DOI

1

Approximate scale invariance in particle systems: A large-dimensional justification - Maimbourg, Thibaud and Kurchan, Jorge

EPL 114, (2016)

Abstract : Systems of particles interacting via inverse-power law potentials have an invariance with respect to changes in length and temperature, implying a correspondence in the dynamics and thermodynamics between different ``isomorphic'' sets of temperatures and densities. In a recent series of works, it has been argued that such correspondences hold to a surprisingly good approximation in a much more general class of potentials, an observation that summarizes many properties that have been observed in the past. In this paper we show that such relations are exact in high-dimensional liquids and glasses, a limit in which the conditions for these mappings to hold become transparent. The special role played by the exponential potential is also confirmed. Copyright (C) EPLA, 2016

EPL 114, (2016)

Abstract : Systems of particles interacting via inverse-power law potentials have an invariance with respect to changes in length and temperature, implying a correspondence in the dynamics and thermodynamics between different ``isomorphic'' sets of temperatures and densities. In a recent series of works, it has been argued that such correspondences hold to a surprisingly good approximation in a much more general class of potentials, an observation that summarizes many properties that have been observed in the past. In this paper we show that such relations are exact in high-dimensional liquids and glasses, a limit in which the conditions for these mappings to hold become transparent. The special role played by the exponential potential is also confirmed. Copyright (C) EPLA, 2016

DOI

2

From non-ergodic eigenvectors to local resolvent statistics and back: A random matrix perspective - Facoetti, Davide and Vivo, Pierpaolo and Biroli, Giulio

EPL 115, (2016)

Abstract : We study the statistics of the local resolvent and non-ergodic properties of eigenvectors for a generalised Rosenzweig-Porter N x N random matrix model, undergoing two transitions separated by a delocalised non-ergodic phase. Interpreting the model as the combination of on-site random energies \a(i)\ and a structurally disordered hopping, we found that each eigenstate is delocalised over N2-gamma sites close in energy vertical bar a(j) - a(i)vertical bar = N1-gamma in agreement with Kravtsov et al. (New J. Phys., 17 (2015) 122002). Our other main result, obtained combining a recurrence relation for the resolvent matrix with insights from Dyson's Brownian motion, is to show that the properties of the non-ergodic delocalised phase can be probed studying the statistics of the local resolvent in a non-standard scaling limit. Copyright (C) EPLA, 2016

EPL 115, (2016)

Abstract : We study the statistics of the local resolvent and non-ergodic properties of eigenvectors for a generalised Rosenzweig-Porter N x N random matrix model, undergoing two transitions separated by a delocalised non-ergodic phase. Interpreting the model as the combination of on-site random energies \a(i)\ and a structurally disordered hopping, we found that each eigenstate is delocalised over N2-gamma sites close in energy vertical bar a(j) - a(i)vertical bar = N1-gamma in agreement with Kravtsov et al. (New J. Phys., 17 (2015) 122002). Our other main result, obtained combining a recurrence relation for the resolvent matrix with insights from Dyson's Brownian motion, is to show that the properties of the non-ergodic delocalised phase can be probed studying the statistics of the local resolvent in a non-standard scaling limit. Copyright (C) EPLA, 2016

DOI

3

Bifurcations of a large-scale circulation in a quasi-bidimensional turbulent flow - Michel, G. and Herault, J. and Petrelis, F. and Fauve, S.

EPL 115, (2016)

Abstract : We report the experimental study of the bifurcations of a large-scale circulation that is formed over a turbulent flow generated by a spatially periodic forcing. After shortly describing how the flow becomes turbulent through a sequence of symmetry-breaking bifurcations, we focus our study on the transitions that occur within the turbulent regime. They are related to changes in the shape of the probability density function (PDF) of the amplitude of the large-scale flow. We discuss the nature of these bifurcations and how to model the shape of the PDF. Copyright (C) EPLA, 2016

EPL 115, (2016)

Abstract : We report the experimental study of the bifurcations of a large-scale circulation that is formed over a turbulent flow generated by a spatially periodic forcing. After shortly describing how the flow becomes turbulent through a sequence of symmetry-breaking bifurcations, we focus our study on the transitions that occur within the turbulent regime. They are related to changes in the shape of the probability density function (PDF) of the amplitude of the large-scale flow. We discuss the nature of these bifurcations and how to model the shape of the PDF. Copyright (C) EPLA, 2016

DOI

4

Liquid-solid transitions in the three-body hard-core model - Comparin, Tommaso and Kapfer, Sebastian C. and Krauth, Werner

EPL 109, (2015)

Abstract : We determine the phase diagram for a generalisation of two- and three-dimensional hard spheres: a classical system with three-body interactions realised as a hard cut-off on the mean-square distance for each triplet of particles. Quantum versions of this model are important in the context of the unitary Bose gas, which is currently under close theoretical and experimental scrutiny. In two dimensions, the three-body hard-core model possesses a conventional atomic liquid phase and a peculiar solid phase formed by dimers. These dimers interact effectively as hard disks. In three dimensions, the solid phase consists of isolated atoms that arrange in a simple-hexagonal lattice. Copyright (C) EPLA, 2015

EPL 109, (2015)

Abstract : We determine the phase diagram for a generalisation of two- and three-dimensional hard spheres: a classical system with three-body interactions realised as a hard cut-off on the mean-square distance for each triplet of particles. Quantum versions of this model are important in the context of the unitary Bose gas, which is currently under close theoretical and experimental scrutiny. In two dimensions, the three-body hard-core model possesses a conventional atomic liquid phase and a peculiar solid phase formed by dimers. These dimers interact effectively as hard disks. In three dimensions, the solid phase consists of isolated atoms that arrange in a simple-hexagonal lattice. Copyright (C) EPLA, 2015

DOI

5

Adsorption-induced strain of a nanoscale silicon honeycomb - Grosman, A. and Puibasset, J. and Rolley, E.

EPL 109, (2015)

Abstract : We report on systematic measurements of both adsorption and anisotropic mechanical deformations of mesoporous silicon, using heptane at room temperature. Porous Si obtained from highly doped (100) Si can be thought of as a nanoscale random honeycomb with pores parallel to the [001] axis. We show that strains is an element of(parallel to) and is an element of(perpendicular to) measured along and transversely to the pore axis exhibit a hysteretic behavior as a function of the fluid pressure, which is due to the hysteresis in fluid adsorption. The pressure dependence of the strains together with the independent measurement of the transverse stress, allows us to determine the biaxial transverse modulus and to estimate the longitudinal Young's modulus of porous Si. We argue that the value of these constants implies that Young's modulus of the 6 nm thick walls of the honeycomb is about 5 times smaller than that of bulk silicon, striking evidence of finite-size effects. Copyright (C) EPLA, 2015

EPL 109, (2015)

Abstract : We report on systematic measurements of both adsorption and anisotropic mechanical deformations of mesoporous silicon, using heptane at room temperature. Porous Si obtained from highly doped (100) Si can be thought of as a nanoscale random honeycomb with pores parallel to the [001] axis. We show that strains is an element of(parallel to) and is an element of(perpendicular to) measured along and transversely to the pore axis exhibit a hysteretic behavior as a function of the fluid pressure, which is due to the hysteresis in fluid adsorption. The pressure dependence of the strains together with the independent measurement of the transverse stress, allows us to determine the biaxial transverse modulus and to estimate the longitudinal Young's modulus of porous Si. We argue that the value of these constants implies that Young's modulus of the 6 nm thick walls of the honeycomb is about 5 times smaller than that of bulk silicon, striking evidence of finite-size effects. Copyright (C) EPLA, 2015

DOI

6

Non-universal Voronoi cell shapes in amorphous ellipsoid packs - Schaller, Fabian M. and Kapfer, Sebastian C. and Hilton, James E. and Cleary, Paul W. and Mecke, Klaus and De Michele, Cristiano and Schilling, Tanja and Saadatfar, Mohammad and Schroeter, Matthias and Delaney, Gary W. and Schroeder-Turk, Gerd E.

EPL 111, (2015)

Abstract : In particulate systems with short-range interactions, such as granular matter or simple fluids, local structure determines the macroscopic physical properties. We analyse local structure metrics derived from the Voronoi diagram of oblate ellipsoids, for various aspect ratios a and global packing fractions phi(g). We focus on jammed static configurations of frictional ellipsoids, obtained by tomographic imaging and by discrete element method simulations. The rescaled distribution of local packing fractions phi(l), defined as the ratio of particle volume and its Voronoi cell volume, is found to be independent of the particle aspect ratio, and coincide with results for sphere packs. By contrast, the typical Voronoi cell shape, quantified by the Minkowski tensor anisotropy index beta = beta(2,0)(0), points towards a difference between random packings of spheres and those of oblate ellipsoids. While the average cell shape beta of all cells with a given value of phi(l) is similar in dense and loose jammed sphere packings, the structure of dense and loose ellipsoid packings differs substantially such that this does not hold true. Copyright (C) EPLA, 2015

EPL 111, (2015)

Abstract : In particulate systems with short-range interactions, such as granular matter or simple fluids, local structure determines the macroscopic physical properties. We analyse local structure metrics derived from the Voronoi diagram of oblate ellipsoids, for various aspect ratios a and global packing fractions phi(g). We focus on jammed static configurations of frictional ellipsoids, obtained by tomographic imaging and by discrete element method simulations. The rescaled distribution of local packing fractions phi(l), defined as the ratio of particle volume and its Voronoi cell volume, is found to be independent of the particle aspect ratio, and coincide with results for sphere packs. By contrast, the typical Voronoi cell shape, quantified by the Minkowski tensor anisotropy index beta = beta(2,0)(0), points towards a difference between random packings of spheres and those of oblate ellipsoids. While the average cell shape beta of all cells with a given value of phi(l) is similar in dense and loose jammed sphere packings, the structure of dense and loose ellipsoid packings differs substantially such that this does not hold true. Copyright (C) EPLA, 2015

DOI

7

Experimental observation of 1/f noise in quasi-bidimensional turbulent flows - Herault, J. and Petrelis, F. and Fauve, S.

EPL 111, (2015)

Abstract : We report the experimental observation of 1/f(alpha) noise in quasi-bidimensional turbulence of an electromagnetically forced flow. The large-scale velocity U-L exhibits this power-law spectrum with alpha approximate to 0.7 over a range of frequencies smaller than both the characteristic turnover frequency and the damping rate of the flow. By studying the statistical properties of sojourn time in each polarity of U-L, we demonstrate that the 1/f(alpha) noise is generated by a renewal process, defined by a two-state model given by the polarities of the large-scale circulation. The statistical properties of this renewal process are shown to control the value of the exponent alpha. Copyright (C) EPLA, 2015

EPL 111, (2015)

Abstract : We report the experimental observation of 1/f(alpha) noise in quasi-bidimensional turbulence of an electromagnetically forced flow. The large-scale velocity U-L exhibits this power-law spectrum with alpha approximate to 0.7 over a range of frequencies smaller than both the characteristic turnover frequency and the damping rate of the flow. By studying the statistical properties of sojourn time in each polarity of U-L, we demonstrate that the 1/f(alpha) noise is generated by a renewal process, defined by a two-state model given by the polarities of the large-scale circulation. The statistical properties of this renewal process are shown to control the value of the exponent alpha. Copyright (C) EPLA, 2015

DOI

8

Event-chain Monte Carlo for classical continuous spin models - Michel, Manon and Mayer, Johannes and Krauth, Werner

EPL 112, (2015)

Abstract : We apply the event-chain Monte Carlo algorithm to classical continuum spin models on a lattice and clarify the condition for its validity. In the two-dimensional XY model, it outperforms the local Monte Carlo algorithm by two orders of magnitude, although it remains slower than the Wolff cluster algorithm. In the three-dimensional XY spin glass model at low temperature, the event-chain algorithm is far superior to the other algorithms. Copyright (C) EPLA, 2015

EPL 112, (2015)

Abstract : We apply the event-chain Monte Carlo algorithm to classical continuum spin models on a lattice and clarify the condition for its validity. In the two-dimensional XY model, it outperforms the local Monte Carlo algorithm by two orders of magnitude, although it remains slower than the Wolff cluster algorithm. In the three-dimensional XY spin glass model at low temperature, the event-chain algorithm is far superior to the other algorithms. Copyright (C) EPLA, 2015

DOI

9

Drifting patterns as field reversals - Petrelis, F. and Laroche, C. and Gallet, B. and Fauve, S.

EPL 112, (2015)

Abstract : One-dimensional patterns generated by the Faraday instability at the surface of a vertically vibrated fluid are investigated when the reflection symmetry in the direction of the pattern is broken. For large symmetry breaking, the stationary instability turns into a Hopf bifurcation at a codimension-2 point. This Hopf bifurcation amounts to a periodic drift of the pattern. Further above the onset of the instability, this drift transition competes with the Eckhaus instability as predicted by the study of a model built upon the Swift-Hohenberg equation. In the presence of noise, the drift becomes random and time series of the pattern amplitude display random reversals (sign changes). We show that these reversals belong to the same class as those observed in a variety of contexts such as magnetic fields generated by dynamo action. Copyright (C) EPLA, 2015

EPL 112, (2015)

Abstract : One-dimensional patterns generated by the Faraday instability at the surface of a vertically vibrated fluid are investigated when the reflection symmetry in the direction of the pattern is broken. For large symmetry breaking, the stationary instability turns into a Hopf bifurcation at a codimension-2 point. This Hopf bifurcation amounts to a periodic drift of the pattern. Further above the onset of the instability, this drift transition competes with the Eckhaus instability as predicted by the study of a model built upon the Swift-Hohenberg equation. In the presence of noise, the drift becomes random and time series of the pattern amplitude display random reversals (sign changes). We show that these reversals belong to the same class as those observed in a variety of contexts such as magnetic fields generated by dynamo action. Copyright (C) EPLA, 2015

DOI

10

Wave-induced motion of magnetic spheres - Gissinger, Christophe

EPL 112, (2015)

Abstract : We report an experimental study of the motion of magnetized beads driven by a travelling-wave magnetic field. For sufficiently large wave speed, we report the existence of a backward motion, in which the sphere can move in the direction opposite to the driving wave. We show that the transition to this new state is strongly subcritical and can lead to chaotic motion of the bead. For some parameters, this counterpropagation of the sphere can be one order of magnitude faster than the driving-wave speed. These results are understood in the framework of a model based on the interplay among solid friction, air resistance and magnetic torque. Copyright (C) EPLA, 2015

EPL 112, (2015)

Abstract : We report an experimental study of the motion of magnetized beads driven by a travelling-wave magnetic field. For sufficiently large wave speed, we report the existence of a backward motion, in which the sphere can move in the direction opposite to the driving wave. We show that the transition to this new state is strongly subcritical and can lead to chaotic motion of the bead. For some parameters, this counterpropagation of the sphere can be one order of magnitude faster than the driving-wave speed. These results are understood in the framework of a model based on the interplay among solid friction, air resistance and magnetic torque. Copyright (C) EPLA, 2015

DOI

11

Comparing open and closed molecular self-assembly - Castelnovo, M. and Verdier, T. and Foret, L.

EPL 105, (2014)

Abstract : We study theoretically in the present work the self-assembly of molecules in an open system, which is fed by monomers and depleted in partial or complete clusters. Such a scenario is likely to occur for example in the context of viral self-assembly. We provide a general formula for the mean-field size distribution which is valid both at equilibrium in a closed system, and in the stationary state in an open system. This allows us to explore in a simple way out-of-equilibrium features for self-assembly and compare them to equilibrium properties. In particular, we identify a region of parameter space for which the out-of-equilibrium size distribution in the presence of external fluxes is equal to the equilibrium size distribution in the absence of external fluxes, up to a constant renormalization factor. The range of validity of this result and its consequences are discussed. Copyright (c) EPLA, 2014

EPL 105, (2014)

Abstract : We study theoretically in the present work the self-assembly of molecules in an open system, which is fed by monomers and depleted in partial or complete clusters. Such a scenario is likely to occur for example in the context of viral self-assembly. We provide a general formula for the mean-field size distribution which is valid both at equilibrium in a closed system, and in the stationary state in an open system. This allows us to explore in a simple way out-of-equilibrium features for self-assembly and compare them to equilibrium properties. In particular, we identify a region of parameter space for which the out-of-equilibrium size distribution in the presence of external fluxes is equal to the equilibrium size distribution in the absence of external fluxes, up to a constant renormalization factor. The range of validity of this result and its consequences are discussed. Copyright (c) EPLA, 2014

DOI

12

Optical signatures of a fully dark exciton condensate - Combescot, Monique and Combescot, Roland and Alloing, Mathieu and Dubin, Francois

EPL 105, (2014)

Abstract : We propose optical means to reveal the presence of a dark exciton condensate that does not yield any photoluminescence at all. We show that i) the dark exciton density can be obtained from the blueshift of the excitonic absorption line induced by dark excitons; ii) the polarization of the dark condensate can be obtained from the blueshift dependence on the probe photon polarization as well as from the Faraday effect. All these effects result from carrier exchanges between dark and bright states. Copyright (C) EPLA, 2014

EPL 105, (2014)

Abstract : We propose optical means to reveal the presence of a dark exciton condensate that does not yield any photoluminescence at all. We show that i) the dark exciton density can be obtained from the blueshift of the excitonic absorption line induced by dark excitons; ii) the polarization of the dark condensate can be obtained from the blueshift dependence on the probe photon polarization as well as from the Faraday effect. All these effects result from carrier exchanges between dark and bright states. Copyright (C) EPLA, 2014

DOI

13

Dynamo action by turbulence in absolute equilibrium - Prasath, Srinivasa Gopalakrishnan Ganga and Fauve, Stephan and Brachet, Marc

EPL 106, (2014)

Abstract : We consider the generation of a large-scale magnetic field by a turbulent flow driven by a small-scale helical forcing in a low magnetic Prandtl number fluid. We provide an estimate of the dynamo threshold that takes into account the presence of large-scale turbulent fluctuations by considering that the scales of the flow that mostly contribute to the dynamo process are roughly in absolute equilibrium. We show that turbulent flows in absolute equilibrium do generate dynamos and we compare their growth rates to their laminar counterparts. Finally, we show that the back reaction of the growing magnetic field modifies the statistical properties of turbulent flow by suppressing its kinetic helicity at large magnetic Reynolds number. Copyright (C) EPLA, 2014

EPL 106, (2014)

Abstract : We consider the generation of a large-scale magnetic field by a turbulent flow driven by a small-scale helical forcing in a low magnetic Prandtl number fluid. We provide an estimate of the dynamo threshold that takes into account the presence of large-scale turbulent fluctuations by considering that the scales of the flow that mostly contribute to the dynamo process are roughly in absolute equilibrium. We show that turbulent flows in absolute equilibrium do generate dynamos and we compare their growth rates to their laminar counterparts. Finally, we show that the back reaction of the growing magnetic field modifies the statistical properties of turbulent flow by suppressing its kinetic helicity at large magnetic Reynolds number. Copyright (C) EPLA, 2014

DOI

14

Evidence for a Bose-Einstein condensate of excitons - Alloing, Mathieu and Beian, Mussie and Lewenstein, Maciej and Fuster, David and Gonzalez, Yolanda and Gonzalez, Luisa and Combescot, Roland and Combescot, Monique and Dubin, Francois

EPL 107, (2014)

Abstract : We report compelling evidence for a ``gray'' condensate of dipolar excitons, electrically polarised in a 25 nm wide GaAs quantum well. The condensate is composed by a macroscopic population of dark excitons coherently coupled to a lower population of bright excitons. To create the exciton condensate we use an all-optical approach in order to produce microscopic traps which confine a dense exciton gas (similar to 10(10) cm(-2)) that yet exhibits an anomalously weak photoemission at sub-kelvin temperatures. This is the first fingerprint for the ``gray'' condensate. It is then confirmed by the macroscopic spatial coherence and the linear polarization of the weak excitonic photoluminescence emitted from the trap, as theoretically predicted. Copyright (C) EPLA, 2014

EPL 107, (2014)

Abstract : We report compelling evidence for a ``gray'' condensate of dipolar excitons, electrically polarised in a 25 nm wide GaAs quantum well. The condensate is composed by a macroscopic population of dark excitons coherently coupled to a lower population of bright excitons. To create the exciton condensate we use an all-optical approach in order to produce microscopic traps which confine a dense exciton gas (similar to 10(10) cm(-2)) that yet exhibits an anomalously weak photoemission at sub-kelvin temperatures. This is the first fingerprint for the ``gray'' condensate. It is then confirmed by the macroscopic spatial coherence and the linear polarization of the weak excitonic photoluminescence emitted from the trap, as theoretically predicted. Copyright (C) EPLA, 2014

DOI

15

Spectral density of the non-backtracking operator on random graphs - Saade, A. and Krzakala, F. and Zdeborova, L.

EPL 107, (2014)

Abstract : The non-backtracking operator was recently shown to provide a significant improvement when used for spectral clustering of sparse networks. In this paper we analyze its spectral density on large random sparse graphs using a mapping to the correlation functions of a certain interacting quantum disordered system on the graph. On sparse, tree-like graphs, this can be solved efficiently by the cavity method and a belief propagation algorithm. We show that there exists a paramagnetic phase, leading to zero spectral density, that is stable outside a circle of radius v., where. is the leading eigenvalue of the non-backtracking operator. We observe a second-order phase transition at the edge of this circle, between a zero and a non-zero spectral density. The fact that this phase transition is absent in the spectral density of other matrices commonly used for spectral clustering provides a physical justification of the performances of the non-backtracking operator in spectral clustering. Copyright (C) EPLA, 2014

EPL 107, (2014)

Abstract : The non-backtracking operator was recently shown to provide a significant improvement when used for spectral clustering of sparse networks. In this paper we analyze its spectral density on large random sparse graphs using a mapping to the correlation functions of a certain interacting quantum disordered system on the graph. On sparse, tree-like graphs, this can be solved efficiently by the cavity method and a belief propagation algorithm. We show that there exists a paramagnetic phase, leading to zero spectral density, that is stable outside a circle of radius v., where. is the leading eigenvalue of the non-backtracking operator. We observe a second-order phase transition at the edge of this circle, between a zero and a non-zero spectral density. The fact that this phase transition is absent in the spectral density of other matrices commonly used for spectral clustering provides a physical justification of the performances of the non-backtracking operator in spectral clustering. Copyright (C) EPLA, 2014

DOI

16

Patterns in biofilms: From contour undulations to fold focussing - Amar, Martine Ben and Wu, Min

EPL 108, (2014)

Abstract : Morphologies of soft materials in growth, swelling or drying have been extensively studied recently. Shape modifications occur as the size varies transforming ordinary spheres, cylinders and thin plates into more or less complex objects. Here we consider the genesis of biofilm patterns when a simple disc containing initially bacteria with moderate adhesion to a rigid substrate grows according to very simple rules. The initial circular geometry is lost during the growth expansion, contour undulations and buckling appear, ultimately a rather regular periodic focussing of folds repartition emerges. We theoretically predict these morphological instabilities as bifurcations of solutions in elasticity, characterized by typical driving parameters established here. The substrate plays a critical role limiting the geometry of the possible modes of instabilities and anisotropic growth, adhesion and toughness compete to eventually give rise to wrinkling, buckling or both. Additionally, due to the substrate, we show that the ordinary buckling modes, vertical deviation of thin films, are not observed in practice and a competitive pattern with self-focussing of folds can be found analytically. These patterns are reminiscent of the blisters of delamination in material sciences and explain recent observations of bacteria biofilms. The model presented here is purely analytical, is based on a neo-Hookean elastic energy, and can be extended without difficulties and applied to polymer materials. editor's choice Copyright (C) EPLA, 2014

EPL 108, (2014)

Abstract : Morphologies of soft materials in growth, swelling or drying have been extensively studied recently. Shape modifications occur as the size varies transforming ordinary spheres, cylinders and thin plates into more or less complex objects. Here we consider the genesis of biofilm patterns when a simple disc containing initially bacteria with moderate adhesion to a rigid substrate grows according to very simple rules. The initial circular geometry is lost during the growth expansion, contour undulations and buckling appear, ultimately a rather regular periodic focussing of folds repartition emerges. We theoretically predict these morphological instabilities as bifurcations of solutions in elasticity, characterized by typical driving parameters established here. The substrate plays a critical role limiting the geometry of the possible modes of instabilities and anisotropic growth, adhesion and toughness compete to eventually give rise to wrinkling, buckling or both. Additionally, due to the substrate, we show that the ordinary buckling modes, vertical deviation of thin films, are not observed in practice and a competitive pattern with self-focussing of folds can be found analytically. These patterns are reminiscent of the blisters of delamination in material sciences and explain recent observations of bacteria biofilms. The model presented here is purely analytical, is based on a neo-Hookean elastic energy, and can be extended without difficulties and applied to polymer materials. editor's choice Copyright (C) EPLA, 2014

DOI

17

High-Reynolds-number turbulence in complex fluids - Kulmatova, D. and Bonn, D. and Kellay, H.

EPL 101, (2013)

Abstract : We here examine the structure of turbulence in the case of a complex fluid made up of water and surfactants. This fluid has the particular property of shear thickening when driven at shear rates above a certain threshold. Through a study of the spectral properties and the structure function scalings, important differences arise with respect to the reference case, i.e., water. The surfactant solution shows strong intermittency at small scales. The large scales are, on the other hand, free of intermittency. While this transition is observed in the structure function scalings, no sign of this transition is seen in the power spectrum of velocity fluctuations which shows a single scaling range. The strongly intermittent small-scale region, despite the scaling of the power spectrum, exhibits properties reminiscent of the near-dissipative range. Copyright (C) EPLA, 2013

EPL 101, (2013)

Abstract : We here examine the structure of turbulence in the case of a complex fluid made up of water and surfactants. This fluid has the particular property of shear thickening when driven at shear rates above a certain threshold. Through a study of the spectral properties and the structure function scalings, important differences arise with respect to the reference case, i.e., water. The surfactant solution shows strong intermittency at small scales. The large scales are, on the other hand, free of intermittency. While this transition is observed in the structure function scalings, no sign of this transition is seen in the power spectrum of velocity fluctuations which shows a single scaling range. The strongly intermittent small-scale region, despite the scaling of the power spectrum, exhibits properties reminiscent of the near-dissipative range. Copyright (C) EPLA, 2013

DOI

18

Universal current fluctuations in the symmetric exclusion process and other diffusive systems - Akkermans, Eric and Bodineau, Thierry and Derrida, Bernard and Shpielberg, Ohad

EPL 103, (2013)

Abstract : Using the macroscopic fluctuation theory of Bertini, De Sole, Gabrielli, Jona-Lasinio, and Landim, one can show that the statistics of the current of the symmetric simple exclusion process (SSEP) connected to two reservoirs on an arbitrary large finite domain in dimension d are the same as in the one-dimensional case. Numerical results on squares support this claim while results on cubes exhibit some discrepancy. We argue that the results of the macroscopic fluctuation theory should be recovered by increasing the size of the contacts. The generalization to other diffusive systems is straightforward. Copyright (C) EPLA, 2013

EPL 103, (2013)

Abstract : Using the macroscopic fluctuation theory of Bertini, De Sole, Gabrielli, Jona-Lasinio, and Landim, one can show that the statistics of the current of the symmetric simple exclusion process (SSEP) connected to two reservoirs on an arbitrary large finite domain in dimension d are the same as in the one-dimensional case. Numerical results on squares support this claim while results on cubes exhibit some discrepancy. We argue that the results of the macroscopic fluctuation theory should be recovered by increasing the size of the contacts. The generalization to other diffusive systems is straightforward. Copyright (C) EPLA, 2013

DOI

19

Homoclinic bifurcations in low-Prandtl-number Rayleigh-Benard convection with uniform rotation - Maity, P. and Kumar, K. and Pal, P.

EPL 103, (2013)

Abstract : We present results of direct numerical simulations on homoclinic gluing and ungluing bifurcations in a low-Prandtl-number (0 <= Pr <= 0.025) Rayleigh-Benard system rotating slowly and uniformly about a vertical axis. We have performed simulations with stress-free top and bottom boundaries for several values of the Taylor number (5 <= Ta <= 50) near the instability onset. We observe a single homoclinic ungluing bifurcation, marked by the spontaneous breaking of a larger limit cycle into two limit cycles with the variation of the reduced Rayleigh number r for smaller values of Ta (< 25). A pair of homoclinic bifurcations, instead of one bifurcation, is observed with the variation of r for slightly higher values of Ta (25 <= Ta <= 50) in the same fluid dynamical system. The variation of the bifurcation threshold with Ta is also investigated. We have also constructed a low-dimensional model which qualitatively captures the dynamics of the system near the homoclinic bifurcations for low rotation rates. The model is used to study the unfolding of bifurcations and the variation of the homoclinic bifurcation threshold with Pr. Copyright (C) EPLA, 2013

EPL 103, (2013)

Abstract : We present results of direct numerical simulations on homoclinic gluing and ungluing bifurcations in a low-Prandtl-number (0 <= Pr <= 0.025) Rayleigh-Benard system rotating slowly and uniformly about a vertical axis. We have performed simulations with stress-free top and bottom boundaries for several values of the Taylor number (5 <= Ta <= 50) near the instability onset. We observe a single homoclinic ungluing bifurcation, marked by the spontaneous breaking of a larger limit cycle into two limit cycles with the variation of the reduced Rayleigh number r for smaller values of Ta (< 25). A pair of homoclinic bifurcations, instead of one bifurcation, is observed with the variation of r for slightly higher values of Ta (25 <= Ta <= 50) in the same fluid dynamical system. The variation of the bifurcation threshold with Ta is also investigated. We have also constructed a low-dimensional model which qualitatively captures the dynamics of the system near the homoclinic bifurcations for low rotation rates. The model is used to study the unfolding of bifurcations and the variation of the homoclinic bifurcation threshold with Pr. Copyright (C) EPLA, 2013

DOI

20

Spooling and disordered packing of elastic rods in cylindrical cavities - Pineirua, M. and Adda-Bedia, M. and Moulinet, S.

EPL 104, (2013)

Abstract : The compaction of elastic rods in rigid cylindrical cavities is experimentally performed. The results show two main packing behaviours: an ordered regime in which the rod spools on the internal surface of the cavity and a disordered phase where the orientation of the coils is randomly distributed. The phase diagram separating these two packing configurations is determined as a function of the aspect ratio of the container and of the intrinsic curvature of the rod. A theoretical stability analysis and an experimental study of the dynamics of the rod at the injection point allow to describe different instability mechanisms that drive the transitions from ordered to disordered packing, leading to the identification of the different disordering scenarios. Copyright (C) EPLA, 2013

EPL 104, (2013)

Abstract : The compaction of elastic rods in rigid cylindrical cavities is experimentally performed. The results show two main packing behaviours: an ordered regime in which the rod spools on the internal surface of the cavity and a disordered phase where the orientation of the coils is randomly distributed. The phase diagram separating these two packing configurations is determined as a function of the aspect ratio of the container and of the intrinsic curvature of the rod. A theoretical stability analysis and an experimental study of the dynamics of the rod at the injection point allow to describe different instability mechanisms that drive the transitions from ordered to disordered packing, leading to the identification of the different disordering scenarios. Copyright (C) EPLA, 2013