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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.

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
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.

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.

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.

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