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Emile O.,University of Rennes 1 | Emile J.,Rennes Institute of Physics
Physical Review Letters | Year: 2011

We report on the deformation of an air-water surface with a totally reflected low-power laser beam, inducing a convex mirror effect on the beam propagation. This bending is stronger close to the critical angle and depends on the polarization of the laser light. A model, leading to a simple dependence between the Goos-Hänchen shift and the radius of curvature of the interface, supports these observations. Bendings with radius of curvature as low as 0.10 m are demonstrated. © 2011 American Physical Society.

Cantat I.,Rennes Institute of Physics
Physics of Fluids | Year: 2013

Many microfluidics devices, coating processes, or diphasic flows involve the motion of a liquid meniscus on a wet wall. This motion induces a specific viscous force, that exhibits a nonlinear dependency in the meniscus velocity. We propose a review of the theoretical and experimental work made on this viscous force, for simple interfacial properties. The interface is indeed assumed either perfectly compressible (mobile interface) or perfectly incompressible (rigid interface). We show that, in the second case, the viscous force exerted by the wall on the meniscus is a combination of two power laws, scaling such as Ca1/3 and Ca2/3, with Ca the capillary number. We provide a prediction for the stress exerted on a foam sliding on a wet solid and compare it with experimental data, for the incompressible case. © 2013 American Institute of Physics.

Bertoni R.,Rennes Institute of Physics
Nature Materials | Year: 2016

Photoinduced phase transformations occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently affecting the functionalities. The transient nature of photoinduced states has thus far severely limited the scope of applications. It is of paramount importance to explore whether structural feedback during the solid deformation has the capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons, which have long been under scrutiny, coherently propagating cell deformations over acoustic timescales have not been explored to a similar degree, particularly with respect to cooperative elastic interactions. Herein we demonstrate, experimentally and theoretically, a self-amplified responsiveness in a spin-crossover material during its delayed volume expansion. The cooperative response at the material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically driven cooperativity triggered by a light pulse offers an efficient route towards the generation and stabilization of photoinduced phases in many volume-changing materials. © 2016 Nature Publishing Group

Le Caer G.,Rennes Institute of Physics
Journal of Statistical Physics | Year: 2010

A constrained diffusive random walk of n steps in Rd and a random flight in Rd, which are equivalent, were investigated independently in recent papers (J. Stat. Phys. 127:813, 2007; J. Theor. Probab. 20:769, 2007, and J. Stat. Phys. 131:1039, 2008). The n steps of the walk are independent and identically distributed random vectors of exponential length and uniform orientation. Conditioned on the sum of their lengths being equal to a given value l, closed-form expressions for the distribution of the endpoint of the walk were obtained altogether for any n for d=1,2,4. Uniform distributions of the endpoint inside a ball of radius l were evidenced for a walk of three steps in 2D and of two steps in 4D. The previous walk is generalized by considering step lengths which have independent and identical gamma distributions with a shape parameter q>0. Given the total walk length being equal to 1, the step lengths have a Dirichlet distribution whose parameters are all equal to q. The walk and the flight above correspond to q=1. Simple analytical expressions are obtained for any d≥2 and n≥2 for the endpoint distributions of two families of walks whose q are integers or half-integers which depend solely on d. These endpoint distributions have a simple geometrical interpretation. Expressed for a two-step planar walk whose q=1, it means that the distribution of the endpoint on a disc of radius 1 is identical to the distribution of the projection on the disc of a point M uniformly distributed over the surface of the 3D unit sphere. Five additional walks, with a uniform distribution of the endpoint in the inside of a ball, are found from known finite integrals of products of powers and Bessel functions of the first kind. They include four different walks in R3, two of two steps and two of three steps, and one walk of two steps in R4. Pearson-Liouville random walks, obtained by distributing the total lengths of the previous Pearson-Dirichlet walks according to some specified probability law are finally discussed. Examples of unconstrained random walks, whose step lengths are gamma distributed, are more particularly considered. © 2010 Springer Science+Business Media, LLC.

Rouxel T.,Rennes Institute of Physics
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2015

The occurrence of damage at the surface of glass parts caused by sharp contact loading is a major issue for glass makers, suppliers and end-users. Yet, it is still a poorly understood problem from the viewpoints both of glass science and solid mechanics. Different microcracking patterns are observed at indentation sites depending on the glass composition and indentation cracks may form during both the loading and the unloading stages. Besides, we do not know much about the fracture toughness of glass and its composition dependence, so that setting a criterion for crack initiation and predicting the extent of the damage yet remain out of reach. In this study, by comparison of the behaviour of glasses from very different chemical systems and by identifying experimentally the individual contributions of the different rheological processes leading to the formation of the imprint-namely elasticity, densification and shear flow-we obtain a fairly straightforward prediction of the type and extent of the microcracks which will most likely form, depending on the physical properties of the glass. Finally, some guidelines to reduce the driving force for microcracking are proposed in the light of the effects of composition, temperature and pressure, and the areas for further research are briefly discussed. © 2015 The Author(s) Published by the Royal Society. All rights reserved.

Fade J.,Rennes Institute of Physics | Alouini M.,Rennes Institute of Physics
Physical Review Letters | Year: 2012

A new concept devoted to sensing the depolarization strength of materials from a single measurement is proposed and successfully validated on a variety of samples. It relies on the measurement of the orthogonality breaking between two orthogonal states of polarization after interaction with the material to be characterized. Due to orthogonality preservation between the two states after propagation in birefringent media, this measurement concept is shown to be perfectly suited to depolarization remote sensing through fibers, opening the way to real-time depolarization endoscopy. © 2012 American Physical Society.

Le Caer G.,Rennes Institute of Physics
Physica A: Statistical Mechanics and its Applications | Year: 2015

Random walks of n steps taken into independent uniformly random directions in a d-dimensional Euclidean space (d≥2), which are characterized by a sum of step lengths which is fixed and taken to be 1 without loss of generality, are named "Dirichlet" when this constraint is realized via a Dirichlet law of step lengths. The latter continuous multivariate distribution, which depends on n positive parameters, generalizes the beta distribution (n=2). It is simply obtained from n independent gamma random variables with identical scale factors. Previous literature studies of these random walks dealt with symmetric Dirichlet distributions whose parameters are all equal to a value q which takes half-integer or integer values. In the present work, the probability density function of the distance from the endpoint to the origin is first made explicit for a symmetric Dirichlet random walk of two steps. It is valid for any positive value of q and for all d≥2. The latter pdf is used in turn to express the related density of a random walk of two steps whose step length is distributed according to an asymmetric beta distribution which depends on two parameters, namely q and q+s where s is a positive integer. © 2015 Elsevier B.V. All rights reserved.

Ghoufi A.,Rennes Institute of Physics
The European physical journal. E, Soft matter | Year: 2013

Many Body Dissipative Particles Dynamics (MDPD) simulation is a novel promising mesoscopic method to model the liquid-vapor interfaces. Based upon works of Paganobarraga and Frenkel (J. Chem. Phys. 15, 5015 (2001)) and Trofimov (J. Chem. Phys. 117, 9383 (2002)) and of Warren (Phys. Rev. E 68, 066702 (2003)) this method has been critically reviewed during this last decade. We propose here to give an overview of the Many Body Dissipative Particles Dynamic simulation within the framework of the liquid-vapor interfaces. We recall the theoretical background of MDPD and we present some recent results of systems of interest such as water liquid-vapor interfaces and salt effect on water surface tension. Additionally we discuss the ability of MDPD to capture the mechanisms at the mesoscopic scale through the formation of micelles and the coalescence of a nanodroplet water on water surface.

Cantat I.,Rennes Institute of Physics
Soft Matter | Year: 2011

The origin of the dissipation in liquid foams is not fully understood, especially in the large deformation, large velocity regime. Numerical simulations, now very accurate in the quasi static regime, are still sparse in the dissipative regime, and are all based on restrictive assumptions or very small bubble numbers. Here we present the results obtained with 2D numerical simulations involving 500 bubbles under simple shear, in a non-quasi static regime. The bubble description is kept as simple as possible and the dissipation is assumed to arise from surface tension variations induced by film area variations. This model leads to a steady state stress under simple shear that is well fitted by a Herschel-Bulkley law with an exponent 0.6. We show that small tension dynamical inhomogeneities induce foam structure modifications responsible for the largest part of the stress increase. © 2011 The Royal Society of Chemistry.

Le Caer G.,Rennes Institute of Physics
Journal of Statistical Physics | Year: 2011

An n-step Pearson-Gamma random walk in ℝd starts at the origin and consists of n independent steps with gamma distributed lengths and uniform orientations. The gamma distribution of each step length has a shape parameter q > 0. Constrained random walks of n steps in ℝd are obtained from the latter walks by imposing that the sum of the step lengths is equal to a fixed value. Simple closed-form expressions were obtained in particular for the distribution of the endpoint of such constrained walks for any d ≥ d0 and any n ≥ 2 when q is either q = d/2 - 1 (d0 = 3) or q = d-1 (d0 = 2) (Le Caër in J. Stat. Phys. 140:728-751, 2010). When the total walk length is chosen, without loss of generality, to be equal to 1, then the constrained step lengths have a Dirichlet distribution whose parameters are all equal to q and the associated walk is thus named a Pearson-Dirichlet random walk. The density of the endpoint position of a n-step planar walk of this type (n ≥ 2), with q = d = 2, was shown recently to be a weighted mixture of 1 + floor(n/2) endpoint densities of planar Pearson-Dirichlet walks with q = 1 (Beghin and Orsingher in Stochastics 82:201-229, 2010). The previous result is generalized to any walk space dimension and any number of steps n ≥ 2 when the parameter of the Pearson-Dirichlet random walk is q = d > 1. We rely on the connection between an unconstrained random walk and a constrained one, which have both the same n and the same q = d, to obtain a closed-form expression of the endpoint density. The latter is a weighted mixture of 1 + floor(n/2) densities with simple forms, equivalently expressed as a product of a power and a Gauss hypergeometric function. The weights are products of factors which depends both on d and n and Bessel numbers independent of d. © 2011 Springer Science+Business Media, LLC.

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