equations of membrane theory modified to account for surface energies, we show that the presence of a liquid drop causes an azimuthal compression over a finite region. This explains the origin of the wrinkling of such membranes observed recently (Huang et al., Science, 2007, 317, 650) and suggests a single parameter that determines the extent of the wrinkled region. While experimental data supports the importance of this single parameter, our theory under-predicts the extent of the wrinkled region observed experimentally. We suggest that this discrepancy is likely to be due to the wrinkling observed here being far from the threshold.
Density of states of colloidal glasses and supercooled liquids - Ghosh, Antina and Mari, Romain and Chikkadi, Vijayakumar and Schall, Peter and Kurchan, Jorge and Bonn, Daniel
SOFT MATTER 63082-3090 (2010)

Abstract : The glass transition is perhaps the greatest unsolved problem in condensed matter physics: the main question is how to reconcile the liquid-like structure with solid-like mechanical properties. In solids, structure and mechanics are related directly through the vibrational density of states of the material. Here, we obtain for the first time the density of states of colloidal glasses and supercooled liquids from a normal-mode analysis of particle displacements measured using confocal microscopy. We find that the spectrum of the (non-linear) vibrations has many `soft', low-frequency modes, more abundant and very different in nature from the usual acoustic vibrations of ordinary solids. This results in an anomalous low-frequency peak in the density of states which approaches zero frequency as one goes deeper into the glass. The observed soft modes are due to collective `swirling' particle motions, that extend over surprisingly long length scales.
Super-diffusion around the rigidity transition: Levy and the Lilliputians - Lechenault, F. and Candelier, R. and Dauchot, O. and Bouchaud, J. -P. and Biroli, G.
SOFT MATTER 63059-3064 (2010)

Abstract : By analyzing the displacement statistics of an assembly of horizontally vibrated bi-disperse frictional grains in the vicinity of the jamming transition experimentally studied before (F. Lechenault, O. Dauchot, G. Biroli and J.-P. Bouchard, Europhys. Lett., 2008, 83, 46003), we establish that their superdiffusive motion is a genuine Levy flight, but with a `jump' size that is very small compared to the diameter of the grains. The vibration induces a broad distribution of jumps that are random in time, but correlated in space, and that can be interpreted as micro-crack events at all scales. As the volume fraction departs from the critical jamming density, this distribution is truncated at a smaller and smaller jump size, inducing a crossover towards standard diffusive motion at long times. This interpretation contrasts with the idea of temporally persistent, spatially correlated currents and raises new issues regarding the analysis of the dynamics in terms of vibrational modes.
Evaporation of macroscopic sessile droplets - Cazabat, Anne-Marie and Guena, Geoffroy
SOFT MATTER 62591-2612 (2010)

Abstract : This review is aimed at presenting the evaporation of macroscopic sessile droplets on inert substrates in normal atmosphere in simple cases, as a basis for more complex analyses.
Dynamics of colloidal crystals in shear flow - Derks, Didi and Wu, Yu Ling and van Blaaderen, Alfons and Imhof, Arnout
SOFT MATTER 51060-1065 (2009)

Abstract : We investigate particle dynamics in nearly hard sphere colloidal crystals submitted to a steady shear flow. Both the fluctuations of single colloids and the collective motion of crystalline layers as a whole are studied by using a home-built counter rotating shear cell in combination with confocal microscopy. Firstly, our real space observations confirm the global structure and orientation as well as the collective zigzag motion as found by early scattering experiments. Secondly, dynamic processes accompanying the shear melting transition are followed on the particle level. Local rearrangements in the crystal are seen to occur more frequently with increasing shear rate. This shear-enhanced particle mobility is quantified by measuring the random particle displacements from time-tracked particle coordinates. We find that shear induced melting takes place when these random displacements reach 12\% of the particle separation, reminiscent of the Lindemann criterion for melting in equilibrium systems. In addition, a dynamic criterion for melting, based on the relative importance of the long time self diffusion compared to the short time self diffusion, is discussed.